diff --git a/docs/generating_csharp.md b/docs/generating_csharp.md
index 7a50918e..bac77a0f 100644
--- a/docs/generating_csharp.md
+++ b/docs/generating_csharp.md
@@ -182,6 +182,12 @@ Eventually we might add translation from the Doxygen comment annotations to thos
 
 This should be enough to at least display those comments in IDEs, but any `@...` doxygen tags will be left as is, instead of being translated into their proper XML form.
 
+## Exposed structs at the P/Invoke boundary
+
+Structs exposed via `--expose-as-struct` (see [the C docs](/docs/generating_c.md#expose-simple-structs-as-structs)) are passed by value between C and C# as blittable C# structs, with one exception. If an exposed struct has exactly one field, and that field is a scalar (an arithmetic type other than `bool`, or an enum), then the `DllImport` declarations use that scalar directly, and the struct is wrapped and unwrapped on the C# side. The public C# API is unaffected, and the C side is unaffected too, because in C both spellings have the same ABI.
+
+This is needed for Unity's IL2CPP on WebAssembly. IL2CPP wraps every C# struct into a union with padding, and on wasm32 Clang only passes and returns single-element structs as plain scalars, which that wrapper no longer is. So the IL2CPP-compiled call site would return such a struct through a hidden pointer and pass it by pointer, while the C library returns and accepts a plain scalar (`wasm-ld` reports this as `function signature mismatch`, and the calls trap or read garbage at runtime). A scalar has the same ABI everywhere. Structs with more than one field don't have this problem, since both sides agree on passing those indirectly.
+
 ## Distributing the C# bindings as a Nuget package
 
 This is not a full explanation, but a rought outline of what you need to do.
diff --git a/src/generators/csharp/generator.cpp b/src/generators/csharp/generator.cpp
index b4de8111..865e8ca2 100644
--- a/src/generators/csharp/generator.cpp
+++ b/src/generators/csharp/generator.cpp
@@ -933,6 +933,56 @@ namespace mrbind::CSharp
         return CppToCSharpIdentifier(name.parts.back());
     }
 
+    std::optional<Generator::ExposedStructSingleScalarField> Generator::GetExposedStructSingleScalarField(const CInterop::TypeKinds::Class &class_desc, const cppdecl::QualifiedName &cpp_class_name)
+    {
+        assert(class_desc.kind == CInterop::ClassKind::exposed_struct);
+
+        // Find the only non-static field, if any.
+        const CInterop::ClassField *field = nullptr;
+        for (const CInterop::ClassField &elem : class_desc.fields)
+        {
+            if (elem.is_static)
+                continue;
+            if (field)
+                return {}; // More than one field.
+            field = &elem;
+        }
+        if (!field)
+            return {};
+
+        cppdecl::Type cpp_type = ParseTypeOrThrow(field->type);
+        if (!cpp_type.modifiers.empty())
+            return {}; // Arrays and pointers.
+        cpp_type.RemoveQualifiers(cppdecl::CvQualifiers::const_);
+        const std::string cpp_type_str = CppdeclToCode(cpp_type);
+
+        std::string csharp_type;
+        if (auto prim = c_desc.platform_info.FindPrimitiveType(cpp_type_str))
+        {
+            // Not `bool`, because the C# struct stores it as a `byte` behind a property, and because `bool` isn't blittable when passed by value anyway.
+            if (prim->kind == PrimitiveTypeInfo::Kind::boolean)
+                return {};
+
+            auto csharp_type_opt = CToCSharpPrimitiveTypeOpt(cpp_type_str, false);
+            if (!csharp_type_opt)
+                return {};
+            csharp_type = std::string(*csharp_type_opt);
+        }
+        else if (auto type_desc = c_desc.FindTypeOpt(cpp_type_str); type_desc && std::holds_alternative<CInterop::TypeKinds::Enum>(type_desc->var))
+        {
+            csharp_type = CppToCSharpEnumName(cpp_type.simple_type.name);
+        }
+        else
+        {
+            return {}; // Nested exposed structs and anything else.
+        }
+
+        return ExposedStructSingleScalarField{
+            .csharp_type = std::move(csharp_type),
+            .csharp_field_name = CppToCSharpFieldName(cpp_class_name, false, field->full_name),
+        };
+    }
+
     std::string Generator::CppToCSharpByValueHelperName(cppdecl::QualifiedName name, bool is_shared)
     {
         // Must make this before adjusting the name.
@@ -1704,15 +1754,40 @@ namespace mrbind::CSharp
                                         const std::string csharp_value_type = CppToCSharpExposedStructName(cpp_effective_type.simple_type.name);
                                         const std::string csharp_in_opt_type = CppToCSharpInOptStructHelperName(cpp_effective_type.simple_type.name);
 
+                                        // Exposed structs with exactly one scalar field are passed through `DllImport` by value as that scalar, not as the struct.
+                                        // In C both spellings have the same ABI on every platform we care about, so the C side is unaffected.
+                                        // This is needed for Unity's IL2CPP on wasm32. IL2CPP wraps every C# struct into a union with padding, and Clang only
+                                        //   passes/returns single-element structs as plain scalars on wasm32, which that wrapper isn't. So the IL2CPP-compiled
+                                        //   call site returns such a struct through a hidden pointer and passes it by pointer, while the C library returns and
+                                        //   accepts a plain scalar (`wasm-ld` warns about `function signature mismatch`, and the calls trap or read garbage).
+                                        //   A scalar has the same ABI on both sides. Structs with several fields are fine, both sides pass those indirectly.
+                                        // The pass-by-pointer variant below (for default arguments) is unaffected, pointers are always the same.
+                                        const std::optional<ExposedStructSingleScalarField> scalar_field = GetExposedStructSingleScalarField(elem, cpp_effective_type.simple_type.name);
+                                        const std::string csharp_dllimport_type = scalar_field ? scalar_field->csharp_type : csharp_value_type;
+
+                                        TypeBinding::ReturnUsage return_usage{
+                                            .cpp_never_throws = true, // Exposed structs must be trivial, which means all their non-deleted SMFs must be trivial too, which implies non-throwing.
+                                            .dllimport_return_type = csharp_dllimport_type,
+                                            .csharp_return_type = csharp_value_type,
+                                            // Default `make_return_statements` is good enough when not unwrapping the scalar.
+                                        };
+                                        if (scalar_field)
+                                        {
+                                            return_usage.make_return_statements = [csharp_value_type, csharp_field_name = scalar_field->csharp_field_name](const std::string &target, const std::string &expr)
+                                            {
+                                                return target + " new " + csharp_value_type + " {" + csharp_field_name + " = " + expr + "};";
+                                            };
+                                        }
+
                                         return CreateBinding({
                                             .param_usage = TypeBinding::ParamUsage{
-                                                .make_strings = [csharp_value_type](const std::string &name, bool /*have_useless_defarg*/)
+                                                .make_strings = [csharp_value_type, csharp_dllimport_type, scalar_field](const std::string &name, bool /*have_useless_defarg*/)
                                                 {
                                                     return TypeBinding::ParamUsage::Strings{
                                                         .cpp_never_throws = true, // Exposed structs must be trivial, which means all their non-deleted SMFs must be trivial too, which implies non-throwing.
-                                                        .dllimport_decl_params = {{.type = csharp_value_type, .name = name}},
+                                                        .dllimport_decl_params = {{.type = csharp_dllimport_type, .name = name}},
                                                         .csharp_decl_params = {{.type = csharp_value_type, .name = name}},
-                                                        .dllimport_args = {name},
+                                                        .dllimport_args = {scalar_field ? name + "." + scalar_field->csharp_field_name : name},
                                                     };
                                                 },
                                             },
@@ -1727,12 +1802,7 @@ namespace mrbind::CSharp
                                                     };
                                                 },
                                             },
-                                            .return_usage = TypeBinding::ReturnUsage{
-                                                .cpp_never_throws = true, // Exposed structs must be trivial, which means all their non-deleted SMFs must be trivial too, which implies non-throwing.
-                                                .dllimport_return_type = csharp_value_type,
-                                                .csharp_return_type = csharp_value_type,
-                                                // Default `make_return_expr` is good enough!
-                                            },
+                                            .return_usage = std::move(return_usage),
                                         });
                                     }
                                     break;
@@ -4807,7 +4877,8 @@ namespace mrbind::CSharp
                     {
                         // You can assign to `this`, and it assigns elementwise! Nice.
                         // See: https://stackoverflow.com/q/10038598/2752075
-                        file.WriteString("this = " + expr + ";\n");
+                        // This goes through the return binding because exposed structs with a single scalar field are returned from C as that scalar, see `GetTypeBindingOpt()`.
+                        file.WriteString(ret_binding->MakeReturnStatements("this =", expr) + "\n");
                     }
                     else
                     {
@@ -4821,7 +4892,8 @@ namespace mrbind::CSharp
 
                             ctor_expr = "(_Underlying *)" + generator.RequestHelper("_Alloc") + "(" + class_size_str + ")";
 
-                            post_ctor_statements = "*(" + generator.CppToCSharpExposedStructName(generator.ParseNameOrThrow(func_like.ret.cpp_type)) + " *)_UnderlyingPtr = " + expr + ";\n";
+                            // This goes through the return binding because exposed structs with a single scalar field are returned from C as that scalar, see `GetTypeBindingOpt()`.
+                            post_ctor_statements = ret_binding->MakeReturnStatements("*(" + generator.CppToCSharpExposedStructName(generator.ParseNameOrThrow(func_like.ret.cpp_type)) + " *)_UnderlyingPtr =", expr) + "\n";
                         }
                         else
                         {
diff --git a/src/generators/csharp/generator.h b/src/generators/csharp/generator.h
index 0400c53d..6c36aecb 100644
--- a/src/generators/csharp/generator.h
+++ b/src/generators/csharp/generator.h
@@ -670,6 +670,19 @@ namespace mrbind::CSharp
         //   since the result can be affected by the properties of the class.
         [[nodiscard]] std::string CppToCSharpUnqualExposedStructName(cppdecl::QualifiedName name);
 
+        // Describes the only field of an exposed struct that has exactly one scalar field. See `GetExposedStructSingleScalarField()`.
+        struct ExposedStructSingleScalarField
+        {
+            // The C# type of the field, e.g. `int`.
+            std::string csharp_type;
+            // The C# name of the field in the exposed struct.
+            std::string csharp_field_name;
+        };
+        // If the exposed struct `class_desc` (named `cpp_class_name` in C++) has exactly one non-static field, and that field is either an arithmetic type
+        //   other than `bool` or an enum, returns the description of that field. Otherwise returns null.
+        // Such structs are passed through `DllImport` by value as that field rather than as the struct itself. See `GetTypeBindingOpt()` for the explanation.
+        [[nodiscard]] std::optional<ExposedStructSingleScalarField> GetExposedStructSingleScalarField(const CInterop::TypeKinds::Class &class_desc, const cppdecl::QualifiedName &cpp_class_name);
+
         // Converts a C++ qualified class name to a C# name of its helper that's used to pass it by value.
         // This only makes sense for classes that use the pass-by enum.
         [[nodiscard]] std::string CppToCSharpByValueHelperName(cppdecl::QualifiedName name, bool is_shared);
diff --git a/test/input/MR/test_csharp.h b/test/input/MR/test_csharp.h
index f2eef0cf..ebed9791 100644
--- a/test/input/MR/test_csharp.h
+++ b/test/input/MR/test_csharp.h
@@ -1065,6 +1065,9 @@ namespace MR::CSharp
         int x;
     };
 
+    // Single-field exposed structs are passed through P/Invoke as their only field. This tests by-value parameters and return values.
+    inline ExposedLayoutC test_exposed_c(ExposedLayoutC a, ExposedLayoutC b = {}) {a.x += b.x; return a;}
+
 
     // Test various array members.
     struct ArrayMembers
diff --git a/test/output_c/include/MR/test_csharp.h b/test/output_c/include/MR/test_csharp.h
index 40486b2a..ebf7f289 100644
--- a/test/output_c/include/MR/test_csharp.h
+++ b/test/output_c/include/MR/test_csharp.h
@@ -6312,6 +6312,11 @@ MR_C_API MR_CSharp_ExposedLayoutB MR_CSharp_ExposedLayoutB_Construct_1(const MR_
 /// Parameter `_2` can not be null. It is a single object.
 MR_C_API bool MR_C_equal_MR_CSharp_ExposedLayoutB(const MR_CSharp_ExposedLayoutB *_1, const MR_CSharp_ExposedLayoutB *_2);
 
+// Single-field exposed structs are passed through P/Invoke as their only field. This tests by-value parameters and return values.
+/// Generated from function `MR::CSharp::test_exposed_c`.
+/// Parameter `b` has a default argument: `{}`, pass a null pointer to use it.
+MR_C_API MR_CSharp_ExposedLayoutC MR_CSharp_test_exposed_c(MR_CSharp_ExposedLayoutC a, const MR_CSharp_ExposedLayoutC *b);
+
 /// Returns a pointer to a member variable of class `MR::CSharp::ArrayMembers` named `i`.
 /// Parameter `_this` can not be null. It is a single object.
 /// The returned pointer will never be null. It is non-owning, do NOT destroy it.
diff --git a/test/output_c/source/MR/test_csharp.cpp b/test/output_c/source/MR/test_csharp.cpp
index 65968fc7..6c0a21e0 100644
--- a/test/output_c/source/MR/test_csharp.cpp
+++ b/test/output_c/source/MR/test_csharp.cpp
@@ -9550,6 +9550,18 @@ bool MR_C_equal_MR_CSharp_ExposedLayoutB(const MR_CSharp_ExposedLayoutB *_1, con
     ) // MRBINDC_TRY
 }
 
+MR_CSharp_ExposedLayoutC MR_CSharp_test_exposed_c(MR_CSharp_ExposedLayoutC a, const MR_CSharp_ExposedLayoutC *b)
+{
+    MRBINDC_TRY(
+    using namespace MR;
+    using namespace CSharp;
+    return MRBINDC_BIT_CAST((MR_CSharp_ExposedLayoutC), ::MR::CSharp::test_exposed_c(
+        MRBINDC_BIT_CAST((MR::CSharp::ExposedLayoutC), a),
+        (b ? MRBINDC_BIT_CAST((MR::CSharp::ExposedLayoutC), *b) : MR::CSharp::ExposedLayoutC(MR::CSharp::ExposedLayoutC{}))
+    ));
+    ) // MRBINDC_TRY
+}
+
 const int *MR_CSharp_ArrayMembers_Get_i(const MR_CSharp_ArrayMembers *_this)
 {
     return std::addressof(((_this ? void() : MRBINDC_THROW("Parameter `_this` can not be null.", void)), *(const MR::CSharp::ArrayMembers *)(_this)).i);
diff --git a/test/output_c_fixed_typedefs/include/MR/test_csharp.h b/test/output_c_fixed_typedefs/include/MR/test_csharp.h
index 4fd33589..3f58c286 100644
--- a/test/output_c_fixed_typedefs/include/MR/test_csharp.h
+++ b/test/output_c_fixed_typedefs/include/MR/test_csharp.h
@@ -6128,6 +6128,11 @@ MR_C_API MR_CSharp_ExposedLayoutB MR_CSharp_ExposedLayoutB_Construct_1(const MR_
 // Parameter `_2` can not be null. It is a single object.
 MR_C_API bool MR_C_equal_MR_CSharp_ExposedLayoutB(const MR_CSharp_ExposedLayoutB *_1, const MR_CSharp_ExposedLayoutB *_2);
 
+// Single-field exposed structs are passed through P/Invoke as their only field. This tests by-value parameters and return values.
+// Generated from function `MR::CSharp::test_exposed_c`.
+// Parameter `b` has a default argument: `{}`, pass a null pointer to use it.
+MR_C_API MR_CSharp_ExposedLayoutC MR_CSharp_test_exposed_c(MR_CSharp_ExposedLayoutC a, const MR_CSharp_ExposedLayoutC *b);
+
 // Returns a pointer to a member variable of class `MR::CSharp::ArrayMembers` named `i`.
 // Parameter `_this` can not be null. It is a single object.
 // The returned pointer will never be null. It is non-owning, do NOT destroy it.
diff --git a/test/output_c_fixed_typedefs/source/MR/test_csharp.cpp b/test/output_c_fixed_typedefs/source/MR/test_csharp.cpp
index 9f0434a2..df275891 100644
--- a/test/output_c_fixed_typedefs/source/MR/test_csharp.cpp
+++ b/test/output_c_fixed_typedefs/source/MR/test_csharp.cpp
@@ -6882,6 +6882,14 @@ bool MR_C_equal_MR_CSharp_ExposedLayoutB(const MR_CSharp_ExposedLayoutB *_1, con
     );
 }
 
+MR_CSharp_ExposedLayoutC MR_CSharp_test_exposed_c(MR_CSharp_ExposedLayoutC a, const MR_CSharp_ExposedLayoutC *b)
+{
+    return MRBINDC_BIT_CAST((MR_CSharp_ExposedLayoutC), ::MR::CSharp::test_exposed_c(
+        MRBINDC_BIT_CAST((MR::CSharp::ExposedLayoutC), a),
+        (b ? MRBINDC_BIT_CAST((MR::CSharp::ExposedLayoutC), *b) : MR::CSharp::ExposedLayoutC(MR::CSharp::ExposedLayoutC{}))
+    ));
+}
+
 const int32_t *MR_CSharp_ArrayMembers_Get_i(const MR_CSharp_ArrayMembers *_this)
 {
     return std::addressof(((_this ? void() : MRBINDC_THROW("Parameter `_this` can not be null.", void)), *(const MR::CSharp::ArrayMembers *)(_this)).i);
diff --git a/test/output_c_fixed_typedefs_64_only/include/MR/test_csharp.h b/test/output_c_fixed_typedefs_64_only/include/MR/test_csharp.h
index 691e4495..dd0f43b8 100644
--- a/test/output_c_fixed_typedefs_64_only/include/MR/test_csharp.h
+++ b/test/output_c_fixed_typedefs_64_only/include/MR/test_csharp.h
@@ -6311,6 +6311,11 @@ MR_C_API MR_CSharp_ExposedLayoutB MR_CSharp_ExposedLayoutB_Construct_1(const MR_
 /// Parameter `_2` can not be null. It is a single object.
 MR_C_API bool MR_C_equal_MR_CSharp_ExposedLayoutB(const MR_CSharp_ExposedLayoutB *_1, const MR_CSharp_ExposedLayoutB *_2);
 
+// Single-field exposed structs are passed through P/Invoke as their only field. This tests by-value parameters and return values.
+/// Generated from function `MR::CSharp::test_exposed_c`.
+/// Parameter `b` has a default argument: `{}`, pass a null pointer to use it.
+MR_C_API MR_CSharp_ExposedLayoutC MR_CSharp_test_exposed_c(MR_CSharp_ExposedLayoutC a, const MR_CSharp_ExposedLayoutC *b);
+
 /// Returns a pointer to a member variable of class `MR::CSharp::ArrayMembers` named `i`.
 /// Parameter `_this` can not be null. It is a single object.
 /// The returned pointer will never be null. It is non-owning, do NOT destroy it.
diff --git a/test/output_c_fixed_typedefs_64_only/source/MR/test_csharp.cpp b/test/output_c_fixed_typedefs_64_only/source/MR/test_csharp.cpp
index c6ba68ac..ea50349e 100644
--- a/test/output_c_fixed_typedefs_64_only/source/MR/test_csharp.cpp
+++ b/test/output_c_fixed_typedefs_64_only/source/MR/test_csharp.cpp
@@ -7138,6 +7138,14 @@ bool MR_C_equal_MR_CSharp_ExposedLayoutB(const MR_CSharp_ExposedLayoutB *_1, con
     );
 }
 
+MR_CSharp_ExposedLayoutC MR_CSharp_test_exposed_c(MR_CSharp_ExposedLayoutC a, const MR_CSharp_ExposedLayoutC *b)
+{
+    return MRBINDC_BIT_CAST((MR_CSharp_ExposedLayoutC), ::MR::CSharp::test_exposed_c(
+        MRBINDC_BIT_CAST((MR::CSharp::ExposedLayoutC), a),
+        (b ? MRBINDC_BIT_CAST((MR::CSharp::ExposedLayoutC), *b) : MR::CSharp::ExposedLayoutC(MR::CSharp::ExposedLayoutC{}))
+    ));
+}
+
 const int *MR_CSharp_ArrayMembers_Get_i(const MR_CSharp_ArrayMembers *_this)
 {
     return std::addressof(((_this ? void() : MRBINDC_THROW("Parameter `_this` can not be null.", void)), *(const MR::CSharp::ArrayMembers *)(_this)).i);
diff --git a/test/output_csharp/src/MR/test_csharp.cs b/test/output_csharp/src/MR/test_csharp.cs
index e810ba01..8de9690d 100644
--- a/test/output_csharp/src/MR/test_csharp.cs
+++ b/test/output_csharp/src/MR/test_csharp.cs
@@ -13643,10 +13643,10 @@ public unsafe ConstBox_ConvCtorExposed(ConstBox_ConvCtorExposed _other) : this(n
                 public unsafe ConstBox_ConvCtorExposed(int _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
                     MR.CS.Misc._Exceptions.Prepare();
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
 
@@ -13683,10 +13683,10 @@ public unsafe Box_ConvCtorExposed(ConstBox_ConvCtorExposed _other) : this(null,
                 public unsafe Box_ConvCtorExposed(int _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
                     MR.CS.Misc._Exceptions.Prepare();
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
 
@@ -13713,9 +13713,9 @@ public struct ConvCtorExposed
                 public unsafe ConvCtorExposed(int _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
                     MR.CS.Misc._Exceptions.Prepare();
-                    this = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    this = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
 
@@ -15720,10 +15720,10 @@ public unsafe ConstBox_ExposedLayoutB(ConstBox_ExposedLayoutB _other) : this(nul
                 public unsafe ConstBox_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
                     MR.CS.Misc._Exceptions.Prepare();
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
 
@@ -15732,10 +15732,10 @@ public unsafe ConstBox_ExposedLayoutB(int _1, int _2) : this(null, is_owning: tr
                 public unsafe ConstBox_ExposedLayoutB(MR.CS.CSharp.Const_A _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
                     MR.CS.Misc._Exceptions.Prepare();
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                     _KeepAlive(_1);
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
@@ -15809,10 +15809,10 @@ public unsafe Box_ExposedLayoutB(ConstBox_ExposedLayoutB _other) : this(null, is
                 public unsafe Box_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
                     MR.CS.Misc._Exceptions.Prepare();
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
 
@@ -15821,10 +15821,10 @@ public unsafe Box_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 public unsafe Box_ExposedLayoutB(MR.CS.CSharp.Const_A _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
                     MR.CS.Misc._Exceptions.Prepare();
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                     _KeepAlive(_1);
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
@@ -15853,9 +15853,9 @@ public struct ExposedLayoutB : System.IEquatable<MR.CS.CSharp.ExposedLayoutB>
                 public unsafe ExposedLayoutB(int _1, int _2)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
                     MR.CS.Misc._Exceptions.Prepare();
-                    this = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    this = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
 
@@ -15864,9 +15864,9 @@ public unsafe ExposedLayoutB(int _1, int _2)
                 public unsafe ExposedLayoutB(MR.CS.CSharp.Const_A _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
                     MR.CS.Misc._Exceptions.Prepare();
-                    this = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    this = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
                 }
 
@@ -25651,6 +25651,19 @@ public static unsafe ref readonly MR.CS.CSharp.ExposedLayoutSh TestExposedCref(i
                 if (__c_ret is not null) return *__c_ret; else return null;
             }
 
+            // Single-field exposed structs are passed through P/Invoke as their only field. This tests by-value parameters and return values.
+            /// Generated from function `MR::CSharp::test_exposed_c`.
+            /// Parameter `b` defaults to `{}`.
+            public static unsafe MR.CS.CSharp.ExposedLayoutC TestExposedC(MR.CS.CSharp.ExposedLayoutC a, MR.CS.CSharp._InOpt_ExposedLayoutC b = default)
+            {
+                [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_test_exposed_c", ExactSpelling = true)]
+                extern static int __MR_CSharp_test_exposed_c(int a, MR.CS.CSharp.ExposedLayoutC *b);
+                MR.CS.Misc._Exceptions.Prepare();
+                var __c_ret = __MR_CSharp_test_exposed_c(a.x, b.HasValue ? &b.Object : null);
+                MR.CS.Misc._Exceptions.ThrowIfNeeded();
+                return new MR.CS.CSharp.ExposedLayoutC {x = __c_ret};
+            }
+
             /// Generated from function `MR::CSharp::test_optint`.
             /// Parameter `b` defaults to `default_optint`.
             /// Parameter `c` defaults to `default_optint`.
diff --git a/test/output_csharp/src/MR/test_declaration_order.cs b/test/output_csharp/src/MR/test_declaration_order.cs
index 302cbef8..a798fdd2 100644
--- a/test/output_csharp/src/MR/test_declaration_order.cs
+++ b/test/output_csharp/src/MR/test_declaration_order.cs
@@ -86,11 +86,11 @@ public unsafe Box_A(ConstBox_A _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.A.B C()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_c", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.A.B __MR_DeclOrder_A_c(_Underlying *_this);
+                    extern static int __MR_DeclOrder_A_c(_Underlying *_this);
                     MR.CS.Misc._Exceptions.Prepare();
                     var __c_ret = __MR_DeclOrder_A_c(_UnderlyingPtr);
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                    return __c_ret;
+                    return new MR.CS.DeclOrder.A.B {bleh = __c_ret};
                 }
 
                 /// Generated from method `MR::DeclOrder::A::d`.
@@ -124,13 +124,13 @@ public struct A
                 public unsafe MR.CS.DeclOrder.A.B C()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_c", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.A.B __MR_DeclOrder_A_c(MR.CS.DeclOrder.A *_this);
+                    extern static int __MR_DeclOrder_A_c(MR.CS.DeclOrder.A *_this);
                     fixed (MR.CS.DeclOrder.A *__ptr__this = &this)
                     {
                         MR.CS.Misc._Exceptions.Prepare();
                         var __c_ret = __MR_DeclOrder_A_c(__ptr__this);
                         MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                        return __c_ret;
+                        return new MR.CS.DeclOrder.A.B {bleh = __c_ret};
                     }
                 }
 
@@ -228,11 +228,11 @@ public unsafe Box_B(ConstBox_B _other) : this(null, is_owning: true)
                     public unsafe MR.CS.DeclOrder.A A()
                     {
                         [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_B_a", ExactSpelling = true)]
-                        extern static MR.CS.DeclOrder.A __MR_DeclOrder_A_B_a(_Underlying *_this);
+                        extern static int __MR_DeclOrder_A_B_a(_Underlying *_this);
                         MR.CS.Misc._Exceptions.Prepare();
                         var __c_ret = __MR_DeclOrder_A_B_a(_UnderlyingPtr);
                         MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                        return __c_ret;
+                        return new MR.CS.DeclOrder.A {blah = __c_ret};
                     }
 
                     /// Generated from method `MR::DeclOrder::A::B::b`.
@@ -265,13 +265,13 @@ public struct B
                     public unsafe MR.CS.DeclOrder.A A()
                     {
                         [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_B_a", ExactSpelling = true)]
-                        extern static MR.CS.DeclOrder.A __MR_DeclOrder_A_B_a(MR.CS.DeclOrder.A.B *_this);
+                        extern static int __MR_DeclOrder_A_B_a(MR.CS.DeclOrder.A.B *_this);
                         fixed (MR.CS.DeclOrder.A.B *__ptr__this = &this)
                         {
                             MR.CS.Misc._Exceptions.Prepare();
                             var __c_ret = __MR_DeclOrder_A_B_a(__ptr__this);
                             MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                            return __c_ret;
+                            return new MR.CS.DeclOrder.A {blah = __c_ret};
                         }
                     }
 
@@ -513,11 +513,11 @@ public unsafe Box_C_False(ConstBox_C_False _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.C_True Blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_false_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_True __MR_DeclOrder_C_false_blah(_Underlying *_this);
+                    extern static int __MR_DeclOrder_C_false_blah(_Underlying *_this);
                     MR.CS.Misc._Exceptions.Prepare();
                     var __c_ret = __MR_DeclOrder_C_false_blah(_UnderlyingPtr);
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                    return __c_ret;
+                    return new MR.CS.DeclOrder.C_True {bleh = __c_ret};
                 }
             }
 
@@ -539,13 +539,13 @@ public struct C_False
                 public unsafe MR.CS.DeclOrder.C_True Blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_false_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_True __MR_DeclOrder_C_false_blah(MR.CS.DeclOrder.C_False *_this);
+                    extern static int __MR_DeclOrder_C_false_blah(MR.CS.DeclOrder.C_False *_this);
                     fixed (MR.CS.DeclOrder.C_False *__ptr__this = &this)
                     {
                         MR.CS.Misc._Exceptions.Prepare();
                         var __c_ret = __MR_DeclOrder_C_false_blah(__ptr__this);
                         MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                        return __c_ret;
+                        return new MR.CS.DeclOrder.C_True {bleh = __c_ret};
                     }
                 }
             }
@@ -699,11 +699,11 @@ public unsafe Box_C_True(ConstBox_C_True _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.C_False Blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_true_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_False __MR_DeclOrder_C_true_blah(_Underlying *_this);
+                    extern static int __MR_DeclOrder_C_true_blah(_Underlying *_this);
                     MR.CS.Misc._Exceptions.Prepare();
                     var __c_ret = __MR_DeclOrder_C_true_blah(_UnderlyingPtr);
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                    return __c_ret;
+                    return new MR.CS.DeclOrder.C_False {bleh = __c_ret};
                 }
             }
 
@@ -725,13 +725,13 @@ public struct C_True
                 public unsafe MR.CS.DeclOrder.C_False Blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_true_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_False __MR_DeclOrder_C_true_blah(MR.CS.DeclOrder.C_True *_this);
+                    extern static int __MR_DeclOrder_C_true_blah(MR.CS.DeclOrder.C_True *_this);
                     fixed (MR.CS.DeclOrder.C_True *__ptr__this = &this)
                     {
                         MR.CS.Misc._Exceptions.Prepare();
                         var __c_ret = __MR_DeclOrder_C_true_blah(__ptr__this);
                         MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                        return __c_ret;
+                        return new MR.CS.DeclOrder.C_False {bleh = __c_ret};
                     }
                 }
             }
diff --git a/test/output_csharp/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs b/test/output_csharp/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
index cf6489c4..b993fce0 100644
--- a/test/output_csharp/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
+++ b/test/output_csharp/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
@@ -68,27 +68,27 @@ public static unsafe implicit operator bool(MR.CS.Std.Const_Function_MRCSharpExp
                 public unsafe MR.CS.CSharp.ExposedLayoutC Call(MR.CS.CSharp.ExposedLayoutC _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutC __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_Underlying *_this, MR.CS.CSharp.ExposedLayoutC _1);
+                    extern static int __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_Underlying *_this, int _1);
                     MR.CS.Misc._Exceptions.Prepare();
-                    var __c_ret = __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_UnderlyingPtr, _1);
+                    var __c_ret = __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_UnderlyingPtr, _1.x);
                     MR.CS.Misc._Exceptions.ThrowIfNeeded();
-                    return __c_ret;
+                    return new MR.CS.CSharp.ExposedLayoutC {x = __c_ret};
                 }
 
                 // Custom extras:
 
                 public delegate MR.CS.CSharp.ExposedLayoutC Delegate(ref readonly MR.CS.CSharp.ExposedLayoutC _1);
 
-                private protected unsafe delegate MR.CS.CSharp.ExposedLayoutC _CDelegate(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value);
+                private protected unsafe delegate int _CDelegate(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value);
 
-                private protected static unsafe MR.CS.CSharp.ExposedLayoutC _CCallWrapper(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value)
+                private protected static unsafe int _CCallWrapper(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value)
                 {
                     try
                     {
                         ref readonly MR.CS.CSharp.ExposedLayoutC _arg_1 = ref *(MR.CS.CSharp.ExposedLayoutC *)sizeof(MR.CS.CSharp.ExposedLayoutC); // Uninitialized ref.
                         _arg_1 = ref *_1;
                         MR.CS.CSharp.ExposedLayoutC _ret = ((Delegate)System.Runtime.InteropServices.GCHandle.FromIntPtr((nint)_userdata).Target!)(in _arg_1);
-                        return _ret;
+                        return _ret.x;
                     }
                     catch (Exception __e)
                     {
diff --git a/test/output_csharp_fixed_typedefs/src/MR/test_csharp.cs b/test/output_csharp_fixed_typedefs/src/MR/test_csharp.cs
index 479f7aad..d798f78a 100644
--- a/test/output_csharp_fixed_typedefs/src/MR/test_csharp.cs
+++ b/test/output_csharp_fixed_typedefs/src/MR/test_csharp.cs
@@ -14066,9 +14066,9 @@ public unsafe ConstBox_ConvCtorExposed(ConstBox_ConvCtorExposed _other) : this(n
                 public unsafe ConstBox_ConvCtorExposed(int _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                 }
 
                 /// <summary>
@@ -14118,9 +14118,9 @@ public unsafe Box_ConvCtorExposed(ConstBox_ConvCtorExposed _other) : this(null,
                 public unsafe Box_ConvCtorExposed(int _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                 }
 
                 /// <summary>
@@ -14156,8 +14156,8 @@ public struct ConvCtorExposed
                 public unsafe ConvCtorExposed(int _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
-                    this = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    this = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                 }
 
                 /// <summary>
@@ -16286,9 +16286,9 @@ public unsafe ConstBox_ExposedLayoutB(ConstBox_ExposedLayoutB _other) : this(nul
                 public unsafe ConstBox_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -16298,9 +16298,9 @@ public unsafe ConstBox_ExposedLayoutB(int _1, int _2) : this(null, is_owning: tr
                 public unsafe ConstBox_ExposedLayoutB(MR.CS.CSharp.Const_A _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -16385,9 +16385,9 @@ public unsafe Box_ExposedLayoutB(ConstBox_ExposedLayoutB _other) : this(null, is
                 public unsafe Box_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -16397,9 +16397,9 @@ public unsafe Box_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 public unsafe Box_ExposedLayoutB(MR.CS.CSharp.Const_A _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -16436,8 +16436,8 @@ public struct ExposedLayoutB : System.IEquatable<MR.CS.CSharp.ExposedLayoutB>
                 public unsafe ExposedLayoutB(int _1, int _2)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
-                    this = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    this = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -16447,8 +16447,8 @@ public unsafe ExposedLayoutB(int _1, int _2)
                 public unsafe ExposedLayoutB(MR.CS.CSharp.Const_A _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
-                    this = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    this = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -27133,6 +27133,18 @@ public static unsafe ref readonly MR.CS.CSharp.ExposedLayoutSh testExposedCref(i
                 if (__c_ret is not null) return *__c_ret; else return null;
             }
 
+            // Single-field exposed structs are passed through P/Invoke as their only field. This tests by-value parameters and return values.
+            /// <summary>
+            /// Generated from function `MR::CSharp::test_exposed_c`.
+            /// Parameter `b` defaults to `{}`.
+            /// </summary>
+            public static unsafe MR.CS.CSharp.ExposedLayoutC testExposedC(MR.CS.CSharp.ExposedLayoutC a, MR.CS.CSharp._InOpt_ExposedLayoutC b = default)
+            {
+                [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_test_exposed_c", ExactSpelling = true)]
+                extern static int __MR_CSharp_test_exposed_c(int a, MR.CS.CSharp.ExposedLayoutC *b);
+                return new MR.CS.CSharp.ExposedLayoutC {x = __MR_CSharp_test_exposed_c(a.x, b.HasValue ? &b.Object : null)};
+            }
+
             /// <summary>
             /// Generated from function `MR::CSharp::test_optint`.
             /// Parameter `b` defaults to `MR::CSharp::default_optint`.
diff --git a/test/output_csharp_fixed_typedefs/src/MR/test_declaration_order.cs b/test/output_csharp_fixed_typedefs/src/MR/test_declaration_order.cs
index 99302f26..92a9b9a4 100644
--- a/test/output_csharp_fixed_typedefs/src/MR/test_declaration_order.cs
+++ b/test/output_csharp_fixed_typedefs/src/MR/test_declaration_order.cs
@@ -112,8 +112,8 @@ public unsafe Box_A(ConstBox_A _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.A.B c()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_c", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.A.B __MR_DeclOrder_A_c(_Underlying *_this);
-                    return __MR_DeclOrder_A_c(_UnderlyingPtr);
+                    extern static int __MR_DeclOrder_A_c(_Underlying *_this);
+                    return new MR.CS.DeclOrder.A.B {bleh = __MR_DeclOrder_A_c(_UnderlyingPtr)};
                 }
 
                 /// <summary>
@@ -154,10 +154,10 @@ public struct A
                 public unsafe MR.CS.DeclOrder.A.B c()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_c", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.A.B __MR_DeclOrder_A_c(MR.CS.DeclOrder.A *_this);
+                    extern static int __MR_DeclOrder_A_c(MR.CS.DeclOrder.A *_this);
                     fixed (MR.CS.DeclOrder.A *__ptr__this = &this)
                     {
-                        return __MR_DeclOrder_A_c(__ptr__this);
+                        return new MR.CS.DeclOrder.A.B {bleh = __MR_DeclOrder_A_c(__ptr__this)};
                     }
                 }
 
@@ -280,8 +280,8 @@ public unsafe Box_B(ConstBox_B _other) : this(null, is_owning: true)
                     public unsafe MR.CS.DeclOrder.A a()
                     {
                         [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_B_a", ExactSpelling = true)]
-                        extern static MR.CS.DeclOrder.A __MR_DeclOrder_A_B_a(_Underlying *_this);
-                        return __MR_DeclOrder_A_B_a(_UnderlyingPtr);
+                        extern static int __MR_DeclOrder_A_B_a(_Underlying *_this);
+                        return new MR.CS.DeclOrder.A {blah = __MR_DeclOrder_A_B_a(_UnderlyingPtr)};
                     }
 
                     /// <summary>
@@ -321,10 +321,10 @@ public struct B
                     public unsafe MR.CS.DeclOrder.A a()
                     {
                         [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_B_a", ExactSpelling = true)]
-                        extern static MR.CS.DeclOrder.A __MR_DeclOrder_A_B_a(MR.CS.DeclOrder.A.B *_this);
+                        extern static int __MR_DeclOrder_A_B_a(MR.CS.DeclOrder.A.B *_this);
                         fixed (MR.CS.DeclOrder.A.B *__ptr__this = &this)
                         {
-                            return __MR_DeclOrder_A_B_a(__ptr__this);
+                            return new MR.CS.DeclOrder.A {blah = __MR_DeclOrder_A_B_a(__ptr__this)};
                         }
                     }
 
@@ -603,8 +603,8 @@ public unsafe Box_C_False(ConstBox_C_False _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.C_True blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_false_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_True __MR_DeclOrder_C_false_blah(_Underlying *_this);
-                    return __MR_DeclOrder_C_false_blah(_UnderlyingPtr);
+                    extern static int __MR_DeclOrder_C_false_blah(_Underlying *_this);
+                    return new MR.CS.DeclOrder.C_True {bleh = __MR_DeclOrder_C_false_blah(_UnderlyingPtr)};
                 }
             }
 
@@ -634,10 +634,10 @@ public struct C_False
                 public unsafe MR.CS.DeclOrder.C_True blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_false_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_True __MR_DeclOrder_C_false_blah(MR.CS.DeclOrder.C_False *_this);
+                    extern static int __MR_DeclOrder_C_false_blah(MR.CS.DeclOrder.C_False *_this);
                     fixed (MR.CS.DeclOrder.C_False *__ptr__this = &this)
                     {
-                        return __MR_DeclOrder_C_false_blah(__ptr__this);
+                        return new MR.CS.DeclOrder.C_True {bleh = __MR_DeclOrder_C_false_blah(__ptr__this)};
                     }
                 }
             }
@@ -823,8 +823,8 @@ public unsafe Box_C_True(ConstBox_C_True _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.C_False blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_true_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_False __MR_DeclOrder_C_true_blah(_Underlying *_this);
-                    return __MR_DeclOrder_C_true_blah(_UnderlyingPtr);
+                    extern static int __MR_DeclOrder_C_true_blah(_Underlying *_this);
+                    return new MR.CS.DeclOrder.C_False {bleh = __MR_DeclOrder_C_true_blah(_UnderlyingPtr)};
                 }
             }
 
@@ -854,10 +854,10 @@ public struct C_True
                 public unsafe MR.CS.DeclOrder.C_False blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_true_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_False __MR_DeclOrder_C_true_blah(MR.CS.DeclOrder.C_True *_this);
+                    extern static int __MR_DeclOrder_C_true_blah(MR.CS.DeclOrder.C_True *_this);
                     fixed (MR.CS.DeclOrder.C_True *__ptr__this = &this)
                     {
-                        return __MR_DeclOrder_C_true_blah(__ptr__this);
+                        return new MR.CS.DeclOrder.C_False {bleh = __MR_DeclOrder_C_true_blah(__ptr__this)};
                     }
                 }
             }
diff --git a/test/output_csharp_fixed_typedefs/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs b/test/output_csharp_fixed_typedefs/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
index dc86ac14..4018f136 100644
--- a/test/output_csharp_fixed_typedefs/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
+++ b/test/output_csharp_fixed_typedefs/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
@@ -75,22 +75,22 @@ public static unsafe implicit operator bool(MR.CS.Std.Const_Function_MRCSharpExp
                 public unsafe MR.CS.CSharp.ExposedLayoutC call(MR.CS.CSharp.ExposedLayoutC _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutC __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_Underlying *_this, MR.CS.CSharp.ExposedLayoutC _1);
-                    return __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_UnderlyingPtr, _1);
+                    extern static int __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_Underlying *_this, int _1);
+                    return new MR.CS.CSharp.ExposedLayoutC {x = __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_UnderlyingPtr, _1.x)};
                 }
 
                 // Custom extras:
 
                 public delegate MR.CS.CSharp.ExposedLayoutC Delegate(ref readonly MR.CS.CSharp.ExposedLayoutC _1);
 
-                private protected unsafe delegate MR.CS.CSharp.ExposedLayoutC _CDelegate(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value);
+                private protected unsafe delegate int _CDelegate(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value);
 
-                private protected static unsafe MR.CS.CSharp.ExposedLayoutC _CCallWrapper(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value)
+                private protected static unsafe int _CCallWrapper(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value)
                 {
                     ref readonly MR.CS.CSharp.ExposedLayoutC _arg_1 = ref *(MR.CS.CSharp.ExposedLayoutC *)sizeof(MR.CS.CSharp.ExposedLayoutC); // Uninitialized ref.
                     _arg_1 = ref *_1;
                     MR.CS.CSharp.ExposedLayoutC _ret = ((Delegate)System.Runtime.InteropServices.GCHandle.FromIntPtr((nint)_userdata).Target!)(in _arg_1);
-                    return _ret;
+                    return _ret.x;
                 }
 
                 /// <summary>
diff --git a/test/output_csharp_fixed_typedefs_64_only/src/MR/test_csharp.cs b/test/output_csharp_fixed_typedefs_64_only/src/MR/test_csharp.cs
index 8175b4d6..8d35f1a3 100644
--- a/test/output_csharp_fixed_typedefs_64_only/src/MR/test_csharp.cs
+++ b/test/output_csharp_fixed_typedefs_64_only/src/MR/test_csharp.cs
@@ -15214,9 +15214,9 @@ public unsafe ConstBox_ConvCtorExposed(ConstBox_ConvCtorExposed _other) : this(n
                 public unsafe ConstBox_ConvCtorExposed(int _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                 }
 
                 /// <summary>
@@ -15266,9 +15266,9 @@ public unsafe Box_ConvCtorExposed(ConstBox_ConvCtorExposed _other) : this(null,
                 public unsafe Box_ConvCtorExposed(int _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    *(MR.CS.CSharp.ConvCtorExposed *)_UnderlyingPtr = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                 }
 
                 /// <summary>
@@ -15304,8 +15304,8 @@ public struct ConvCtorExposed
                 public unsafe ConvCtorExposed(int _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ConvCtorExposed_Construct", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ConvCtorExposed __MR_CSharp_ConvCtorExposed_Construct(int _1);
-                    this = __MR_CSharp_ConvCtorExposed_Construct(_1);
+                    extern static int __MR_CSharp_ConvCtorExposed_Construct(int _1);
+                    this = new MR.CS.CSharp.ConvCtorExposed {x = __MR_CSharp_ConvCtorExposed_Construct(_1)};
                 }
 
                 /// <summary>
@@ -17526,9 +17526,9 @@ public unsafe ConstBox_ExposedLayoutB(ConstBox_ExposedLayoutB _other) : this(nul
                 public unsafe ConstBox_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -17538,9 +17538,9 @@ public unsafe ConstBox_ExposedLayoutB(int _1, int _2) : this(null, is_owning: tr
                 public unsafe ConstBox_ExposedLayoutB(MR.CS.CSharp.Const_A _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -17625,9 +17625,9 @@ public unsafe Box_ExposedLayoutB(ConstBox_ExposedLayoutB _other) : this(null, is
                 public unsafe Box_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -17637,9 +17637,9 @@ public unsafe Box_ExposedLayoutB(int _1, int _2) : this(null, is_owning: true)
                 public unsafe Box_ExposedLayoutB(MR.CS.CSharp.Const_A _1) : this(null, is_owning: true)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
                     _UnderlyingPtr = (_Underlying *)MR.CS.Misc._Alloc(4);
-                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    *(MR.CS.CSharp.ExposedLayoutB *)_UnderlyingPtr = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -17676,8 +17676,8 @@ public struct ExposedLayoutB : System.IEquatable<MR.CS.CSharp.ExposedLayoutB>
                 public unsafe ExposedLayoutB(int _1, int _2)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_2", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
-                    this = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_2(int _1, int _2);
+                    this = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_2(_1, _2)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -17687,8 +17687,8 @@ public unsafe ExposedLayoutB(int _1, int _2)
                 public unsafe ExposedLayoutB(MR.CS.CSharp.Const_A _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_ExposedLayoutB_Construct_1", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutB __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
-                    this = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr);
+                    extern static int __MR_CSharp_ExposedLayoutB_Construct_1(MR.CS.CSharp.Const_A._Underlying *_1);
+                    this = new MR.CS.CSharp.ExposedLayoutB {x = __MR_CSharp_ExposedLayoutB_Construct_1(_1._UnderlyingPtr)};
                 }
 
                 // This gets a lifetime annotation from `--infer-lifetime-constructors`, but it should be a no-op in an exposed struct.
@@ -28358,6 +28358,18 @@ public static unsafe ref readonly MR.CS.CSharp.ExposedLayoutSh testExposedCref(i
                 if (__c_ret is not null) return *__c_ret; else return null;
             }
 
+            // Single-field exposed structs are passed through P/Invoke as their only field. This tests by-value parameters and return values.
+            /// <summary>
+            /// Generated from function `MR::CSharp::test_exposed_c`.
+            /// Parameter `b` defaults to `{}`.
+            /// </summary>
+            public static unsafe MR.CS.CSharp.ExposedLayoutC testExposedC(MR.CS.CSharp.ExposedLayoutC a, MR.CS.CSharp._InOpt_ExposedLayoutC b = default)
+            {
+                [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_CSharp_test_exposed_c", ExactSpelling = true)]
+                extern static int __MR_CSharp_test_exposed_c(int a, MR.CS.CSharp.ExposedLayoutC *b);
+                return new MR.CS.CSharp.ExposedLayoutC {x = __MR_CSharp_test_exposed_c(a.x, b.HasValue ? &b.Object : null)};
+            }
+
             /// <summary>
             /// Generated from function `MR::CSharp::test_optint`.
             /// Parameter `b` defaults to `MR::CSharp::default_optint`.
diff --git a/test/output_csharp_fixed_typedefs_64_only/src/MR/test_declaration_order.cs b/test/output_csharp_fixed_typedefs_64_only/src/MR/test_declaration_order.cs
index 99302f26..92a9b9a4 100644
--- a/test/output_csharp_fixed_typedefs_64_only/src/MR/test_declaration_order.cs
+++ b/test/output_csharp_fixed_typedefs_64_only/src/MR/test_declaration_order.cs
@@ -112,8 +112,8 @@ public unsafe Box_A(ConstBox_A _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.A.B c()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_c", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.A.B __MR_DeclOrder_A_c(_Underlying *_this);
-                    return __MR_DeclOrder_A_c(_UnderlyingPtr);
+                    extern static int __MR_DeclOrder_A_c(_Underlying *_this);
+                    return new MR.CS.DeclOrder.A.B {bleh = __MR_DeclOrder_A_c(_UnderlyingPtr)};
                 }
 
                 /// <summary>
@@ -154,10 +154,10 @@ public struct A
                 public unsafe MR.CS.DeclOrder.A.B c()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_c", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.A.B __MR_DeclOrder_A_c(MR.CS.DeclOrder.A *_this);
+                    extern static int __MR_DeclOrder_A_c(MR.CS.DeclOrder.A *_this);
                     fixed (MR.CS.DeclOrder.A *__ptr__this = &this)
                     {
-                        return __MR_DeclOrder_A_c(__ptr__this);
+                        return new MR.CS.DeclOrder.A.B {bleh = __MR_DeclOrder_A_c(__ptr__this)};
                     }
                 }
 
@@ -280,8 +280,8 @@ public unsafe Box_B(ConstBox_B _other) : this(null, is_owning: true)
                     public unsafe MR.CS.DeclOrder.A a()
                     {
                         [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_B_a", ExactSpelling = true)]
-                        extern static MR.CS.DeclOrder.A __MR_DeclOrder_A_B_a(_Underlying *_this);
-                        return __MR_DeclOrder_A_B_a(_UnderlyingPtr);
+                        extern static int __MR_DeclOrder_A_B_a(_Underlying *_this);
+                        return new MR.CS.DeclOrder.A {blah = __MR_DeclOrder_A_B_a(_UnderlyingPtr)};
                     }
 
                     /// <summary>
@@ -321,10 +321,10 @@ public struct B
                     public unsafe MR.CS.DeclOrder.A a()
                     {
                         [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_A_B_a", ExactSpelling = true)]
-                        extern static MR.CS.DeclOrder.A __MR_DeclOrder_A_B_a(MR.CS.DeclOrder.A.B *_this);
+                        extern static int __MR_DeclOrder_A_B_a(MR.CS.DeclOrder.A.B *_this);
                         fixed (MR.CS.DeclOrder.A.B *__ptr__this = &this)
                         {
-                            return __MR_DeclOrder_A_B_a(__ptr__this);
+                            return new MR.CS.DeclOrder.A {blah = __MR_DeclOrder_A_B_a(__ptr__this)};
                         }
                     }
 
@@ -603,8 +603,8 @@ public unsafe Box_C_False(ConstBox_C_False _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.C_True blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_false_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_True __MR_DeclOrder_C_false_blah(_Underlying *_this);
-                    return __MR_DeclOrder_C_false_blah(_UnderlyingPtr);
+                    extern static int __MR_DeclOrder_C_false_blah(_Underlying *_this);
+                    return new MR.CS.DeclOrder.C_True {bleh = __MR_DeclOrder_C_false_blah(_UnderlyingPtr)};
                 }
             }
 
@@ -634,10 +634,10 @@ public struct C_False
                 public unsafe MR.CS.DeclOrder.C_True blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_false_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_True __MR_DeclOrder_C_false_blah(MR.CS.DeclOrder.C_False *_this);
+                    extern static int __MR_DeclOrder_C_false_blah(MR.CS.DeclOrder.C_False *_this);
                     fixed (MR.CS.DeclOrder.C_False *__ptr__this = &this)
                     {
-                        return __MR_DeclOrder_C_false_blah(__ptr__this);
+                        return new MR.CS.DeclOrder.C_True {bleh = __MR_DeclOrder_C_false_blah(__ptr__this)};
                     }
                 }
             }
@@ -823,8 +823,8 @@ public unsafe Box_C_True(ConstBox_C_True _other) : this(null, is_owning: true)
                 public unsafe MR.CS.DeclOrder.C_False blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_true_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_False __MR_DeclOrder_C_true_blah(_Underlying *_this);
-                    return __MR_DeclOrder_C_true_blah(_UnderlyingPtr);
+                    extern static int __MR_DeclOrder_C_true_blah(_Underlying *_this);
+                    return new MR.CS.DeclOrder.C_False {bleh = __MR_DeclOrder_C_true_blah(_UnderlyingPtr)};
                 }
             }
 
@@ -854,10 +854,10 @@ public struct C_True
                 public unsafe MR.CS.DeclOrder.C_False blah()
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_DeclOrder_C_true_blah", ExactSpelling = true)]
-                    extern static MR.CS.DeclOrder.C_False __MR_DeclOrder_C_true_blah(MR.CS.DeclOrder.C_True *_this);
+                    extern static int __MR_DeclOrder_C_true_blah(MR.CS.DeclOrder.C_True *_this);
                     fixed (MR.CS.DeclOrder.C_True *__ptr__this = &this)
                     {
-                        return __MR_DeclOrder_C_true_blah(__ptr__this);
+                        return new MR.CS.DeclOrder.C_False {bleh = __MR_DeclOrder_C_true_blah(__ptr__this)};
                     }
                 }
             }
diff --git a/test/output_csharp_fixed_typedefs_64_only/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs b/test/output_csharp_fixed_typedefs_64_only/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
index dc86ac14..4018f136 100644
--- a/test/output_csharp_fixed_typedefs_64_only/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
+++ b/test/output_csharp_fixed_typedefs_64_only/src/std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC.cs
@@ -75,22 +75,22 @@ public static unsafe implicit operator bool(MR.CS.Std.Const_Function_MRCSharpExp
                 public unsafe MR.CS.CSharp.ExposedLayoutC call(MR.CS.CSharp.ExposedLayoutC _1)
                 {
                     [System.Runtime.InteropServices.DllImport("bleh", EntryPoint = "MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call", ExactSpelling = true)]
-                    extern static MR.CS.CSharp.ExposedLayoutC __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_Underlying *_this, MR.CS.CSharp.ExposedLayoutC _1);
-                    return __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_UnderlyingPtr, _1);
+                    extern static int __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_Underlying *_this, int _1);
+                    return new MR.CS.CSharp.ExposedLayoutC {x = __MR_C_std_function_MR_CSharp_ExposedLayoutC_from_MR_CSharp_ExposedLayoutC_call(_UnderlyingPtr, _1.x)};
                 }
 
                 // Custom extras:
 
                 public delegate MR.CS.CSharp.ExposedLayoutC Delegate(ref readonly MR.CS.CSharp.ExposedLayoutC _1);
 
-                private protected unsafe delegate MR.CS.CSharp.ExposedLayoutC _CDelegate(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value);
+                private protected unsafe delegate int _CDelegate(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value);
 
-                private protected static unsafe MR.CS.CSharp.ExposedLayoutC _CCallWrapper(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value)
+                private protected static unsafe int _CCallWrapper(MR.CS.CSharp.ExposedLayoutC *_1, void *_userdata, void **_cleanup_value)
                 {
                     ref readonly MR.CS.CSharp.ExposedLayoutC _arg_1 = ref *(MR.CS.CSharp.ExposedLayoutC *)sizeof(MR.CS.CSharp.ExposedLayoutC); // Uninitialized ref.
                     _arg_1 = ref *_1;
                     MR.CS.CSharp.ExposedLayoutC _ret = ((Delegate)System.Runtime.InteropServices.GCHandle.FromIntPtr((nint)_userdata).Target!)(in _arg_1);
-                    return _ret;
+                    return _ret.x;
                 }
 
                 /// <summary>