Make readonly field is possible

src/Apache.Arrow/Arrays/Bool8Array.cs

@@ -50,7 +50,7 @@ public override bool TryCreateType(IArrowType storageType, string metadata, out
     /// </summary>
     public class Bool8Type : ExtensionType
     {
-        public static Bool8Type Default = new Bool8Type();
+        public static readonly Bool8Type Default = new Bool8Type();
 
         public override string Name => "arrow.bool8";
         public override string ExtensionMetadata => "";

src/Apache.Arrow/Arrays/GuidArray.cs

@@ -50,7 +50,7 @@ public override bool TryCreateType(IArrowType storageType, string metadata, out
     /// </summary>
     public class GuidType : ExtensionType
     {
-        public static GuidType Default = new GuidType();
+        public static readonly GuidType Default = new GuidType();
 
         internal const int ByteWidth = 16;
 

src/Apache.Arrow/Arrays/VariantArray.cs

@@ -29,7 +29,7 @@ namespace Apache.Arrow
     /// </summary>
     public class VariantExtensionDefinition : ExtensionDefinition
     {
-        public static VariantExtensionDefinition Instance = new VariantExtensionDefinition();
+        public static readonly VariantExtensionDefinition Instance = new VariantExtensionDefinition();
 
         public override string ExtensionName => VariantType.ExtensionName;
 
@@ -87,7 +87,7 @@ public class VariantType : ExtensionType
     {
         internal const string ExtensionName = "arrow.parquet.variant";
 
-        public static VariantType Default = new VariantType();
+        public static readonly VariantType Default = new VariantType();
 
         public override string Name => ExtensionName;
         public override string ExtensionMetadata => "";

src/Apache.Arrow/Types/StringType.cs

@@ -18,7 +18,7 @@ namespace Apache.Arrow.Types
 {
     public sealed class StringType : ArrowType
     {
-        public static StringType Default = new StringType();
+        public static readonly StringType Default = new StringType();
 
         public override ArrowTypeId TypeId => ArrowTypeId.String;
         public override string Name => "utf8";

src/Apache.Arrow/Types/StringViewType.cs

@@ -18,7 +18,7 @@ namespace Apache.Arrow.Types
 {
     public sealed class StringViewType : ArrowType
     {
-        public static StringViewType Default = new StringViewType();
+        public static readonly StringViewType Default = new StringViewType();
 
         public override ArrowTypeId TypeId => ArrowTypeId.StringView;
         public override string Name => "utf8view";