Concatenate Lists in C#: net10.0, collection expression, parameterised benchmark

[vladimir-pecanac-main]

Sample update for the article "Concatenate Lists in C#: How to Combine Two Lists" (https://code-maze.com/csharp-concatenate-lists/).

Changes, all inside collections-lists/ConcatenateLists:

• All three projects move from net6.0 to net10.0.
• New UsingCollectionExpression method ([.. firstList, .. secondList]) with a test and a benchmark.
• New UsingAddNoCapacity and UsingAddRangeNoCapacity methods with tests and benchmarks, so the article's list-capacity comparison is measured by committed code.
• ConcatenatorBenchmark: [Params(50_000, 1_000_000)] on a Size field, and inputs built in [GlobalSetup] as distinct strings (Code0..CodeN and Maze0..MazeN). The old Enumerable.Repeat inputs made Union return two elements while every other method returned two million.
• Count() replaced by the Count property (seven call sites in Concatenator.cs, six in Tests.cs).
• UsingCopyTo: the string[] variable is named combinedArray and both CopyTo calls use the two-argument overload.
• ConcatenateLists/Program.cs converted to top-level statements; Console.ReadLine removed.
• New test: Union returns one element for ["a", "a"] and ["a"].

Package versions, re-queried on NuGet on 2026-10-10 (latest listed stable):

• Microsoft.NET.Test.Sdk 18.10.1
• xunit 2.9.3
• xunit.runner.visualstudio 4.0.1
• coverlet.collector 10.1.0
• BenchmarkDotNet 0.15.8

Local results on SDK 10.0.302 / runtime 10.0.10: dotnet build -c Release 0 warnings 0 errors; dotnet test 10 passed, 0 failed; dotnet list package --vulnerable --include-transitive reports no vulnerable packages in any project.