Fix TestContext.Properties to return null instead of throwing KeyNotFoundException

[Copilot]

MSTest 4.x introduced a breaking change where accessing TestContext.Properties with non-existent keys throws KeyNotFoundException instead of returning null as in 3.x. This affects both direct dictionary access and property accessors.

Root Cause

The issue had two parts:

1. GetProperty method: Used conditional compilation with platform-specific behavior
   • WINDOWS_UWP/WIN_UI: Used TryGetValue (correct)
   • Other platforms: Used dictionary indexer Properties[name] (throws on missing keys)
2. Direct dictionary access: Properties returned raw Dictionary<string, object?> which throws on missing keys

Changes

• src/TestFramework/TestFramework.Extensions/TestContext.cs

  • Removed platform-specific behavior, unified to use TryGetValue across all platforms
  • Fixes property accessors like TestRunDirectory, DeploymentDirectory, ResultsDirectory

• src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs

  • Added NullReturningDictionary wrapper class that implements IDictionary<string, object?>
  • Overrides indexer getter to return null for non-existent keys instead of throwing
  • All dictionary operations (Add, TryGetValue, indexer) delegate to underlying dictionary
  • Removed redundant _properties field - all operations go through the wrapper
  • Constructor uses local variable for initialization before wrapping
  • IsReadOnly property properly delegates to underlying dictionary
  • Added comprehensive documentation explaining backwards compatibility requirements
  • Updated Properties property to return the wrapper
  • Updated TryGetPropertyValue and AddProperty methods to use wrapper

• Added comprehensive tests

  • Unit test: Verifies both direct dictionary access (Properties["NonExistent"]) and property accessors return null when unset
  • Acceptance test: Tests both direct dictionary access (Properties["NonExistent"]) and property accessors in end-to-end scenario

Example

// MSTest 3.x and now fixed 4.x
var value = testContext.Properties["NonExistent"];  // Returns null
var dir = testContext.TestRunDirectory;              // Returns null when not set

// MSTest 4.x before fix
var value = testContext.Properties["NonExistent"];  // Throws KeyNotFoundException
var dir = testContext.TestRunDirectory;              // Throws KeyNotFoundException

Original prompt

---

This section details on the original issue you should resolve

<issue_title>MSTest 4.x breaking change causes TestContext.Properties to throw KeyNotFoundException instead of returning null</issue_title>
<issue_description>## Describe the bug

During our migration from MSTest 3.x to 4.x, I've noticed a breaking change where calls to TextContext.Properties["NonExistent"] throw an exception instead of returning null. We have a lot of code that depends on this returning null.

Steps To Reproduce

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <IsPackable>false</IsPackable>
    <IsTestProject>true</IsTestProject>
    <EnableMSTestRunner>true</EnableMSTestRunner>
    <OutputType>Exe</OutputType>
    
    <!-- Set MSTestVersion property to switch versions: 3 or 4 (default) -->
    <MSTestVersion Condition="'$(MSTestVersion)' == ''">4</MSTestVersion>
  </PropertyGroup>

  <!-- MSTest 3.x -->
  <ItemGroup Condition="'$(MSTestVersion)' == '3'">
    <PackageReference Include="MSTest" Version="3.6.3" />
  </ItemGroup>

  <!-- MSTest 4.x (default) -->
  <ItemGroup Condition="'$(MSTestVersion)' == '4'">
    <PackageReference Include="MSTest" Version="4.0.2" />
  </ItemGroup>

</Project>

using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace MSTestPropertyRepro;

[TestClass]
public class PropertyAccessTests
{
    public TestContext? TestContext { get; set; }

    [TestMethod]
    public void TestAccessingNonExistentProperty()
    {
        // Try to access a property that doesn't exist
        try
        {
            var nonExistent = TestContext?.Properties["NonExistentProperty"];
            Console.WriteLine($"NonExistentProperty returned: {nonExistent ?? "null"}");
            
            if (nonExistent == null)
            {
                Console.WriteLine("✓ MSTest 3.x behavior: Returns null for missing properties");
            }
            else
            {
                Console.WriteLine($"✗ Unexpected: Got value '{nonExistent}'");
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine($"✗ MSTest 4.x behavior: Throws exception for missing properties");
            Console.WriteLine($"Exception: {ex.GetType().Name} - {ex.Message}");
            throw;
        }
    }
}

MSTest 3.x

MSTest v3.6.3 (UTC 11/11/2024) [win-x64 - .NET 10.0.0]

Test run summary: Passed! - bin\Debug\net10.0\MSTestPropertyRepro.dll (net10.0|x64)
  total: 1
  failed: 0
  succeeded: 1
  skipped: 0

✅ Returns null when accessing non-existent properties - Test PASSES

MSTest 4.x

MSTest v4.0.0-preview.25358.7 (UTC 7/8/2025) [win-x64 - .NET 10.0.0]

Test run summary: Failed! - bin\Debug\net10.0\MSTestPropertyRepro.dll (net10.0|x64)
  total: 1
  failed: 1
  succeeded: 0
  skipped: 0
  
Exception: System.Collections.Generic.KeyNotFoundException: 
The given key 'NonExistentProperty' was not present in the dictionary.

❌ Throws KeyNotFoundException when accessing non-existent properties - Test FAILS

Breaking Change

// MSTest 3.x
var value = TestContext.Properties["NonExistent"]; // Returns null

// MSTest 4.x
var value = TestContext.Properties["NonExistent"]; // Throws KeyNotFoundException
```</issue_description>

<agent_instructions>Fix the implementation to return `null` instead of throwing if the `key` doesn't exist</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes MSTest 4.x breaking change causes TestContext.Properties to throw KeyNotFoundException instead of returning null #6952

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[Evangelink]

Moving back to draft so we can discuss with @Youssef1313 as it seems the issue reported internally is different.

[nohwnd]

This is documented, and better behavior than before. And good time to do breaking change between major versions.

IDictionary also documents that key not found will throw exception, so hiding a type behind that interface that works differently is a change that will surprise new users.