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