The PreferDisposeOverTestCleanupFixer was incorrectly handling partial classes where a Dispose method exists in one partial declaration and a TestCleanup method exists in another. This resulted in the codefix creating a duplicate Dispose method, causing compiler error CS0111.

Problem

Consider this partial class scenario:

public partial class MyTestClass : IDisposable
{
    public void Dispose()
    {
        int x = 1;
    }
}

[TestClass]
public partial class MyTestClass
{
    [TestCleanup]
    public void MyTestCleanup()
    {
        int y = 1;
    }
}

The old fixer logic only searched for Dispose methods within the current partial class declaration (containingType.Members), so it would not find the existing Dispose method in the other partial declaration. This caused it to create a new Dispose method in the second partial class, resulting in:

// Incorrect output with CS0111 error
[TestClass]
public partial class MyTestClass : IDisposable
{
    public void Dispose()  // ❌ Duplicate method definition
    {
        int y = 1;
    }
}

Solution

Updated the fixer to use typeSymbol.GetMembers("Dispose") which searches across all parts of a partial class. The fix now correctly:

1. Finds the existing Dispose method in any partial declaration
2. Appends the TestCleanup method body to the existing Dispose method
3. Removes the TestCleanup method

Result:

public partial class MyTestClass : IDisposable
{
    public void Dispose()
    {
        int x = 1;
        int y = 1;  // ✅ TestCleanup body appended
    }
}

[TestClass]
public partial class MyTestClass
{
    // ✅ TestCleanup method removed
}

Changes

• PreferDisposeOverTestCleanupFixer.cs: Enhanced to detect Dispose methods across all partial class declarations using semantic model
• PreferDisposeOverTestCleanupAnalyzerTests.cs: Updated test expectation from broken behavior (CS0111 error) to correct behavior
• Added safeguard to only handle Dispose methods in the same document

Fixes #4181.

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