Add analyzer for duplicate test method attributes (MSTEST0059)

[Copilot]

Methods decorated with multiple TestMethodAttribute-derived attributes compile but only the first attribute returned by reflection will be used, which can be confusing. For example:

public class MyTestMethod : TestMethodAttribute { }

[TestClass]
public class MyTestClass
{
    [TestMethod]
    [MyTestMethod]  // Warning: multiple test method attributes
    public void M() { }
}

Changes

Analyzer Implementation

• Created DuplicateTestMethodAttributeAnalyzer that detects methods with >1 TestMethodAttribute-derived attribute
• Reports warning-level diagnostic (MSTEST0059) on the method
• Memory-optimized using counter instead of list allocation

Resources & Localization

• Added diagnostic ID, title, message, and description to Resources.resx
• Updated all 13 .xlf localization files with new entries
• Updated AnalyzerReleases.Unshipped.md
• Messages clarify that only the first attribute returned by reflection will be used when multiple attributes are present

Tests

• Coverage for single/multiple attributes, derived types, DataTestMethod combinations, edge cases

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add analyzer for duplicate test method attribute</issue_title>
<issue_description>The following code will compile successfully:

public class MyTestMethod : TestMethodAttribute
{
    public MyTestMethod([CallerFilePath] string callerFilePath = "", [CallerLineNumber] int callerLineNumber = -1) : base(callerFilePath, callerLineNumber)
    {
    }
}

[TestClass]
public class MyTestClass
{
    [TestMethod]
    [MyTestMethod]
    public void M() { }
}

We should produce a warning so that user picks one of the two attributes.</issue_description>

<agent_instructions>Create a new analyzer that will detect that a method is marked with a single attribute that is or inherits from [TestMethod]</agent_instructions>

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

• Fixes Add analyzer for duplicate test method attribute #7041

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[Evangelink]

It's not so much unexpected behavior. The problem is that only one attribute will be called (the first returned by reflection).

[Evangelink]

@copilot please update the messages.

[Copilot]

Updated the messages to clarify that only the first attribute returned by reflection will be used, rather than saying "unexpected behavior". Changes committed in b4de188.