When C# syntax errors occur in RoslynCodeTaskFactory tasks, error messages now point to the original MSBuild project file instead of temporary generated files, making debugging much easier for users.

Before this change

<Project>
  <UsingTask TaskName="SyntaxErrorTask"
             TaskFactory="RoslynCodeTaskFactory"
             AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll" >
    <Task>
      <Code Type="Fragment" Language="cs">
        syntax error
      </Code>
    </Task>
  </UsingTask>

  <Target Name="Go">
    <SyntaxErrorTask />
  </Target>
</Project>

Error output pointed to temporary files:

C:\Users\user\AppData\Local\Temp\MSBuildTemp\tmp058945a478fe48dbb624f76b9f3f981b.tmp(36,21): error CS1002: ; expected
test.proj(15,5): error : The source file for this compilation can be found at: "C:\Users\user\AppData\Local\Temp\MSBuildTemp\tmp058945a478fe48dbb624f76b9f3f981b.tmp"

After this change

Error output now points to the original source:

test.proj(3,9): error CS1002: ; expected
test.proj(15,5): error : The source file for this compilation can be found at: "C:\Users\user\AppData\Local\Temp\MSBuildTemp\tmp058945a478fe48dbb624f76b9f3f981b.tmp"

Implementation Details

• Added #line directives to generated code for both Fragment and Method code types
• Preserved compilation caching by keeping original source code for cache keys while adding line directives during compilation
• Extracted file path and line information from build context and XML parsing with line info
• Updated verification tests to reflect the new generated code structure

The solution injects #line directives like #line 2 "/path/to/project.proj" before user code in the generated C# files, which instructs the compiler to map errors back to the original source location.

This applies to RoslynCodeTaskFactoryCodeType.Fragment and RoslynCodeTaskFactoryCodeType.Method code types. The Class code type already works correctly since it uses the complete user-provided class.

Fixes #12176.

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.