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Asked: 2026-05-10T23:34:08+00:00

I came across an issue that makes me think there is bug in the

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I came across an issue that makes me think there is bug in the 3.0 framework. When I try to use extension methods I get the following error: 

Missing compiler required member 'System.Runtime.CompilerServices.ExtensionAttribute..ctor'

When using this simple code:

public static class StringUtils {     static void TestExtension(this String targetString) {      } }

The only way to make this compile error go away is to add the following code:

namespace System.Runtime.CompilerServices {     public class ExtensionAttribute : Attribute { } }

It’s been a few months since I have used extensions methods, but I’m pretty sure I didn’t have to do this. Has anyone else come across this issue?

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  1. I just ran into this problem myself. In my case, it was because I converted a VS 2005/.Net 2.0 project to a VS 2008/.Net 3.5 project. The conversion tool kept references to System.Core 2.0, and I couldn’t find an easy way to change the references to System.Core 3.5.

    I ended up re-creating the project in VS 2008 from scratch, and it was created with proper references to System.Core 3.5

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