title: Error when using extension methods in C#
body: <p>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: </p>
  
  <pre><code>Missing compiler required member
  'System.Runtime.CompilerServices.ExtensionAttribute..ctor'
  </code></pre>
  
  <p>When using this simple code:</p>
  
  <pre><code>public static class StringUtils {
      static void TestExtension(this String targetString) {
  
      }
  }
  </code></pre>
  
  <p>The only way to make this compile error go away is to add the following code:</p>
  
  <pre><code>namespace System.Runtime.CompilerServices {
      public class ExtensionAttribute : Attribute { }
  }
  </code></pre>
  
  <p>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?</p>