<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>

Your framework isn't high enough for Extension Methods.
That's a
hack
for making extension methods work without being in 3.5