Extension methods don’t compile? No panic!

I came across to a really strange problem today. Some extension methods that I have copied from another project just did not want to be compiled at all.

‘string’ does not contain a definition for ‘MethodName’.

I have checked everything, including clearing the temporary ASP.NET files within framework folder (C:\Windows\Microsoft.NET\Framework\…), restarting Visual Studio and even restarting the OS. The target framework of the projects were all set to 3.5 as well. But unfortunately, nothing helped. Finally, when I compared what kind of configuration I could be missing from web.config, I have found out the missing (and the killer) part.

If you have a similar problem, try adding the following configuration section in your project’s web.config file.

  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" warningLevel="4" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
        <providerOption name="CompilerVersion" value="v3.5"/>
        <providerOption name="WarnAsError" value="false"/>
      </compiler>
    </compilers>
  </system.codedom>