title: BC30560: 'ExtensionAttribute' is ambiguous in the namespace 'System.Runtime.CompilerServices'
body: <p>I have asp.net project (in .net 2.0) and I converted project to .net 4.0.
  After I built the project successfully, I launched the website on browser, it throws error as following:</p>
  
  <blockquote>
    <p>Compilation Error</p>
    
    <p>Description: An error occurred during the compilation of a resource
    required to service this request. Please review the following specific
    error details and modify your source code appropriately. </p>
    
    <p>Compiler Error Message: BC30560: 'ExtensionAttribute' is ambiguous in
    the namespace 'System.Runtime.CompilerServices'.</p>
    
    <p>Source Error:</p>
    
    <p>[No relevant source lines]</p>
    
    <p>Source File: InternalXmlHelper.vb    Line: 9 </p>
    
    <p>........</p>
  </blockquote>
  
  <p>Please give me some idea to fix it.</p>