title: Add reference problem in silverlight: You can't add a reference to dll as it was not built against the silverlight runtime
body: <p>I have a silverlight application which has a class library project in it. th class library is running in .Net framework 4 and the silverlight project is running in silverlight 3. when I want to add a reference in the silverlight proj to the class library I get this message:</p>
  
  <blockquote>
    <p>"You can't add a reference to Classlibrary1.dll as it was not built against the silverlight runtime. silverlight project will only work with silverlight asseblies."</p>
  </blockquote>
  
  <p>I've tried to change the framework the classlibrary1 used to .net framework 3.5, 3, and 2 but it does not help. ( and my silverlight application can only run in silverlight 3)</p>