title: ASP.NET Web API error after upgrading to Visual Studio 2012 RC: Method not found: 'Void System.Net.Http.Headers.HttpHeaders.AddWithoutValidation
body: <p>After upgrading to Visual Studio 2012 I can no longer access any of my ApiControllers, the following error is thrown:</p>
  
  <pre><code>Server Error in '/' Application.
  
  Method not found: 'Void System.Net.Http.Headers.HttpHeaders.AddWithoutValidation(System.String, System.Collections.Generic.IEnumerable`1&lt;System.String&gt;)'.
  
  Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
  
  Exception Details: System.MissingMethodException: Method not found: 'Void System.Net.Http.Headers.HttpHeaders.AddWithoutValidation(System.String, System.Collections.Generic.IEnumerable`1&lt;System.String&gt;)'.
  
  Source Error: 
  
  An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
  
  Stack Trace: 
  
  
  [MissingMethodException: Method not found: 'Void System.Net.Http.Headers.HttpHeaders.AddWithoutValidation(System.String, System.Collections.Generic.IEnumerable`1&lt;System.String&gt;)'.]
     System.Web.Http.WebHost.HttpControllerHandler.AddHeaderToHttpRequestMessage(HttpRequestMessage httpRequestMessage, String headerName, String[] headerValues) +0
     System.Web.Http.WebHost.HttpControllerHandler.ConvertRequest(HttpContextBase httpContextBase) +248
     System.Web.Http.WebHost.HttpControllerHandler.BeginProcessRequest(HttpContextBase httpContextBase, AsyncCallback callback, Object state) +79
     System.Web.Http.WebHost.HttpControllerHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) +48
     System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +268
     System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&amp; completedSynchronously) +155
  
  Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.17626
  </code></pre>
  
  <p>Anyone knows how to fix this?</p>