Carlos Quintero blog

The strange case of "Set property 'System.Windows.ResourceDictionary.DeferrableContent' threw an exception."

In the recent days, each time that I clicked the New Project button of the Visual Studio 2012 IDE, I got this exception:

"Set property 'System.Windows.ResourceDictionary.DeferrableContent' threw an exception."

I have been clueless about this problem until today. When a problem happens in Visual Studio, the recommended approach is to launch it in "Safe mode", because maybe an extension (add-in, package, etc.) is causing it. As a previous step, what I did today is to take a look at the Add-In Manager and I noticed that I had an add-in (a test runner that I created to perform integration tests of my MZ-Tools add-in) marked to load on startup. I unmarked it and then the problem disappeared. Why was this add-in causing this problem?

After some isolation, it happened that this add-in was setting an event handler for the AppDomain.AssemblyResolve event (to locate required assemblies) and a silenced NullReferenceException was happening in the event handler. The following minimal add-in reproduces the issue:

    public class Connect : IDTExtensibility2
    {
      private DTE2 _applicationObject;
      private AddIn _addInInstance;
      private AppDomain _appDomain;
 
      public void OnConnection(object application, ext_ConnectMode connectMode, object addInInst, ref Array custom)
        {
            _applicationObject = (DTE2)application;
            _addInInstance = (AddIn)addInInst;

         switch (connectMode)
         {
            case ext_ConnectMode.ext_cm_Startup:
               // OnStartupComplete will be called
               break;

            case ext_ConnectMode.ext_cm_AfterStartup:
               InitializeAddIn();
               break;
         }
      }

      public void OnStartupComplete(ref Array custom)
      {
         InitializeAddIn();
      }

      private void InitializeAddIn()
      {
         _appDomain = AppDomain.CurrentDomain;
         _appDomain.AssemblyResolve += AppDomain_AssemblyResolve;
      }

      public void OnDisconnection(ext_DisconnectMode disconnectMode, ref Array custom)
        {
         _appDomain.AssemblyResolve -= AppDomain_AssemblyResolve;
      }

        public void OnAddInsUpdate(ref Array custom)
        {
        }

        public void OnBeginShutdown(ref Array custom)
        {
        }

      private Assembly AppDomain_AssemblyResolve(object sender, ResolveEventArgs args)
      {
         Assembly objAssembly = null;
         AssemblyName objAssemblyName = null;

         // Force a NullReferenceException
         if (objAssemblyName.Name == "")
         {
         }

         return objAssembly;
      }
    }