DragDrop registration did not succeed

Question

System.InvalidOperationException: DragDrop registration did not succeed. ---> System.Threading.ThreadStateException:

What does this except

Answer 1

This exception means that the thread that owns the Panel (the Panel being added) has been initialized using the MTA threading model. The drag/drop system requires that the calling thread use the STA thread model (particularly it requires that COM be initialized via OleInitialize). Threading models are an unfortunate vestige of COM, a predecessor of the .NET platform.

If you have the [STAThread] attribute on your Main function, then the main program thread should already be STA. The most likely explanation, then, is that this exception is happening on a different thread. Look at the Threads window in Visual Studio (Debug | Windows | Threads) when the exception occurs and see if you are on a thread other than the main thread. If you are, the solution is probably as simple as setting the thread model for that new thread, which you can do as follows (add this code to the thread where the control is being created):

Thread.CurrentThread.SetApartmentState( ApartmentState.STA )

(Thread and ApartmentState are members of System.Threading)

That code will need to happen before you actually start the new thread. As noted by @Tomer, you can also specify this declaratively using the [STAThread] attribute.

If you find that the exception is happening on the main thread, post back and let us know, and maybe we can help more. A stack trace at the time of the exception may help track down the problem.