Here is a sequence of events that causes a mysterious crash (mysterious to me anyway). 1. Initialize D3D as normal in a C# project. 2. Display a message box with MessageBox.Show() for some error. (This step was just added recently). 3. Show the form that is associated with Direct3D. Now, when I call Present(), it will throw an exception with the message "Error in the application." I check TestCooperativeLevel and CheckCooperativeLevel, and both are successful at all times up to this point. The key to the problem is step 2. If I take it out, everything will work fine - it seems as though showing a message box before the main form is shown will do something bad, but I don't know how to remedy this or what is going on. I have had this project working for over a year, and this is something I haven't encountered before.

Thanks for the idea, but that didn't seem to do anything. Do you think it should for a specific reason?

Well, I doubt this is it then since I could always see the MessageBoxes (with or without that style bit set). Thanks, though for the idea.