If you encounter this warning while compiling a Windows Form application in Visual Studio: The service System.Windows.Forms.Design.IEventHandlerService already exists in the service container. Parameter name: serviceType The solution is not very obvious, and the help provides no guidance.  After a little experimenting, I discovered an easy solution: Exit Visual Studio. Open Explorer, and navigate to the folder Related posts: Visual Studio 2008 and .NET 3.5 Service Pack 1 Released Adding Assemblies to the Visual Studio "Add Reference" Dialog Web Service Stumper: “Ambiguous Type”