Can't load type from DLL.

Hello, I wrote a DLL named "GameInput.dll" that references "DirectInput.dll". When I put "GameInput.dll" into Unity and add a reference to Visual Studio I gain access to all of the code in "GameInput.dll". Although I don't get any errors in Visual Studio when I go back to Unity I am getting a "TypeLoadException: Could not load type from assembly" error. If I go back to "GameInput.dll" and remove all usages of the "Microsoft.DirectX.DirectInput" namespace it removes all errors in Unity. Does anyone know what the problem is? Thank you