Original Post
Whenever I add a new variable, be it local or global or as a class member, ANYTHING, my program crashes on these lines of code for the mouse using DI: code: DIMOUSESTATE mouse_state; if (FAILED(m_mouse->GetDeviceState(sizeof(DIMOUSESTATE), (LPVOID)&mouse_state))) { ::MessageBox(0, "m_mouse->GetDeviceState() - Not so good Al", 0, 0); ::PostQuitMessage(0); } Debugging tells me this: "An unhandled exception of type 'System.NullReferenceException' occurred in 3D Flying Thing.exe Additional information: Object reference not set to an instance of an object." and points to the if statement. The weird thing is, this only occurs if I add any new variables. At the moment with the amount I've got it's perfectly happy to work as it's supposed to but as soon as anything new is added, this happens. If I comment out the if statement the program will run fine with new variables. I am utterly stumped by this, it makes no sense (to me). There is no correlation between these variables and that if statment (that I can see) so why is this happening?