I'm building a tool that needs quite a few free floating child windows. I'm not an expert at Win32 but from the little bits of documentation i've found I need to use WS_EX_TOOLWINDOW window type. So i'm doing this.

	WNDCLASSEX wc; 
	wc.cbSize        = sizeof( WNDCLASSEX );
	wc.style         = CS_HREDRAW | CS_VREDRAW;
	wc.lpfnWndProc   = TestCallback;
	wc.cbClsExtra    = 0;
	wc.cbWndExtra    = 0;
	wc.hInstance     = m_hInstance;
	wc.hIcon         = ::LoadIcon(0, IDI_APPLICATION);
	wc.hCursor       = ::LoadCursor(0, IDC_ARROW);
	wc.hbrBackground = (HBRUSH)::GetStockObject(LTGRAY_BRUSH);
	wc.lpszMenuName  = 0;
	wc.lpszClassName = "TestToolWindow";


	if( RegisterClassEx( &wc ) == 0 )
	{
		DWORD error = GetLastError( );
		Logging::Instance( )->Printf( "Failed to register window, ERROR: %d", error );
	}

For some reason RegisterClassEx fails with error code 87 ( The classic Invalid Paramter ). Now if I use all the same settings and switch it over to the non ex versions it works fine and I can open all the windows I want. According to the docs they should be identical minus a few extra options. At the time this fails I do already have a main window open and updating. This window uses a DirectX device for rendering. Any ideas? Thanks Adam

You haven't set the hIconSm member of the WNDCLASSEX structure for one (there might be others, I didn't check). Try this:

WNDCLASSEX wc = {};
/// rest is the same

That'll initialize everything to zero to begin with.

wow, yup thats all I was missing, thanks for catching that.