Question

Assuming that GetDC(hWnd) is called as the first instruction on WM_CREATE, for window hWnd, is it possible for GetDC() to return NULL?

I am concerned about the possibility of GetDC() failing due to lack of resources. When that happens should I check for errors? And what should I do if the call fails? Do the Win32 API function raise exceptions or should I raise exceptions?

Answer 1

GetDC() can indeed fail, no matter when or where you call it. You ought to take a rather pessimistic view when dealing with the API and be prepared for any API function to fail. As you suggest, one possible reason for failure is exhaustion of system resources, e.g. kernel handles, GDI objects etc.

So you should always check for errors. And not just GetDC(), every single call to an API function should have its return value checked.

The Windows API does not signal errors by raising exceptions. Instead the errors are always signalled through the values returned by the API function. You need to consult the documentation to understand how each individual function reports errors.

If you encounter a failure in a GDI function like GetDC() then there’s not a lot you can do. In the scenario you describe I would log the error or report it to the user, and then terminate execution. There’s generally no recovery from a failure of GetDC().

The pain of checking for errors every time an API function is called is one of the reasons why we have so many frameworks that wrap the low level API. A good framework will do the error checking for you and convert any errors into exceptions. Using a good framework allows you to concentrate on the normal flow of execution and not littering your code with handling code for exceptional cases.