Invalidate（）调试断言失败消息（MFC，VC ++）

Question

I've made a custom control, and when I want it to repaint on the screen I call Invalidate(), and afterwards UpdateWindow(), but i get message:

debug assertion failed for a file afxwin2.inl in line 150 which is:

AFXWIN_INLINE void CWnd::Invalidate(BOOL bErase)

    { ASSERT(::IsWindow(m_hWnd)); ::InvalidateRect(m_hWnd, NULL, bErase); }

The thing is that when I run the same app in release mode, it doesn't report any message! So this clue makes me think it's about some environment configuration I should change.

What do you think?

Thanks.

Answer 1

Well,

ASSERT(::IsWindow(m_hWnd));

is an assertion. Assertions are statements which verify that something is true and kill your program if it's not. They're intended to be used for debugging and development rather than for being in the program once it has been released, so they are normally only compiled in in debug builds. So, it wouldn't be there in a release build, and you wouldn't get the error message. That does not mean that there isn't a problem in the release build. It just means that that it's not running the statement to check whether there's a problem.

I don't know a lot about the error in question, but looking at it,

::IsWindow(m_hWnd)

is obviously false (hence the error message). The documentation for IsWindow() would appear to indicate that the problem is that the window handle in question is not a handle for a valid window. Perhaps it hasn't been created properly, or it has already been destroyed. You'll have to figure out why your window handle is invalid.

A quick google search for "mfc iswindow" brings up this thread on msdn which might be of help to you.

Answer 2

You call Invalidate before window is created or after window is destroyed. Quick fix is to test for ::IsWindow(m_hWnd) before Invalidate call. To really fix this bug, find why Invalidate is called when window doesn't exist. For example, attempt to invalidate window from its constructor causes this assertion.

Answer 3

You have called Invalidate() on an CWnd-derived class, but that window's m_hWnd member has not been built yet. You should call the Create (or CreateEx) method first, in order to build it (or use a method that does all that for you, like DoModal() ).