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I'm trying to copy a bitmap from one on-screen DC to another.
The code below is in the WM_PAINT handler but it doesn't work

invoke BeginPaint,hWnd,addr ps

invoke GetDC,hBoard

mov memDC,eax

invoke GetClientRect,hBoard,addr rect

mov eax,rect.bottom

sub eax,rect.top

mov ecx,rect.right

sub ecx,rect.left

invoke BitBlt,ps.hdc,0,0,ecx,eax,memDC,0,0,SRCCOPY

invoke ReleaseDC,hBoard,memDC

invoke EndPaint,hWnd,addr ps

The code below is in the handler for a button and it works. The only difference between it and the code above is the destination DC is obtained differently

invoke GetDC,hWnd

mov hDC,eax              ;destination DC

invoke GetDC,hBoard

mov memDC,eax        ;source DC(not a memory DC)

invoke GetClientRect,hBoard,addr rect

mov eax,rect.bottom

sub eax,rect.top

mov ecx,rect.right

sub ecx,rect.left

invoke BitBlt,hDC,0,0,ecx,eax,memDC,0,0,SRCCOPY

invoke ReleaseDC,hBoard,memDC

invoke ReleaseDC,hWnd,hDC

Any ideas why the first code doesn't work?

Both code snippets are in the same window procedure.
The first is in the WM_PAINT section
and the second is in the WM_COMMAND section.

You don't have to subtract the top and left because they're always 0.
Make sure that at least the window identified by hBoard or the other window has a class or private device context. Otherwise, it will be the same DC and you'll just copy the contents of hBoard's client area over itself.