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Invalidating the Client Area

The system is not the only source of WM_PAINT messages. The InvalidateRect or InvalidateRgn function can indirectly generate WM_PAINT messages for your windows. These functions mark all or part of a client area as invalid (that must be redrawn).

In the following example, the window procedure invalidates the entire client area when processing WM_CHAR messages. This allows the user to change the figure by typing a number and view the results; these results are drawn as soon as there are no other messages in the application's message queue.

RECT rc;
POINT aptPentagon[6] = {50,2, 98,35, 79,90, 21,90, 2,35, 50,2}, 
      aptHexagon[7]  = {50,2, 93,25, 93,75, 50,98, 7,75, 7,25, 50,2}; 
POINT *ppt = aptPentagon; 
int cpt = 6; 
case WM_CHAR: 
    switch (wParam) 
        case '5': 
            ppt = aptPentagon; 
            cpt = 6; 
        case '6': 
            ppt = aptHexagon; 
            cpt = 7; 
    InvalidateRect(hwnd, NULL, TRUE); 
    return 0L; 
case WM_PAINT: 
    hdc = BeginPaint(hwnd, &ps); 
    GetClientRect(hwnd, &rc); 
    SetMapMode(hdc, MM_ANISOTROPIC); 
    SetWindowExtEx(hdc, 100, 100, NULL); 
    SetViewportExtEx(hdc, rc.right, rc.bottom, NULL); 
    Polyline(hdc, ppt, cpt); 
    EndPaint(hwnd, &ps); 
    return 0L; 

In this example, the NULL argument used by InvalidateRect specifies the entire client area; the TRUE argument causes the background to be erased. If you do not want the application to wait until the application's message queue has no other messages, use the UpdateWindow function to force the WM_PAINT message to be sent immediately. If there is any invalid part of the client area, UpdateWindow sends the WM_PAINT message for the specified window directly to the window procedure.