BitBlt not updating WM_PAINT in Windows C++ API

Question

I have the following function to draw a loaded bitmap to a window.

void OnPaint(HWND hwnd) {
    PAINTSTRUCT     ps;
    HDC             hdc;
    BITMAP          bitmap;
    HDC             hdcMem;
    HGDIOBJ         oldBitmap;

    hdc = BeginPaint(hwnd, &ps);

    hdcMem = CreateCompatibleDC(hdc);
    HBITMAP bmp = mainBitmap;
    oldBitmap = SelectObject(hdcMem, mainBitmap);

    GetObject(bmp, sizeof(bitmap), &bitmap);

    x += 64;
    RECT rect;
    rect.left = x;
    rect.top = 0;
    rect.right = x+64;
    rect.bottom = 64;

    HBITMAP hBmp = CreateCompatibleBitmap(
        hdc,                    // Handle to a device context
        rect.right - rect.left, // Bitmap width
        rect.bottom - rect.top  // Bitmap height
    );

    BitBlt(
        hdc,                    // Destination rectangle context handle
        0,                      // Destination rectangle x-coordinate
        0,                      // Destination rectangle y-coordinate
        rect.right - rect.left, // Destination rectangle width
        rect.bottom - rect.top, // Destination rectangle height
        hdcMem,                 // A handle to the source device context
        rect.left,              // Source rectangle x-coordinate
        rect.top,               // Source rectangle y-coordinate
        SRCCOPY                 // Raster-operation code
    );

    SelectObject(hdcMem, oldBitmap);
    DeleteDC(hdcMem);

    EndPaint(hwnd, &ps);
}

And I have the following image loaded into HBITMAP mainBitmap:

The image is drawn when the window opens successfully, and I see the first icon in the sprite bitmap (yellow grapple hook), but my issue is, when I press 'C' to re-paint the window, the image does not change to the next icon in the sprite image.

Things I Know

• On initialization, x = 64;

• Every time I press 'C', paint is called. (Confirmed in Visual Studio Debugger)

• x is incremented by 64 each time OnPaint is called.

Why is the graphic not changing?

---

Here is my WindowsProc function to handle the WM_PAINT message:

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
    switch (uMsg) {
        HANDLE_MSG(hwnd, WM_PAINT, OnPaint);
    }
    return DefWindowProc(hwnd, uMsg, wParam, lParam);
}

Answer 1

Try to call function InvalidateRect() to update region.