BeginPaint与EndPaint函数的介绍(来至MSDN)

The BeginPaint function automatically sets the clipping region of the device context to exclude any area outside the update region. The update region is set by the InvalidateRect or InvalidateRgn function and by the system after sizing, moving, creating, scrolling, or any other operation that affects the client area. If the update region is marked for erasing, BeginPaint sends a WM_ERASEBKGND message to the window.

An application should not call BeginPaint except in response to a WM_PAINT message. Each call to BeginPaint must have a corresponding call to the EndPaint function.

If the caret is in the area to be painted, BeginPaint automatically hides the caret to prevent it from being erased.

If the window's class has a background brush, BeginPaint uses that brush to erase the background of the update region before returning.

The EndPaint function marks the end of painting in the specified window. This function is required for each call to the BeginPaint function, but only after painting is complete.