Common Display Device Contexts

A common DC is used for drawing in the client area of the window. The Win32 API provides a common DC by default for any window whose window class does not explicitly specify a display DC style. Common DCs are typically used with windows that can be drawn without extensive changes to the DC attributes. Common DCs are convenient because they do not require additional memory or system resources, but they can be inconvenient if the application must set up many attributes before using them.

Windows retrieves all common DCs from the display DC cache. An application can retrieve a common DC immediately after the window is created. Because the common DC is from the cache, the application must always release the DC as soon as possible after drawing. After the common DC is released, it is no longer valid and the application must not attempt to draw with it. To draw again, the application must retrieve a new common DC, and continue to retrieve and release a common DC each time it draws in the window. If the application retrieves the DC handle by using the GetDC function, it must use the ReleaseDC function to release the handle. Similarly, for each BeginPaint function, the application must use a corresponding EndPaint function.

When the application retrieves the DC, Windows adjusts the origin so that it aligns with the upper left corner of the client area. It also sets the clipping region so that output to the DC is clipped to the client area. Any output that would otherwise appear outside the client area is clipped. If the application retrieves the common DC by using BeginPaint, Windows also includes the update region in the clipping region to further restrict the output.

When an application releases a common DC, Windows restores the default values for the attributes of the DC. An application that modifies attribute values must do so each time it retrieves a common DC. Releasing the DC releases any drawing objects the application may have selected into it, so the application need not release these objects before releasing the DC. In all cases, an application must never assume that the common DC retains nondefault selections after being released.

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