一个是句柄，一个是对象。

CDC::CreateCompatibleDC 

virtual BOOL CreateCompatibleDC( CDC* pDC );

Return Value

Nonzero if the function is successful; otherwise 0.

重要的在这

It can be used to prepare images in memory before copying them to the actual device surface of the compatible device.

可以进行图像缓冲

When a memory device context is created, GDI automatically selects a 1-by-1 monochrome stock bitmap for it. GDI output functions can be used with a memory device context only if a bitmap has been created and selected into that context.

只有图像被作为资源引入并选入该容器，GDI导出函数才可能被使用。

原来是操作GDI的利器

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