CDC,CPaintDC,CWindowDC,CClientDC

它们之间的的关系如下：

CObject 

public |------CDC  public |------|------CClientDC  public |------|------CPaintDC  public |------|------CWindowDC 

public |------|------CMetaFileDC 

CDC

msdn上关于CDC是这样描述的

The CDC object provides member functions for working with a device context, such as a display or printer, as well as members for working with a display context associated with the client area of a window.

(翻译：CDC对象提供用于处理设备上下文的成员函数，例如显示器或打印机，以及用于处理与窗口的客户区域相关联的显示上下文的成员。)

可以使用GetDC() 来获取设备的DC。同时，在使用完后，记得调用ReleaseDC()来释放DC

CPaintDC

It performs a CWnd::beginpaint at construction time and CWnd::EndPaint at destruction time.A CPaintDC object can only be used when responding to a WM_PAINT message, usually in your OnPaint message-handler member function.

(翻译：它在构造时调用CWnd :: BeginPaint，在析构时CWnd :: EndPaint结束。CPaintDC对象只能在响应WM_PAINT消息时使用，通常在您的OnPaint消息处理程序成员函数中。)

来处理窗口的绘制消息

CClientDC

This means that the device context associated with a CClientDC object is the client area of a window.

(翻译：这意味着与CClientDC对象相关联的设备上下文是窗口的客户端区域。)

其构造函数中包含了GetDC,析构函数中包含了ReleaseDC。所以在每次显示的去调用它们。

CWindowDC

Calls the Windows function GetWindowDC（）at construction time and ReleaseDC() at destruction time. This means that a CWindowDC object accesses the entire screen area of a CWnd (both client and nonclient areas).

(翻译：在构建时调用Windows函数GetWindowDC，并在析构时调用ReleaseDC。这意味着一个CWindowDC对象访问CWnd的整个屏幕区域（客户端和非客户端区域）)

CWindowDC 和CClientDC一样。不需要显示的去调用GetDC和ReleaseDC函数。CWindowDC可以访问整个窗口区域，包括了客户区和非客户区。

其中，CPaintDC和CClientDC的坐标原点是位于客户区的左上角。而CWindowDC的坐标原点是位于屏幕的左上角。CpaintD