先調用GetWindowRect后再調用ScreenToClient,這個時候得到的rect和直接使用GetClientRect得到的值是相等的。有時候需要獲得窗口矩形的大小和客戶區矩形的大小二者的值，故需要分別調用GetWindowRect和GetClientRect。如果只需要獲得客戶區矩形的大小，調用GetClientRect就行了。GetWindowRect和GetClientRect函數的說明如下：
CWnd::GetClientRect
void GetClientRect( LPRECT lpRect ) const;
Parameters:
lpRect
Points to a RECT structure or a CRect object to receive the client coordinates. The left and top members will be 0. The right and bottom members will contain the width and height of the window.
Remarks:
Copies the client coordinates of the CWnd client area into the structure pointed to by lpRect. The client coordinates specify the upper-left and lower-right corners of the client area. Since client coordinates are relative to the upper-left corners of the CWnd client area, the coordinates of the upper-left corner are (0,0).
CWnd::GetWindowRect
void GetWindowRect( LPRECT lpRect ) const;
Parameters:
lpRect
Points to a CRect object or a RECT structure that will receive the screen coordinates of the upper-left and lower-right corners.
Remarks:
Copies the dimensions of the bounding rectangle of the CWnd object to the structure pointed to by lpRect. The dimensions are given in screen coordinates relative to the upper-left corner of the display screen. The dimensions of the caption, border, and scroll bars, if present, are included.
GetWindowRect() 得到的是在屏幕坐標系下的RECT；（即以屏幕左上角為原點）
GetClientRect() 得到的是在客戶區坐標系下的RECT； （即以所在窗口左上角為原點）
GetWindowRect()取的是整個窗口的矩形；
GetClientRect()取的僅是客戶區的矩形，也就是說不包括標題欄，外框等；
第一個函數獲得的是窗口在屏幕上的位置，得到的結果可能是這樣CRect(10,10,240,240);
第二個函數和它不同，它只獲得了客戶區的大小，因此得到的結果總是這樣CRect(0,0,width,height);
ScreenToClient() 就是把屏幕坐標系下的RECT坐標轉換為客戶區坐標系下的RECT坐標。
The GetClientRect function retrieves the coordinates of a window's client area. The client coordinates specify the upper-left and lower-right corners of the client area. Because client coordinates are relative to the upper-left corner of a window's client area, the coordinates of the upper-left corner are (0,0).
GetClientRect得到的是客戶區的大小，也就是說這樣得到的左上角永遠是（0，0）
The GetWindowRect function retrieves the dimensions of the bounding rectangle of the specified window. The dimensions are given in screen coordinates that are relative to the upper-left corner of the screen.
GetWindowRect 是窗口相對于整個屏幕的坐標，屏幕左上點為0，0
相互轉化用ScreenToClient 或者 ClientToScreen
ClientToScreen
The ClientToScreen function converts the client coordinates of a specified point to screen coordinates.
BOOL ClientToScreen(
HWND hWnd, // window handle for source coordinates
LPPOINT lpPoint // pointer to structure containing screen coordinates
);
Parameters
hWnd
Handle to the window whose client area is used for the conversion.
lpPoint
Pointer to a POINT structure that contains the client coordinates to be converted. The new screen coordinates are copied into this structure if the function succeeds.
Return Values
If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero.
雖然存在調用GetWindowRect后再調用ScreenToClient==GetClientRect，但ScreenToClient（）和ClientToScreen()兩者都是屬于WINDOWS API函數，可能是存在一定的冗余設計，但意義不同。
不過在.Net Framework下對WINDOWS API函數進行了重新整理和優化，在獲取控件或窗口的屏幕坐標和客戶區坐標時更方便的多，只需要得到與控件或窗口相對應屏幕坐標和客戶區坐標屬性值就可以了。
ScreenToClient
The ScreenToClient function converts the screen coordinates of a specified point on the screen to client coordinates.
BOOL ScreenToClient(
HWND hWnd, // window handle for source coordinates
LPPOINT lpPoint // address of structure containing coordinates
);
Parameters：
hWnd
Handle to the window whose client area will be used for the conversion.
lpPoint
Pointer to a POINT structure that contains the screen coordinates to be converted.
Return Values：
If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero.