WM_NCCALCSIZE Overview  Group

The WM_NCCALCSIZE message is sent when the size and position of a window’s client area must be calculated. By processing this message, an application can control the content of the window’s client area when the size or position of the window changes.

WM_NCCALCSIZE 

fCalcValidRects = (BOOL) wParam;        // valid area flag 

lpncsp = (LPNCCALCSIZE_PARAMS) lParam;  // points to size calculation data 

   or 

lpncsp = (LPRECT) lParam;               // points to new window coordinates 

 

Parameters

fCalcValidRects

Value of wParam. A Boolean value that, if TRUE, specifies that the application should indicate which part of the client area contains valid information. The operating system copies the valid information to the specified area within the new client area. Also, if this parameter is TRUE, lParam points to an NCCALCSIZE_PARAMS structure.

If this parameter is FALSE, the application does not need to indicate the valid part of the client area. Also, if this parameter is FALSE, lParam points to a RECT structure.

lpncsp

Value of lParam.

If wParam  is TRUE, lParam points to an NCCALCSIZE_PARAMS structure that contains information an application can use to calculate the new size and position of the client rectangle.

Return Values

If the fCalcValidRects parameter is FALSE, the application should return zero.

If fCalcValidRects is TRUE, the application can return zero or a valid combination of the following values:

If fCalcValidRects is TRUE and an application returns zero, the old client area is preserved and is aligned with the upper-left corner of the new client area.

Default Action

The window may be redrawn, depending on whether the CS_HREDRAW or CS_VREDRAW class style is specified. This is the default, backward-compatible processing of this message by the DefWindowProc function (in addition to the usual client rectangle calculation described in the preceding table).

See Also

DefWindowProc, MoveWindow, SetWindowPos, NCCALCSIZE_PARAMS, RECT 

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