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c#.netwinapipinvoke
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Reputation: 45

Call Win32 CalculatePopupWindowPosition from C#

I'm trying to call the WinAPI function CalculatePopupWindowPosition in C# using P/Invoke. From

http://msdn.microsoft.com/en-us/library/windows/desktop/dd565861(v=vs.85).aspx

I see that it's syntax is:

BOOL WINAPI CalculatePopupWindowPosition(  
  _In_      const POINT *anchorPoint,  
  _In_      const SIZE *windowSize,  
  _In_      UINT flags,  
  _In_opt_  RECT *excludeRect,  
  _Out_     RECT *popupWindowPosition  
);

I then tried to import it using the following code in C#

[DllImport("User32.dll", SetLastError = true, CallingConvention = CallingConvention.StdCall)]
public static extern bool CalculatePopupWindowPosition
(
    [In] ref POINT anchorPoint,
    [In] ref SIZE windowSize,
    [In] ref UInt32 flags,
    [In,Optional] ref RECT excludeRect,
    [Out] out SIZE popupWindowPosition
);

I also implemented the RECT, POINT and SIZE structures and initialized them. Finally I called the function like so.

CalculatePopupWindowPosition(ref nIconPos, ref windowSize, ref flags, ref nIconRect, out windowSize);

This doesn't seem to work though, windowSize contains nothing but zeros, which it shouldn't. Any ideas what I'm doing wrong here?

Upvotes: 0

Views: 455

Answers (1)

David Heffernan
David Heffernan

Reputation: 612993

The flags parameter needs to be passed by value rather than by reference:

[DllImport("User32.dll", SetLastError = true)]
public static extern bool CalculatePopupWindowPosition
(
    ref POINT anchorPoint,
    ref SIZE windowSize,
    uint flags,
    ref RECT excludeRect,
    out RECT popupWindowPosition
);

Some general advice. When an API call fails, check the return value. In this case if the function returns false then call Marshal.GetLastWin32Error to find out the error status code.

Upvotes: 2

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