C# Signature:

/// <summary>
/// Changes an attribute of the specified window. The function also sets the 32-bit (long) value at the specified offset into the extra window memory.
/// </summary>
/// <param name="hWnd">A handle to the window and, indirectly, the class to which the window belongs..</param>

/// <param name="nIndex">The zero-based offset to the value to be set. Valid values are in the range zero through the number of bytes of extra window memory, minus the size of an integer. To set any other value, specify one of the following values: GWL_EXSTYLE, GWL_HINSTANCE, GWL_ID, GWL_STYLE, GWL_USERDATA, GWL_WNDPROC </param>

/// <param name="dwNewLong">The replacement value.</param>
/// <returns>If the function succeeds, the return value is the previous value of the specified 32-bit integer.
/// If the function fails, the return value is zero. To get extended error information, call GetLastError. </returns>
[DllImport("user32.dll")]
static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

VB.NET Signature:

<DllImport("user32.dll")> _
Private Shared Function SetWindowLong( _
     ByVal hWnd As IntPtr, _
     ByVal nIndex As Integer, _
     ByVal dwNewLong As IntPtr) As Integer
End Function

User-Defined Types:

None.

Notes:

None.

Tips & Tricks:

Certain window data is cached, so changes you make using SetWindowLong will not take effect until you call the SetWindowPos function. Specifically, if you change any of the frame styles, you must call SetWindowPos with the SWP_FRAMECHANGED flag for the cache to be updated properly.

Sample Code:

The following sample uses the GetWindowLong and SetWindowLong to remove the 3D border in the MDI client area.

    Private Declare Auto Function SetWindowLong Lib "User32.Dll" (ByVal hWnd As IntPtr, ByVal nIndex As Integer, ByVal dwNewLong As Integer) As Integer
    Private Declare Auto Function GetWindowLong Lib "User32.Dll" (ByVal hWnd As System.IntPtr, ByVal nIndex As Integer) As Integer
    Private Const GWL_EXSTYLE = (-20)
    Private Const WS_EX_CLIENTEDGE = &H200

    Private Sub RemoveMdiBorder()

    For Each c As Control In Me.Controls
        If TypeOf c Is MdiClient Then
        Dim windowLong As Long = GetWindowLong(c.Handle, GWL_EXSTYLE)
        windowLong = windowLong And (Not WS_EX_CLIENTEDGE)
        SetWindowLong(c.Handle, GWL_EXSTYLE, windowLong)
        c.Width = c.Width + 1
        Exit For
        End If
    Next

    End Sub

Alternative Managed API:

Do you know one? Please contribute it!

The above signature is incorrect. The dwNewLong parameter must be an IntPtr, not an int, because the native function takes a DWORD (unsigned int), not an integer.

- This is not quite true: passing int and int32 into SetWindowLong does still work.

-- This functions takes a LONG as third argument and return type, and LONG is declared as a 32-bit signed integer. The function SetWindowLongPtr supersedes this function and has the LONG_PTR type for the third argument and return value. LONG_PTR is 32-bits for a 32-bits OS and 64-bits on a 64-bit OS.

- You should also be aware that a LONG in the .NET framework is defined as a 8-Byte (64-Bit) Integer! While the Integer (Int32) is defined as 4-Byte Integer.

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