[RESOLVED] How do I change LParam in a Windows Message?

People have posted code to drag a borderless form using a Windows Message, both on this forum and in the CodeBank. This method has a drawback: you can't drag the form higher than the top of the screen -- or rather you can, but it immediately flips back again when you release the mouse button. I would like to change that behaviour.

After some time poring over the stream of windows messages, I found that the flip-back occurs whenever there is a WM_CAPTURECHANGED message immediately followed by a WM_WINDOWPOSCHANGING. The latter message contains a pointer to a WINDOWPOS Structure in its LParam field. Apparently it is possible to annul the impending change of position by setting the NOMOVE flag of that WINDOWPOS.

Here's how I've defined WINDOWPOS:

vb.net Code:

<System.Runtime.InteropServices.StructLayout(System.Runtime.InteropServices.LayoutKind.Sequential)> _
    Public Structure WindowPos
        Public hWnd As IntPtr
        Public hWndInsertAfter As IntPtr
        Public x As Integer
        Public y As Integer
        Public w As Integer
        Public h As Integer
        Public flags As UInt32
    End Structure

Here's my WndProc Sub:

vb.net Code:

Private DontMove As Boolean
   Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
        If Me.Top < 0 Then
            If m.Msg = WM_CAPTURECHANGED Then
                DontMove = True
            ElseIf m.Msg = WM_WINDOWPOSCHANGING AndAlso DontMove = True Then
                Dim wPos As WindowPos = DirectCast(m.GetLParam((New WindowPos).GetType), WindowPos)
                wPos.flags = wPos.flags Or CUInt(SWP_NOMOVE)
                m.LParam = New IntPtr(AddressOf wPos)
                DontMove = False
            Else
                DontMove = False
            End If
        End If   
        MyBase.WndProc(m)
End Sub

I have two syntax problems here. Line 7 gets past the syntax checker but it seems a rather elaborate way to get the WINDOWPOS. I'm not even sure if it works yet. Is there a more direct way to get the structure through the LParam pointer? Line 9 gives a syntax error because AddressOf apparently requires a method as its argument. Is there another way to set m.LParam to point at my modified WINDOWPOS structure?

Advice and any comments gratefully received. BB

Follow the CodeBank link in my signature and check out the final code in my Immovable Form thread. It contains an example of changing the LParam of a Windows message.

... and there was the perfect answer. I could virtually paste it in without change. Huge thanks, jmci. BB


edit: for the benefit of others, the answer was to use PtrToStructure and StructureToPtr from the InteropServices.Marshal class.