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

[boops boops]

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:

<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:

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

[jmcilhinney]

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.

[boops boops]

... 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.