SendMessage WM_GETCONTROLNAME PInvoke Error

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VB

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Evening All,

Hoping you can offer some advice here. I am trying to get the control name of a window from another application. I have managed to get the hWnd via a class pre-written by another user (Obtained from Codeproject).

So i am declaring the SendMessage as follows:

Declare Function SendMessageS Lib "user32" Alias "SendMessageA" (ByVal hwnd As IntPtr, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As String) As Long

I have put this into a dummy application so chucked into form_load event. I loop through the top level windows, grab the handle and then enumerate child windows ensuring he MainWindowTitle matches a specific string value.

Then from this i enumerate all the child windows and try to perform a 'SendMessage' to get the control name from it. However when it hits this point it then throws a PInvoke error message as follows:

A call to PInvoke function 'WindowsApplication1!WindowsApplication1.Form1::SendMessageS' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.

Here is my Form_Load code, appreciate it if you could point me in the right direction.

Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
    Dim EnumWind As New WindowEnumeration
    Dim clsName As String = "WindowsForms10.Window.8.app.0.378734a"

    'Get a List of Top Window Belonging to: "WindowsForms10.Window.8.app.0.378734a"
    For Each TopWin As ApiWindow In EnumWind.GetTopLevelWindows(clsName)
        'Check the title of the Screen
        If (TopWin.MainWindowTitle = "Card Payment Screen") Then
            'We have the payment Screen

            'Enumerate all Child Windows (Controls) Passing Parent hWnd
            For Each ChildWin As ApiWindow In EnumWind.GetChildWindows(TopWin.hWnd)
                Dim WM_GETCONTROLNAME As Long = 50721
                Dim sText As String = Space(100)
                Try
                    'We get a stack PInvoke Error Here
                    SendMessageS(ChildWin.hWnd, WM_GETCONTROLNAME, 100, sText)
                Catch ex As Exception

                End Try
            Next
        End If
    Next
End Sub

Many Thanks
Dev