Using the ShowWindow API on a VB6 App from .Net

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Thursday, February 12. 2009

Using the ShowWindow API on a VB6 App from .Net

I was helping someone with an issue restoring vb6 windows from a .Net application over on VBForums. The issue ended up being that the MainWindowHandle of the process was returning the window handle for the ThunderRT6Main window. The ThunderRT6Main is a hidden window that all VB6 apps have that is used for message processing. When we called ShowWindow against this window handle, nothing would happen. To get around this, I ended up calling the GetWindow API with a GW_HWNDPREV to get the window handle of the form window and then called ShowWindow on that.

 Private SW_SHOWNORMAL As Integer = &H1
    <Runtime.InteropServices.DllImport("user32", EntryPoint:="ShowWindow")> _
    Private Shared Function ShowWindow(ByVal hwnd As IntPtr, ByVal nCmdShow As Integer) As Integer
    End Function

    Declare Auto Function GetWindow Lib "user32.dll" ( _
ByVal hWnd As IntPtr, ByVal uCmd As UInt32) As IntPtr

    Const GW_HWNDPREV As Integer = 3

    Dim proc As New Process

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        ' Start the process
        With proc
            .StartInfo.FileName = "c:\temp\project1.exe"
            .Start()
        End With

        ' Wait for it to finish loading and put the window handle into the form caption
        proc.WaitForInputIdle(10000)
        Me.Text = proc.MainWindowHandle.ToString("x8")

    End Sub


    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        ' Call GetWindow with GW_HWNDPREV to get the form from the ThunderRT6Main window
        Dim vb6Window As IntPtr = GetWindow(proc.MainWindowHandle, GW_HWNDPREV)

        ' Call ShowWindow on the window we got back from the last call, which should be the ThunderRT6FormDC window
        ShowWindow(vb6Window, SW_SHOWNORMAL)

    End Sub
Posted by JohnKoer at 22:14 | Comment (1) | Trackbacks (0)

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You are not guaranteed to get the correct window handle by calling GetWindow with GW_HWNDPREV if the target application creates any forms prior to displaying the main form. In my case the target app creates a login dialog and a splash screen prior to displaying the main form. These windows are hidden but kept in memory when the main window is displayed. I have no control over this because the app is not mine. I can see with Spy++ that the previous window to the one returned from MainWindowHandle is the splash screen, the one prior to that is the login dialog and the one prior to that is main vb6 application window.

The main vb6 application window's owner is the one returned by MainWindowHandle so I'm wondering if that relationship could be used to retrieve the main window properly. Unfortunately the 'owner' of the hidden login and splash screen is also the one returned by MainWindowHandle so I can't loop backwards until I find a window that has MainWindowHandle as that owner.
#1 Doug Kaye on 2009-06-12 16:32 (Reply)

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