VB.Net Systemweit aktuelles Steuerelement wie TextBox abfragen

Imports System.Runtime.InteropServices
Imports System.Text
Public Class Form1
    <DllImport("user32.dll")> _  
    Public Shared Function GetForegroundWindow() As IntPtr
    End Function
    <DllImport("user32.dll")> _  
    Public Shared Function GetWindowThreadProcessId(hWnd As IntPtr, ProcessId As IntPtr) As IntPtr
    End Function
    <DllImport("user32.dll")> _  
    Public Shared Sub AttachThreadInput(idAttach As IntPtr, idAttachTo As IntPtr, fAttach As Boolean)
    End Sub
    <DllImport("user32.dll")> _  
    Public Shared Function GetFocus() As IntPtr
    End Function
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _  
    Public Shared Function GetWindowText(ByVal hwnd As IntPtr, ByVal lpString As StringBuilder, ByVal cch As Integer) As Integer
    End Function
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _  
    Public Shared Function GetWindowTextLength(ByVal hwnd As IntPtr) As Integer
    End Function
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _  
    Public Shared Function SendMessage(hWnd As IntPtr, Msg As UInt32, wParam As IntPtr, lParam As StringBuilder) As IntPtr
    End Function

    Private Sub Timer1_Tick(sender As System.Object, e As System.EventArgs) Handles Timer1.Tick
        lblHwnd.Text = GetTextRaw(ControlInFocusedWindow())
    End Sub

    Private Function ControlInFocusedWindow() As IntPtr
        Dim activeWindowHandle As IntPtr = GetForegroundWindow()
        Dim activeWindowThread As IntPtr = GetWindowThreadProcessId(activeWindowHandle, IntPtr.Zero)
        Dim thisWindowThread As IntPtr = GetWindowThreadProcessId(Me.Handle, IntPtr.Zero)
        AttachThreadInput(activeWindowThread, thisWindowThread, True)
        Dim focusedControlHandle As IntPtr = GetFocus()
        AttachThreadInput(activeWindowThread, thisWindowThread, False)
        Return focusedControlHandle
    End Function

    Public Function GetTextRaw(hWnd As IntPtr) As String
        Const WM_GETTEXTLENGTH = &HE
        Const WM_GETTEXT = &HD
        Dim length As Integer = SendMessage(hWnd, WM_GETTEXTLENGTH, IntPtr.Zero, New StringBuilder)
        Dim sb As StringBuilder = New StringBuilder(length + 1)
        SendMessage(hWnd, WM_GETTEXT, sb.Capacity, sb)
        Return sb.ToString()
    End Function

    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        Timer1.Start()
    End Sub
End Class