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using System;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.Diagnostics;
using System.Threading;

namespace AttachThreadGetFocus
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }

    Process notepadProcess;
    int maxSB = 0xff;
    uint winProcessId;
    IntPtr handle;

    private void Form1_Shown(object sender, EventArgs e)
    {
      this.Show(); Application.DoEvents();
      notepadProcess = Process.Start("notepad");
      notepadProcess.WaitForInputIdle();
      handle = notepadProcess.MainWindowHandle;
    }

    void GetText(int capability, IntPtr handle, ref StringBuilder sb)
    {
      sb = new StringBuilder(capability);
      int retVal = SendMessage(handle, WM_GETTEXT, new IntPtr(capability), sb);
    }

    #region PInvoke Deklarationen

    [DllImport("user32.dll", SetLastError = true)]
    static extern uint GetWindowThreadProcessId(
      IntPtr hWnd, out uint lpdwProcessId);

    [DllImport("user32.dll")]
    static extern IntPtr GetForegroundWindow();

    [DllImport("user32.dll", SetLastError = true)]
    static extern bool AttachThreadInput(uint idAttach,
      uint idAttachTo, bool fAttach);

    [DllImport("user32.dll")]
    static extern IntPtr GetFocus();

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    static extern int SendMessage(IntPtr hWnd, int msg,
      IntPtr wParam, StringBuilder lParam);

    public const int WM_GETTEXT = 0x000D;

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

    [DllImport("user32.dll")]
    public static extern bool SetForegroundWindow(IntPtr hWnd);

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    static extern int GetWindowTextLength(IntPtr hWnd);

    [DllImport("kernel32.dll")]
    static extern uint GetCurrentThreadId();

    #endregion

    private void button1_Click(object sender, EventArgs e)
    {
      IntPtr foreWindow = handle;
      IntPtr pt = base.Handle;
      if (pt == foreWindow) return;

      uint winPID = GetWindowThreadProcessId(foreWindow, out winProcessId);
      uint idAttach = GetWindowThreadProcessId(base.Handle, out winProcessId);
      if (AttachThreadInput(idAttach, winPID, true))
      {
        SetForegroundWindow(handle);
        IntPtr focus = GetFocus();
        if (focus != IntPtr.Zero)
        {
          Control ctrl = Control.FromHandle(focus);
          if (ctrl != null)
          {
            this.Text = ctrl.Name;
          }
          else
          {
            StringBuilder lpClassName = new StringBuilder(maxSB);
            // GetText(maxSB, handle, ref lpClassName); //SendMessage        
            GetWindowText(handle, lpClassName, maxSB);  //GetWindowText 
            SetForegroundWindow(this.Handle);
            this.Text = lpClassName.ToString();
            if (this.Text.Contains(notepadProcess.MainWindowTitle))
            {
              lblGetWindowText_funktioniert.Text =
                "GetWindowText funktioniert auf externen " +
                "Prozessen mit AttachThreadInput!";
            }
            Application.DoEvents();
          }
        }
        AttachThreadInput(idAttach, winProcessId, false);
      }
    }

    public string GetText(IntPtr hWnd)
    {
      int length = GetWindowTextLength(hWnd);
      StringBuilder sb = new StringBuilder(length + 1);
      GetWindowText(hWnd, sb, sb.Capacity);
      return sb.ToString();
    }

    private void Form1_FormClosing(object sender, FormClosingEventArgs e)
    {
      if (!notepadProcess.HasExited)
        notepadProcess.Kill(); // Dont do that, nur für Test-Zwecke
    }
  }
}