C#

public partial class Form1 : Form
  {
      public const int GW_HWNDNEXT = 2; // The next window is below the specified window
      public const int GW_HWNDPREV = 3; // The previous window is above

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

      [DllImport("user32.dll")]
      [return: MarshalAs(UnmanagedType.Bool)]
      static extern bool IsWindowVisible(IntPtr hWnd);

      [DllImport("user32.dll", CharSet = CharSet.Auto, EntryPoint = "GetWindow", SetLastError = true)]
      public static extern IntPtr GetNextWindow(IntPtr hwnd, [MarshalAs(UnmanagedType.U4)] int wFlag);

      public Form1()
      {
          InitializeComponent();

          //how to get the return value from here???
      }

      public static Form GetTopMostWindow(IntPtr hWnd_mainFrm)
      {
          Form frm = null;

          IntPtr hwnd = GetTopWindow((IntPtr)null);
          if (hwnd != IntPtr.Zero)
          {
              while ((!IsWindowVisible(hwnd) || frm == null) && hwnd != hWnd_mainFrm)
              {
                  // Get next window under the current handler
                  hwnd = GetNextWindow(hwnd, GW_HWNDNEXT);

                  try
                  {
                      frm = (Form)Form.FromHandle(hwnd);
                  }
                  catch
                  {
                      // Weird behaviour: In some cases, trying to cast to a Form a handle of an object
                      // that isn't a form will just return null. In other cases, will throw an exception.
                  }
              }
          }

          return frm;
      }



  }

[Serializable]
public class MyCollection : ArrayList {
    public enum MyUpdateFields {
        MyGroupName
    }
}