Send message to a Windows process (not its main window)

Here’s how I’ve done this:

using System;
using System.Runtime.InteropServices;
using System.Threading;
using System.Windows.Forms;
public partial class MainForm : Form
{
    #region Dll Imports
    private const int HWND_BROADCAST = 0xFFFF;

    private static readonly int WM_MY_MSG = RegisterWindowMessage( "WM_MY_MSG" );

    [DllImport( "user32" )]
    private static extern bool PostMessage(IntPtr hwnd, int msg, IntPtr wparam, IntPtr lparam);

    [DllImport( "user32" )]
    private static extern int RegisterWindowMessage(string message);
    #endregion Dll Imports
    static Mutex _single = new Mutex(true, "{4EABFF23-A35E-F0AB-3189-C81203BCAFF1}");
    [STAThread]
    static void Main()
    {
        // See if an instance is already running...
        if (_single.WaitOne(TimeSpan.Zero, true)) {
            // No...start up normally.
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            try {
                Application.Run(new MainForm());
            } catch (Exception ex) {
                // handle exception accordingly
            } finally {
                _single.ReleaseMutex();
            }
        } else {
            // Yes...Bring existing instance to top and activate it.
            PostMessage(
                (IntPtr) HWND_BROADCAST,
                WM_MY_MSG,
                new IntPtr(0xCDCD),
                new IntPtr(0xEFEF));
        }
    }

    protected override void WndProc(ref Message m)
    {
        if (m.Msg == WM_MY_MSG) {
            if ((m.WParam.ToInt32() == 0xCDCD) && (m.LParam.ToInt32() == 0xEFEF)) {
                if (WindowState == FormWindowState.Minimized) {
                    WindowState = FormWindowState.Normal;
                }
                // Bring window to front.
                bool temp = TopMost;
                TopMost = true;
                TopMost = temp;
                // Set focus to the window.
                Activate();
            }
        } else {
            base.WndProc(ref m);
        }
    }
}

I hope I’ve transcribed this correctly. I had to leave out a lot of other stuff, but I think I got what is necessary. What I have works for me without fail. If you have a problem, let me know, and I’ll see what I’ve missed.