Question

Here is the scenario, i opened my application then the tray icon shows, if i double-clicked the tray icon, the main interface will be shown. if i open again my application, the main interface should be given focus or if it is not yet shown then it should be shown instead of opening another instance of my app.

here is how my code looks like:

//Program.cs
.....

if(myAppIsNotRunningYet) //the program has not been open yet
{
MyTray = new MyTray();
Application.Run();
}

else //the program is already on the tray
{
//the code to give focus to the mainForm or open it up if not yet open
}

//MyTray.cs
.....
public MyTray()
{
notifyIcon = new NotifyIcon();
....
notifyIcon.Visible = true;
}

private void notifyIcon_DoubleClick(object sender, EventArgs e)
{
MainForm mainForm = new MainForm();
mainForm.ShowDialog();
}

Answer 1

EDIT: OK, It seems that to override WndProc correctly, you have to use a form which is/has been visible. So below is a different solution using a MessageFilter. This does work, so hopefully you are good to go from here!

 internal sealed class Program
 { 
  /// <summary>
  /// Program entry point.
  /// </summary>
  [STAThread]
  public static void Main(string[] args)
  {
    bool newMutex;
    System.Threading.Mutex mutex = new System.Threading.Mutex(true, "{9F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F}", out newMutex);

     // Attempt aquire the mutex
     if(newMutex)
     {
      // If we are able to aquire the mutex, it means the application is not running.
         Application.EnableVisualStyles();
         Application.SetCompatibleTextRenderingDefault(false);

         // Create the new tray icon
         MyTray myTray = new MyTray();

         Application.AddMessageFilter(myTray);
         Application.Run();

         // Release the mutex on exit
         mutex.ReleaseMutex();
     }
     else
     {
        // If the aquire attempt fails, the application is already running
        // so we broadcast a windows message to tell it to wake up.
         NativeMethods.PostMessage((IntPtr)NativeMethods.HWND_BROADCAST, NativeMethods.WM_SHOWME, IntPtr.Zero, IntPtr.Zero);
     }
     }
 }
}

internal class NativeMethods
 {
     public const int HWND_BROADCAST = 0xffff;
     public static readonly int WM_SHOWME = RegisterWindowMessage("WM_SHOWME");

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

     [DllImport("user32")]     
     public static extern int RegisterWindowMessage(string message);
 }

 public class MyTray : IMessageFilter
 {
  private NotifyIcon notifyIcon = new NotifyIcon();
  private Form myForm = new Form();

  public MyTray()
  {
     this.notifyIcon.Icon = System.Drawing.Icon.FromHandle(new System.Drawing.Bitmap(16,16).GetHicon());
     this.notifyIcon.Visible = true;
     this.notifyIcon.DoubleClick += delegate(object sender, EventArgs e) { ShowForm(); };
  }

     void ShowForm()
     {
        this.notifyIcon.Visible = false;            
        this.myForm.ShowDialog();   
        this.notifyIcon.Visible = true;
      }

    public bool PreFilterMessage(ref Message m)
    {
       // If the message is the 'show me' message, then hide the icon and show the form.
       if(m.Msg == NativeMethods.WM_SHOWME)
       {
            if (!this.myForm.Visible)
            {
                ShowForm();
                return true; // Filter the message
            }
       }

       return false; // Forward the message
    }
 }

---

EDIT: I put together a sample which is closer to your scenario:

 internal sealed class Program
 {
  static System.Threading.Mutex mutex = new System.Threading.Mutex(true, "{8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F}");

  /// <summary>
  /// Program entry point.
  /// </summary>
  [STAThread]
  private static void Main(string[] args)
  {
   // Attempt aquire the mutex
         if(mutex.WaitOne(TimeSpan.Zero, true))
         {
          // If we are able to aquire the mutex, it means the application is not running.
             Application.EnableVisualStyles();
             Application.SetCompatibleTextRenderingDefault(false);

             // Create the new tray icon
             MyTray myTray = new MyTray();

             Application.Run();

             // Release the mutex on exit
             mutex.ReleaseMutex();
         }
         else
         {
            // If the aquire attempt fails, the application is already running
            // so we broadcast a windows message to tell it to wake up.
             NativeMethods.PostMessage((IntPtr)NativeMethods.HWND_BROADCAST, NativeMethods.WM_SHOWME, IntPtr.Zero, IntPtr.Zero);
         }
     }
 }

 internal class NativeMethods
 {
     public const int HWND_BROADCAST = 0xffff;
     public static readonly int WM_SHOWME = RegisterWindowMessage("WM_SHOWME");

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

     [DllImport("user32")]     
     public static extern int RegisterWindowMessage(string message);
 }

 public class MyTray : Control
 {
  private NotifyIcon notifyIcon = new NotifyIcon();

  public MyTray()
  {
   this.notifyIcon.Visible = true;
  }

  /// <summary>
  /// This method listens to all windows messages either broadcast or sent to this control
  /// </summary>
  protected override void WndProc(ref Message m)
  {
   // If the message is the 'show me' message, then hide the icon and show the form.
   if(m.Msg == NativeMethods.WM_SHOWME)
   {
    this.notifyIcon.Visible = false;
    using (Form mainForm = new Form())
    {
     mainForm.ShowDialog();
     this.notifyIcon.Visible = true;
    }  
   }
   else
   {
    base.WndProc(ref m);    
   }   
  }
 }

---

EDIT: I found an example in C# that combines the mutex and windows message:

C# .NET Single Instance Application

---

A mutex is probably the best way to go. Combine this with a custom windows message which your application listens for to bring itself into focus (see VB.NET, VB6 and C# Interprocess communication via Window Messaging).

Check out this sample:
C# – Single Application Instance