Enforcing Single Instance of a Hidden-Window Application

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Runtime.InteropServices;
using System.Diagnostics;

namespace SingleInstance
{
	/// <summary>
	/// Summary description for Form1.
	/// </summary>
	public class SingleInstanceForm : System.Windows.Forms.Form
	{
		private System.Windows.Forms.NotifyIcon notifyIcon1;
		private System.Windows.Forms.Label label2;
		private System.ComponentModel.IContainer components;

		// these are used to enforce single instance
		private static Event evnt;
		public EventSignalledHandler eventSignalled;

		[DllImport("user32.dll", CharSet=CharSet.Auto)] static extern bool SetForegroundWindow(IntPtr hWnd);

		public SingleInstanceForm()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();

/*
			Trace.Listeners.Clear();
			TextWriterTraceListener listener = new TextWriterTraceListener( "logfile.txt" );
			Trace.Listeners.Add( listener );
			Trace.AutoFlush = true;
*/
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#region Windows Form Designer generated code
		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			this.components = new System.ComponentModel.Container();
			System.Resources.ResourceManager resources = new System.Resources.ResourceManager(typeof(SingleInstanceForm));
			this.notifyIcon1 = new System.Windows.Forms.NotifyIcon(this.components);
			this.label2 = new System.Windows.Forms.Label();
			this.SuspendLayout();
			// 
			// notifyIcon1
			// 
			this.notifyIcon1.Icon = ((System.Drawing.Icon)(resources.GetObject("notifyIcon1.Icon")));
			this.notifyIcon1.Text = "Click icon to exit";
			this.notifyIcon1.Visible = true;
			this.notifyIcon1.Click += new System.EventHandler(this.notifyIcon1_Click);
			// 
			// label2
			// 
			this.label2.Location = new System.Drawing.Point(24, 80);
			this.label2.Name = "label2";
			this.label2.Size = new System.Drawing.Size(232, 32);
			this.label2.TabIndex = 1;
			this.label2.Text = "Another instance tried to start";
			// 
			// SingleInstanceForm
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(292, 273);
			this.Controls.Add(this.label2);
			this.Name = "SingleInstanceForm";
			this.Text = "Form1";
			this.Closing += new System.ComponentModel.CancelEventHandler(this.SingleInstanceForm_Closing);
			this.ResumeLayout(false);

		}
		#endregion

		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main() 
		{
			// this object provides single instance feature
			evnt = new Event();
				
			// already exists means another instance is running
			if ( evnt.EventAlreadyExists() )
			{
				// signal the other instance to come to foreground
				evnt.SignalEvent();
			}
			else
			{
				// otherwise start normally
				SingleInstanceForm f = new SingleInstanceForm();
				// important: access the Handle so .net will create it
				IntPtr dummy = f.Handle;
				f.eventSignalled = new EventSignalledHandler( f.evnt_EventSignalled );
				evnt.SetObject( f );

				Application.Run();
			}
		}


		// this is called by a seperate thread created by the Event class
		// Invoke does the cross thread magic for us
		public void evnt_EventSignalled()
		{
			Show();
			WindowState = FormWindowState.Normal;

			// XP will flash the 1st instance instead of bring
			// to foreground, unless the 1st instance is hidden
			// W2K brings to foreground, unless the window is hidden
			// in which case it shows but doesn't highlight title bar
			SetForegroundWindow( Handle );
		}

		private void notifyIcon1_Click(object sender, System.EventArgs e)
		{
			// click on the sys tray icon causes app to exit

			// call Environment class verion so that worker thread in
			// Event will die too.

			Environment.Exit(0);

		}

		private void SingleInstanceForm_Closing(object sender, System.ComponentModel.CancelEventArgs e)
		{
			Environment.Exit( 0 );
		}
	}
}