How to: Execute only one instance of application concurrently

There was a requirement that the application can't run more than one instance, and if already running then show the message to the user and set that window as active.

For this purpose we need System.Threading.Mutex, which synchronizes the execution of threads across process boundaries. 

Also, we can use SetForegroundWindow function which brings the thread that created the specified window into the foreground and activates the window. Keyboard input is directed to the window, and various visual cues are changed for the user.

Here is a small example code:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Windows.Forms;

using System.Threading;

using System.Diagnostics;

using System.Runtime.InteropServices;

namespace OnlyOneInstanceOfApp

{

    static class Program

    {

        //boolean to indicate application has initial ownership of mutex

        private static bool isNew;

        [DllImport("user32.dll")]

        private static extern bool SetForegroundWindow(IntPtr hWnd);

        /// <summary>

        /// The main entry point for the application.

        /// </summary>

        [STAThread]

        static void Main()

        {

            //Create mutex at application start and try to get the ownership

            using (var m = new Mutex(true, "OnlyOneInstanceOfApp", out isNew))

            {

                //If application owns the mutex, continue the execution

                if (isNew)

                {

                    Application.EnableVisualStyles();

                    Application.SetCompatibleTextRenderingDefault(false);

                    Application.Run(new Form1());

                }

                //else show user message that application is running and set focus to that application window

                else

                {

                    MessageBox.Show("Application already running");

                    Process current = Process.GetCurrentProcess();

                    foreach (Process process in Process.GetProcessesByName(current.ProcessName))

                    {

                        if (process.Id != current.Id)

                        {

                            SetForegroundWindow(process.MainWindowHandle);

                            break;

                        }

                    }

                }

            }

        }

    }

}

When we run the application we see

 

And now if we again run the application we see the message: