It may happen that your Winforms application must run only in one instance, so you should inform the user about it if he/she would like to start it again.

This useful code could be built as a DLL file and imported in every requested project. I suggest you to use the checking in your Program.cs file. Here’s the code:

using System;
using System.Diagnostics;
using System.Threading;

namespace CheckRunningInstance
{
    public static class RunningApplication
    {
        public static bool CHECK(string applicationName)
        {
            bool createdNew = true;
            Mutex mutex = new Mutex(true, applicationName, out createdNew);
            if (!createdNew)
            {
                Process current = Process.GetCurrentProcess();
                foreach (Process process in Process.GetProcessesByName(current.ProcessName))
                {
                    if (process.Id != current.Id)
                    {
                        throw new ApplicationException("An instance of the application has been already started");
                    }
                }
                return false;
            }
            return createdNew;
        }
    }
}

In your Program.cs file of your application, use it this way:

• first add the DLL as a reference in your project

• include it in your Program.cs file

• run the method in a try-catch block. Handle both if the the method throws its own exception or it returns false

That’s all. Now try to start your application twice. You should get this message: