Use this code:

[STAThread] static void Main() { using(Mutex mutex = new Mutex(false, "Global\\" + appGuid)) { if(!mutex.WaitOne(0, false)) { MessageBox.Show("Instance already running"); return; } Application.Run(new Form1()); } } 

from The Misunderstood Mutex

In WPF, you can use this code in your App.xaml.cs file:

private static System.Threading.Mutex _mutex = null; protected override void OnStartup(StartupEventArgs e) { string mutexId = ((System.Runtime.InteropServices.GuidAttribute)System.Reflection.Assembly.GetExecutingAssembly().GetCustomAttributes(typeof(System.Runtime.InteropServices.GuidAttribute), false).GetValue(0)).Value.ToString(); _mutex = new System.Threading.Mutex(true, mutexId, out bool createdNew); if (!createdNew) Current.Shutdown(); else Exit += CloseMutexHandler; base.OnStartup(e); } protected virtual void CloseMutexHandler(object sender, EventArgs e) { _mutex?.Close(); } 

• https://www.c-sharpcorner.com/UploadFile/f9f215/how-to-restrict-the-application-to-just-one-instance/
• https://social.msdn.microsoft.com/Forums/vstudio/en-US/eced1b92-43c5-405d-84b1-780e774c9d3e/how-can-i-prevent-launching-my-app-multiple-times?forum=wpf

There is really good topic on that matter. You can find it here: using Mutex.

When starting you application, main always calls Application.Run(). Look in your STAThread-Main method and before Application.Run test, if there are running instances of your .exe.

Process p = Process.GetProcesses(); //check for your .exe 

See this post here.

There are times **Mutex** is not working in some areas. Like using in console application. So I tried using WMI Query.

Try this and it will work.

 /// <summary> /// The main entry point for the application. /// </summary> [STAThread] static void Main() { if (!isStillRunning()) { Application.Run(new Form1()); } else { MessageBox.Show("Previous process still running.", "Application Halted", MessageBoxButtons.OK, MessageBoxIcon.Asterisk); Application.Exit(); } } //***Uses WMI Query static bool isStillRunning() { string processName = Process.GetCurrentProcess().MainModule.ModuleName; ManagementObjectSearcher mos = new ManagementObjectSearcher(); mos.Query.QueryString = @"SELECT * FROM Win32_Process WHERE Name = '" + processName + @"'"; if (mos.Get().Count > 1) { return true; } else return false; } 

Hope it helps.

what i always using is

bool checkSingleInstance() { string procName = Process.GetCurrentProcess().ProcessName; // get the list of all processes by that name Process[] processes = Process.GetProcessesByName(procName); if (processes.Length > 1) { return true; } else { return false; } } 

solution in Windows form application Prohibit again run application(reopen application).

1- first add Class RunAlready.cs

2-Call method processIsRunning() with Name Process from RunAlready.cs in Program.cs

Program.cs:

using System; using System.Collections.Generic; using System.Linq; using System.Threading.Tasks; using System.Windows.Forms; namespace Tirage.MainStand { static class Program { /// <summary> /// The main entry point for the application. /// </summary> [STAThread] static void Main() { PublicClass.Class.RunAlready RunAPP = new PublicClass.Class.RunAlready(); string outApp = RunAPP.processIsRunning("Tirage.MainStand"); Application.EnableVisualStyles(); Application.SetCompatibleTextRenderingDefault(false); MainStand_FrmLogin fLogin = new MainStand_FrmLogin(); if (outApp.Length == 0) { if (fLogin.ShowDialog() == DialogResult.OK) { Application.Run(new MainStand_masterFrm()); } } else MessageBox.Show( "Instance already running"); } } } 

class RunAlready:

using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Threading.Tasks; namespace PublicClass.Class { public class RunAlready { public string processIsRunning(string process) { string xdescription = ""; System.Diagnostics.Process[] processes = System.Diagnostics.Process.GetProcessesByName(process); foreach (System.Diagnostics.Process proc in processes) { var iddd = System.Diagnostics.Process.GetCurrentProcess().Id; if (proc.Id != System.Diagnostics.Process.GetCurrentProcess().Id) { xdescription = "Application Run At time:" + proc.StartTime.ToString() + System.Environment.NewLine; xdescription += "Current physical memory : " + proc.WorkingSet64.ToString() + System.Environment.NewLine; xdescription += "Total processor time : " + proc.TotalProcessorTime.ToString() + System.Environment.NewLine; xdescription += "Virtual memory size : " + proc.VirtualMemorySize64.ToString() + System.Environment.NewLine; } } return xdescription; } } } 

 if (Process.GetProcesses().Count(p => p.ProcessName == "exe name") > 1) { foreach (var process in Process.GetProcessesByName("exe name")) { process.Kill(); } }