How to Check If Another Instance of the Application Is Running

How to check if another instance of the application is running

Want some serious code? Here it is.

var exists = System.Diagnostics.Process.GetProcessesByName(System.IO.Path.GetFileNameWithoutExtension(System.Reflection.Assembly.GetEntryAssembly().Location)).Count() > 1;

This works for any application (any name) and will become true if there is another instance running of the same application.

Edit: To fix your needs you can use either of these:

if (System.Diagnostics.Process.GetProcessesByName(System.IO.Path.GetFileNameWithoutExtension(System.Reflection.Assembly.GetEntryAssembly().Location)).Count() > 1) return;

from your Main method to quit the method... OR

if (System.Diagnostics.Process.GetProcessesByName(System.IO.Path.GetFileNameWithoutExtension(System.Reflection.Assembly.GetEntryAssembly().Location)).Count() > 1) System.Diagnostics.Process.GetCurrentProcess().Kill();

which will kill the currently loading process instantly.

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You need to add a reference to System.Core.dll for the .Count() extension method. Alternatively, you can use the .Length property.

How can I check if my program is already running?

Have a look at using a mutex.

static class Program {
    static Mutex mutex = new Mutex(true, "{8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F}");
    [STAThread]
    static void Main() {
        if(mutex.WaitOne(TimeSpan.Zero, true)) {
            try
            {
             Application.EnableVisualStyles();
             Application.SetCompatibleTextRenderingDefault(false);
             Application.Run(new Form1());
            }
            finally
            {
             mutex.ReleaseMutex();
            }
        } else {
            MessageBox.Show("only one instance at a time");
        }
    }
}

If our app is running, WaitOne will return false, and you'll get a message box.

As @Damien_The_Unbeliever pointed out correctly, you should change the Guid of the mutex for each application you write!

Source: http://sanity-free.org/143/csharp_dotnet_single_instance_application.html

How to check if another instance of the app/binary is already running

The standard Unix solution for this is to create a "run file". When you start up, you try to create that file and write your pid to it if it doesn't exist; if it does exist, read the pid out of it, and if there's a running program with that pid and your process name, wait/exit/whatever.

The question is, where do you put that file, and what do you call it?

Well, first, you have to decide what exactly "already running" means. Obviously not "anywhere in the world", but it could be anything from "anywhere on the current machine" to "in the current desktop session". (For example, if User A starts your program, pauses it, then User B comes along and takes over the computer via Fast User Switching, should she be able to run the program, or not?)

For pretty much any reasonable answer to that question, there's an obvious pathname pattern. For example, on a Mac, /tmp is shared system-wide, while $TMPDIR is specific to a given session, so, e.g., /tmp/${ARGV[0]}.pid is a good way to say "only one copy on the machine, period", while ${TMPDIR}/${ARGV[0]}.pid is a good way to say "only one copy per session".

Detecting if another instance of the application is already running

This will probably do just what you want. It has the nice additional feature of bringing the already running instance forward.

EDIT: updated the code to determine the application title automatically.

using System;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Runtime.InteropServices;

static void Main()
{
    if (!EnsureSingleInstance())
    {
        return;
    }

    //...
}

static bool EnsureSingleInstance()
{
    Process currentProcess = Process.GetCurrentProcess();

    var runningProcess = (from process in Process.GetProcesses()
                          where
                            process.Id != currentProcess.Id &&
                            process.ProcessName.Equals(
                              currentProcess.ProcessName,
                              StringComparison.Ordinal)
                          select process).FirstOrDefault();

    if (runningProcess != null)
    {
        ShowWindow(runningProcess.MainWindowHandle, SW_SHOWMAXIMIZED);
        SetForegroundWindow(runningProcess.MainWindowHandle);

        return false;
    }

    return true;
}

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

[DllImport("user32.dll")]
private static extern Boolean ShowWindow(IntPtr hWnd, Int32 nCmdShow);

private const int SW_SHOWMAXIMIZED = 3;

How to determine if a previous instance of my application is running?

Jeroen already answered this, but the best way by far is to use a Mutex... not by Process. Here's a fuller answer with code.

I've updated this answer after seeing some comments about a race condition to address that by instead using the Mutex Constructor

Boolean createdNew;
Mutex mutex;

try
{      
   mutex = new Mutex(false, "SINGLEINSTANCE" out createdNew);
   if (createdNew == false)
   {
      Console.WriteLine("Error : Only 1 instance of this application can run at a time");
      Application.Exit();
   }

   // Run your application
}
catch (Exception e)
{
    // Unable to open the mutex for various reasons
}
finally 
{
    // If this instance created the mutex, ensure that
    // it's cleaned up, otherwise we can't restart the
    // application
    if (mutex && createdNew) 
    {
        mutex.ReleaseMutex();
        mutex.Dispose();
    }
}

Notice the try{} finally{} block. If you're application crashes or exits cleanly but you don't release the Mutex then you may not be able to restart it again later.

How can I tell if another instance of my program is already running?

You can create a Semaphore and stop execution (put the code into your *.dpr file) and bring you running application to the screen.

var
  Semafor: THandle;

begin
  { Don't start twice ... if already running bring this instance to front }
  Semafor := CreateSemaphore(nil, 0, 1, 'MY_APPLICATION_IS_RUNNING');
  if ((Semafor <> 0) and { application is already running }
     (GetLastError = ERROR_ALREADY_EXISTS)) then 
  begin
    RestoreWindow('TMyApplication');
    CloseHandle(Semafor);
    Halt;
  end;

  Application.CreateForm(....);    
  Application.Initialize;
  Application.Run;
  CloseHandle(Semafor);
end;

EDIT (added the RestoreWindow method):

The aFormName is the name of your main form class in your application.

procedure RestoreWindow(aFormName: string);
var
  Wnd,
  App: HWND;    
begin
  Wnd := FindWindow(PChar(aFormName), nil);
  if (Wnd <> 0) then 
  begin { Set Window to foreground }
    App := GetWindowLong(Wnd, GWL_HWNDPARENT);
    if IsIconic(App) then 
      ShowWindow(App, SW_RESTORE);

    SetForegroundwindow(App);
  end;
end;

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