Prevent multiple instances of a given app in .NET?

Question

In .NET, what's the best way to prevent multiple instances of an app from running at the same time? And if there's no "best" technique, what are some of the caveats to consider with each solution?

Answer 1

This is the code for VB.Net

Private Shared Sub Main()
    Using mutex As New Mutex(False, appGuid)
        If Not mutex.WaitOne(0, False) Then
              MessageBox.Show("Instance already running", "ERROR", MessageBoxButtons.OK, MessageBoxIcon.Error)
            Return
        End If

        Application.Run(New Form1())
    End Using
End Sub

This is the code for C#

private static void Main()
{
    using (Mutex mutex = new Mutex(false, appGuid)) {
        if (!mutex.WaitOne(0, false)) {
            MessageBox.Show("Instance already running", "ERROR", MessageBoxButtons.OK, MessageBoxIcon.Error);
            return;
        }

        Application.Run(new Form1());
    }
}

Answer 2

(Note: this is a fun-solution! It works but uses bad GDI+ design to achieve this.)

Put an image in with your app and load it on startup. Hold it until the app exits. The user wont be able to start a 2nd instance. (Of course the mutex solution is much cleaner)

private static Bitmap randomName = new Bitmap("my_image.jpg");

Answer 3

Simply using a StreamWriter, how about this?

System.IO.File.StreamWriter OpenFlag = null;   //globally

and

try
{
    OpenFlag = new StreamWriter(Path.GetTempPath() + "OpenedIfRunning");
}
catch (System.IO.IOException) //file in use
{
    Environment.Exit(0);
}

Answer 4

Here is the code you need to ensure that only one instance is running. This is the method of using a named mutex.

public class Program
{
    static System.Threading.Mutex singleton = new Mutex(true, "My App Name");

    static void Main(string[] args)
    {
        if (!singleton.WaitOne(TimeSpan.Zero, true))
        {
            //there is already another instance running!
            Application.Exit();
        }
    }
}

Answer 5

Hanselman has a post on using the WinFormsApplicationBase class from the Microsoft.VisualBasic assembly to do this.

Answer 6

This worked for me in pure C#. the try/catch is when possibly a process in the list exits during your loop.

using System.Diagnostics;
....
[STAThread]
static void Main()
{
...
        int procCount = 0;
        foreach (Process pp in Process.GetProcesses())
        {
            try
            {
                if (String.Compare(pp.MainModule.FileName, Application.ExecutablePath, true) == 0)
                {
                    procCount++;                        
                    if(procCount > 1) {
                       Application.Exit();
                       return;
                    }
                }
            }
            catch { }
        }
        Application.Run(new Form1());
}