[c#] How to check if another instance of the application is running

Could someone show how it is possible to check whether another instance of the program (e.g. test.exe) is running and if so stop the application from loading if there is an existing instance of it.

It's not sure what you mean with 'the program', but if you want to limit your application to one instance then you can use a Mutex to make sure that your application isn't already running.

[STAThread]
static void Main()
{
    Mutex mutex = new System.Threading.Mutex(false, "MyUniqueMutexName");
    try
    {
        if (mutex.WaitOne(0, false))
        {
            // Run the application
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new MainForm());
        }
        else
        {
            MessageBox.Show("An instance of the application is already running.");
        }
    }
    finally
    {
        if (mutex != null)
        {
            mutex.Close();
            mutex = null;
        }
    }
}

The Process static class has a method GetProcessesByName() which you can use to search through running processes. Just search for any other process with the same executable name.

Here are some good sample applications. Below is one possible way.

public static Process RunningInstance() 
{ 
    Process current = Process.GetCurrentProcess(); 
    Process[] processes = Process.GetProcessesByName (current.ProcessName); 

    //Loop through the running processes in with the same name 
    foreach (Process process in processes) 
    { 
        //Ignore the current process 
        if (process.Id != current.Id) 
        { 
            //Make sure that the process is running from the exe file. 
            if (Assembly.GetExecutingAssembly().Location.
                 Replace("/", "\\") == current.MainModule.FileName) 

            {  
                //Return the other process instance.  
                return process; 

            }  
        }  
    } 
    //No other instance was found, return null.  
    return null;  
}


if (MainForm.RunningInstance() != null)
{
    MessageBox.Show("Duplicate Instance");
    //TODO:
    //Your application logic for duplicate 
    //instances would go here.
}

Many other possible ways. See the examples for alternatives.

First one.

Second One.

Third One

EDIT 1: Just saw your comment that you have got a console application. That is discussed in the second sample.

You can try this

Process[] processes = Process.GetProcessesByName("processname");
foreach (Process p in processes)
{
    IntPtr pFoundWindow = p.MainWindowHandle;
    // Do something with the handle...
    //
}