Checking if my Windows application is running

The recommended way is to use a Mutex. You can check out a sample here : http://www.codeproject.com/KB/cs/singleinstance.aspx

In specific the code:

        /// 
        /// check if given exe alread running or not
        /// 
        /// returns true if already running
        private static bool IsAlreadyRunning()
        {
            string strLoc = Assembly.GetExecutingAssembly().Location;
            FileSystemInfo fileInfo = new FileInfo(strLoc);
            string sExeName = fileInfo.Name;
            bool bCreatedNew;

            Mutex mutex = new Mutex(true, "Global\\"+sExeName, out bCreatedNew);
            if (bCreatedNew)
                mutex.ReleaseMutex();

            return !bCreatedNew;
        }

public partial class App : System.Windows.Application
{
    public bool IsProcessOpen(string name)
    {
        foreach (Process clsProcess in Process.GetProcesses()) 
        {
            if (clsProcess.ProcessName.Contains(name))
            {
                return true;
            }
        }

        return false;
    }

    protected override void OnStartup(StartupEventArgs e)
    {
        // Get Reference to the current Process
        Process thisProc = Process.GetCurrentProcess();

        if (IsProcessOpen("name of application.exe") == false)
        {
            //System.Windows.MessageBox.Show("Application not open!");
            //System.Windows.Application.Current.Shutdown();
        }
        else
        {
            // Check how many total processes have the same name as the current one
            if (Process.GetProcessesByName(thisProc.ProcessName).Length > 1)
            {
                // If ther is more than one, than it is already running.
                System.Windows.MessageBox.Show("Application is already running.");
                System.Windows.Application.Current.Shutdown();
                return;
            }

            base.OnStartup(e);
        }
    }

For my WPF application i've defined global app id and use semaphore to handle it.

public partial class App : Application
{      
    private const string AppId = "c1d3cdb1-51ad-4c3a-bdb2-686f7dd10155";

    //Passing name associates this sempahore system wide with this name
    private readonly Semaphore instancesAllowed = new Semaphore(1, 1, AppId);

    private bool WasRunning { set; get; }

    private void OnExit(object sender, ExitEventArgs e)
    {
        //Decrement the count if app was running
        if (this.WasRunning)
        {
            this.instancesAllowed.Release();
        }
    }

    private void OnStartup(object sender, StartupEventArgs e)
    {
        //See if application is already running on the system
        if (this.instancesAllowed.WaitOne(1000))
        {
            new MainWindow().Show();
            this.WasRunning = true;
            return;
        }

        //Display
        MessageBox.Show("An instance is already running");

        //Exit out otherwise
        this.Shutdown();
    }
}