using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Diagnostics;
using System.Linq;
using System.Reflection;
using System.Threading;
using System.Windows;

namespace mytest
{
    /// <summary>
    /// Логика взаимодействия для App.xaml
    /// </summary>
    public partial class App : Application
    {


        [System.STAThreadAttribute()]
        [System.Diagnostics.DebuggerNonUserCodeAttribute()]
        [System.CodeDom.Compiler.GeneratedCodeAttribute("PresentationBuildTasks", "4.0.0.0")]
        public static void Main()
        {
            Mutex _mutexSingleInstance = new Mutex(true, "MonitorMeSingleInstance");
            if (_mutexSingleInstance.WaitOne(TimeSpan.Zero, true))
            {
                try
                {

                    mytest.App app = new mytest.App();
                    app.InitializeComponent();
                    app.Run();
                }
                finally
                {

                    _mutexSingleInstance.ReleaseMutex();
                    _mutexSingleInstance.Close();
                }
            }
            else
            {
                var processes = Process.GetProcessesByName(Assembly.GetEntryAssembly().GetName().Name);
                {
                    if (processes.Length > 1)
                    {
                        foreach (var process in processes)
                        {
                            if (process.Id != Process.GetCurrentProcess().Id)
                            {
                                WindowHelper.SetForegroundWindow(process.MainWindowHandle);
                                
                            }
                        }
                    }
                }

            }

        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows.Interop;
using System.Windows.Threading;

namespace mytest
{
    class WindowHelper
    {
        [DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool SetForegroundWindow(IntPtr hWnd);
    }
}