private static void SingleInstanceCallback(object sender, InstanceCallbackEventArgs args)
{
    if (args == null || _mainFrm == null) return;
    Action<bool> d = (bool x) =>
    {
        _mainFrm.ApendArgs(args.CommandLineArgs);
        _mainFrm.Activate(x);
    };
    _mainFrm.Invoke(d, true);
}

private void NamedPipeManager_ReceiveString(string obj)
{
    main.WindowState = FormWindowState.Minimized;
    main.WindowState = FormWindowState.Normal;
    main.Activate();
}

using System;
using System.IO;
using System.Linq;
using System.Collections.Generic;
using System.ComponentModel;
using System.Text;
using System.Windows.Forms;
using System.Diagnostics;
using System.Runtime.InteropServices;

namespace WindowsFormsTest1
{
    public partial class Form1 : Form
    {
        [DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool SetForegroundWindow(IntPtr hWnd);

        [DllImport("user32.dll")]
        static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);

        const int ShowWindow_Restore = 9;

        [DllImport("user32.dll")]
        public static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, ref COPYDATASTRUCT lParam);

        [StructLayout(LayoutKind.Sequential)]
        public struct COPYDATASTRUCT
        {
            public IntPtr dwData;
            public int cbData;
            public IntPtr lpData;
        }

        const uint WM_COPYDATA = 0x004A;

        public Form1()
        {
            InitializeComponent();

            Process this_process = Process.GetCurrentProcess();

            //найти все процессы с таким же именем
            Process[] other_processes =
                Process.GetProcessesByName(this_process.ProcessName).Where(pr => pr.Id != this_process.Id).ToArray();

            foreach (var pr in other_processes)
            {
                pr.WaitForInputIdle(1000); //на случай, если процесс еще не загрузился

                //берем первый процесс с окном
                IntPtr hWnd = pr.MainWindowHandle;
                if (hWnd == IntPtr.Zero) continue;

                //отправляем командную строку
                string command_line = "/activate";
                var cds = new COPYDATASTRUCT();
                cds.dwData = (IntPtr)1;
                cds.cbData = (command_line.Length + 1) * 2;
                cds.lpData = Marshal.StringToHGlobalUni(command_line);
                SendMessage(hWnd, WM_COPYDATA, IntPtr.Zero, ref cds);
                Marshal.FreeHGlobal(cds.lpData);

                //активируем окно и выходим
                ShowWindow(hWnd, ShowWindow_Restore);
                SetForegroundWindow(hWnd);
                Environment.Exit(0);
            }
            //если ничего не найдено, продолжаем работу
        }

        protected override void WndProc(ref Message m)
        {
            if (m.Msg == WM_COPYDATA)
            {
                COPYDATASTRUCT data = new COPYDATASTRUCT();
                data = (COPYDATASTRUCT)Marshal.PtrToStructure(m.LParam, data.GetType());
                textBox1.Text = Marshal.PtrToStringUni(data.lpData);
            }
            base.WndProc(ref m);
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            textBox1.Focus();
            textBox1.Select();
        }

    }

}

static Mutex Mutex;
 static NamedPipeManager NamedPipeManager;
 static Form1 main;

Mutex = new Mutex(true, "MyApplicationName", out bool Is);
 if (!Is)
 {
     NamedPipeManager.Write("1");
     Application.Exit();
     return;
 }
 NamedPipeManager = new NamedPipeManager();
 NamedPipeManager.ReceiveString += NamedPipeManager_ReceiveString;
 NamedPipeManager.Start();

private static void NamedPipeManager_ReceiveString(string obj)
 {
     if (main.WindowState == FormWindowState.Minimized)
         main.WindowState = FormWindowState.Normal;
     main.Activate();
 }

[DllImport("user32.dll")]
    private static extern IntPtr GetForegroundWindow();

    [DllImport("user32.dll")]
    private static extern uint GetWindowThreadProcessId(IntPtr hWnd, IntPtr ProcessId);

    [DllImport("user32.dll")]
    private static extern bool AttachThreadInput(uint idAttach, uint idAttachTo, bool fAttach);

    public static void xActivateAndBringToFront(this Form form) {

        // activate window
        var currentForegroundWindow = GetForegroundWindow();
        var thisWindowThreadId = GetWindowThreadProcessId(form.Handle, IntPtr.Zero);
        var currentForegroundWindowThreadId = GetWindowThreadProcessId(currentForegroundWindow, IntPtr.Zero);
        AttachThreadInput(currentForegroundWindowThreadId, thisWindowThreadId, true);
        form.Activate(); // or: SetForegroundWindow(form.Handle);
        AttachThreadInput(currentForegroundWindowThreadId, thisWindowThreadId, false);

        // set window to front
        form.TopMost = true;
        form.TopMost = false;
    }

Application.UserAppDataRegistry.SetValue(CORE_PROCESS_ID, Process.GetCurrentProcess().Id);

int processID = (int)Application.UserAppDataRegistry.GetValue(CORE_PROCESS_ID);
bool b =  AllowSetForegroundWindow(processID);
// .. сообщение ядру ..
// .. выход ..