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);
}

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();
 }

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

[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;

static void Main()
{
    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, активируем его и выходим
        IntPtr hWnd = pr.MainWindowHandle;
        if (hWnd == IntPtr.Zero) continue;
        ShowWindow(hWnd, ShowWindow_Restore);
        SetForegroundWindow(hWnd);
        Environment.Exit(0);
    }
    //если ничего не найдено, продолжаем работу...
}

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

    //[DllImport("user32.dll")]
    //public static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);

    [DllImport("User32")]
    private static extern int SetForegroundWindow(IntPtr hwnd);

    [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 xBringToFront2(this Form form) {

        var currentForegroundWindow = GetForegroundWindow();
        var thisWindowThreadId = GetWindowThreadProcessId(form.Handle, IntPtr.Zero);
        var currentForegroundWindowThreadId = GetWindowThreadProcessId(currentForegroundWindow, IntPtr.Zero);
        AttachThreadInput(currentForegroundWindowThreadId, thisWindowThreadId, true);
        //SetWindowPos(form.Handle, new IntPtr(0), 0, 0, 0, 0, SWP_NOSIZE | SWP_NOMOVE | SWP_SHOWWINDOW);

        // !!! активирует окно и устанавливает фокус ввода
        form.Activate(); // или (нет разницы): SetForegroundWindow(form.Handle);
        AttachThreadInput(currentForegroundWindowThreadId, thisWindowThreadId, false);
        //form.Show();
        //form.Activate();

        // !!! выносит окно на передний план
        form.TopMost = true;
        form.TopMost = false;
    }