我在WPF中为单实例应用程序创建了一个示例项目。可能有帮助：

WPF单实例应用程序

你需要以下本机方法：

public static class NativeMethod
{
    public static IntPtr BroadcastHandle => (IntPtr)0xffff;
    public static int ReactivationMessage => RegisterWindowMessage("WM_SHOWME");

    [DllImport("user32")]
    public static extern bool PostMessage(IntPtr hwnd, int msg, IntPtr wparam, IntPtr lparam);
    [DllImport("user32")]
    public static extern int RegisterWindowMessage(string message);
}

在应用程序主窗口中，向窗口消息添加钩子，当你收到激活消息时，应将主窗口置于其他窗口的顶部。像这样：

public partial class MainWindow
{
    public MainWindow()
    {
        InitializeComponent();
        RegisterWndProcCallback();
    }

    private void RegisterWndProcCallback()
    {
        var handle = new WindowInteropHelper(this).EnsureHandle();
        var hock = new HwndSourceHook(WndProc);
        var source = HwndSource.FromHwnd(handle);

        source.AddHook(hock);
    }

    private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
    {
        if (msg == NativeMethod.ReactivationMessage)
            ReactivateMainWindow();

        return IntPtr.Zero;
    }

    private void ReactivateMainWindow()
    {
        if (WindowState == WindowState.Minimized)
            WindowState = WindowState.Normal;

        Topmost = !Topmost;
        Topmost = !Topmost;
    }
}

在应用启动时，你应该检查应用实例，如果需要，你应该向旧实例发送一条消息并激活它。像这样：

public partial class App
{
    private static readonly Mutex _singleInstanceMutex =
        new Mutex(true, "{40D7FB99-C91E-471C-9E34-5D4A455E35E1}");

    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        if (_singleInstanceMutex.WaitOne(TimeSpan.Zero, true))
        {
            Current.MainWindow = new MainWindow();
            Current.MainWindow.Show();
            Current.MainWindow.Activate();

            _singleInstanceMutex.ReleaseMutex();
        }
        else
        {
            NativeMethod.PostMessage(
                NativeMethod.BroadcastHandle,
                NativeMethod.ReactivationMessage,
                IntPtr.Zero,
                IntPtr.Zero);

            Current.Shutdown();
        }
    }
}