C#

<Application x:Class="Technewlogic.Samples.WpfModalDialog.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	Startup="Application_Startup">
	
    <Application.Resources>
    </Application.Resources>
	
</Application>

using System;
using System.Runtime.InteropServices;
using System.Security;
using System.Threading;
using System.Windows;

namespace Technewlogic.Samples.WpfModalDialog
{	
	/// <summary>
	/// Interaction logic for App.xaml
	/// </summary>
	public partial class App : Application
	{
        // 24.09.2015: Timer in WPF
        System.Windows.Threading.DispatcherTimer dispatcherTimer = new System.Windows.Threading.DispatcherTimer();
        private void dispatcherTimer_Tick(object sender, EventArgs e)
        {
            TopMost = true;
            this.TopMost = true;
            //MessageBox.Show("Popup: dispatcherTimer_Tick");
            InitializeComponent();
        }

        // 24.09.2015: Keep my application on top
        public bool TopMost { get; set; }
        // couldn't find a .NET-way to bring the window
        // to top, reverting to old Win32 functions...
        [SuppressUnmanagedCodeSecurity]
        public class SafeNativeMethods
        {
            [System.Runtime.InteropServices.DllImport("User32.dll", CharSet = CharSet.Unicode)]
            public static extern Int32 FindWindow(String lpClassName, String lpWindowName);
            [System.Runtime.InteropServices.DllImport("User32.dll", CharSet = CharSet.Unicode)]
            public static extern Int32 SetForegroundWindow(int hWnd);
        }

        private void triMain_Click(object sender, System.EventArgs e)
        {
            int hWnd = SafeNativeMethods.FindWindow(null, "Test"); // Win32 API, don't want to use this
            SafeNativeMethods.SetForegroundWindow(hWnd); // Win32 API, don't want to use this
            TopMost = true;
        }

        // Only One Instance of App allowed
        private void Application_Startup(object sender, StartupEventArgs e)
		{
            // 24.09.2015: Keep my application on top + Timer in WPF
            dispatcherTimer.Tick += new EventHandler(dispatcherTimer_Tick);
            dispatcherTimer.Interval = new TimeSpan(0, 0, 1);
            dispatcherTimer.Start();

            bool createdNew;
      		Mutex mutex = new Mutex(true, this.GetType().GUID.ToString(), out createdNew);
      		if (createdNew)
      			{
                	Window window = new MainWindow();
        			window.Closed += (sender2, args) => mutex.Close(); ;
        			window.Show();
                }
      		    else
      			{
        			// MessageBox.Show("App already running...");
        			mutex.Close();
        			Application.Current.Shutdown();
      			}
		}
    }
}

public class SafeNativeMethods
{
    [System.Runtime.InteropServices.DllImport("User32.dll", CharSet = CharSet.Unicode)]
    public static extern Int32 FindWindow(String lpClassName, String lpWindowName);
    [System.Runtime.InteropServices.DllImport("User32.dll", CharSet = CharSet.Unicode)]
    public static extern Int32 SetForegroundWindow(int hWnd);
}

using System;
using System.Diagnostics;
using System.Windows;
using System.Runtime.InteropServices;

namespace Technewlogic.Samples.WpfModalDialog
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>

    public partial class MainWindow : Window
	{
        // 28.09.2015: C# Check If Application Is Already Running Then Set Focus
        [DllImport("user32.dll")]
        private static extern bool SetForegroundWindow(IntPtr hWnd);
        [DllImport("user32.dll")]
        private static extern bool ShowWindowAsync(IntPtr hWnd, int nCmdShow);
        [DllImport("user32.dll")]
        private static extern bool IsIconic(IntPtr hWnd);
            /*
            const int SW_HIDE = 0;
            const int SW_SHOWNORMAL = 1;
            const int SW_SHOWMINIMIZED = 2;
            const int SW_SHOWMAXIMIZED = 3;
            const int SW_SHOWNOACTIVATE = 4;
            const int SW_RESTORE = 9;
            const int SW_SHOWDEFAULT = 10;
            */
        const int swRestore = 9;

        // 25.09.2015: Timer fiering
        System.Windows.Threading.DispatcherTimer dispatcherTimer = new System.Windows.Threading.DispatcherTimer();
        private void dispatcherTimer_Tick(object sender, EventArgs e)
        {
            //MessageBox.Show("Popup: MainWindow_TimerTick");
            // 28.09.2015: C# Check If Application Is Already Running Then Set Focus
            var me = Process.GetCurrentProcess();
            var arrProcesses = Process.GetProcessesByName(me.ProcessName);

            if (arrProcesses.Length > 1)
            {
                for (var i = 0; i < arrProcesses.Length; i++)
                {
                    if (arrProcesses[i].Id != me.Id && arrProcesses[i].SessionId == me.SessionId)
                    {
                        // get the window handle
                        IntPtr hWnd = arrProcesses[i].MainWindowHandle;
                        // if iconic, we need to restore the window
                        if (IsIconic(hWnd))
                        {
                            ShowWindowAsync(hWnd, swRestore);
                        }
                        // bring it to the foreground
                        SetForegroundWindow(hWnd);
                        break;
                    }
                }
            }
        }

        // Main Window
        public MainWindow()
		{
			
			//Alt-Tab preventing
			Window w = new Window(); // Create helper window
        	w.Top = -100; // Location of new window is outside of visible part of screen
        	w.Left = -100;
        	w.Width = 1; // size of window is enough small to avoid its appearance at the beginning
        	w.Height = 1;
        	w.WindowStyle = WindowStyle.ToolWindow; // Set window style as ToolWindow to avoid its icon in AltTab 
        	w.Show(); // We need to show window before set is as owner to our main window
        	this.Owner = w; // Okey, this will result to disappear icon for main window.
        	w.Hide(); // Hide helper window just in case

            // 25.09.2015: Timer settings
            dispatcherTimer.Tick += dispatcherTimer_Tick;
            dispatcherTimer.Interval = new TimeSpan(0, 0, 5);
            dispatcherTimer.Start();
            //Initialisierung der Komponenten
            InitializeComponent();
			ModalDialog.SetParent(ModalDialogParent);
		}

        // ModalDialog
        private void ShowModalDialog_Click(object sender, RoutedEventArgs e)
    	{
            var res = ModalDialog.ShowHandlerDialog(MessageTextBox.Text);
			var resultMessagePrefix = "Result: ";
			if (res)
				ResultText.Text = resultMessagePrefix + "OK";
			else
				ResultText.Text = resultMessagePrefix + "Break";
		}
	}
	
}

********************** Timer_Tick-Anfang ********************** 
Setze ShowWindowAsync für den System.Diagnostics.Process (Test)-Handle 0 mit dem SW_RESTORE-Parameter 9 
Setze ForegroundWindow für System.Diagnostics.Process (Test) mit dem Handle 0 
Setze TOPMOST für Prozess System.Diagnostics.Process (Test) mit dem Handle 0 
Setze TOP für Prozess System.Diagnostics.Process (Test) mit dem Handle 0 
hWnd: 0 
********************** Timer_Tick-Ende ************************ 

InitializeComponent();
ModalDialog.SetParent(ModalDialogParent);

Benjamin.Buhr wrote:

I think the other problem ist that I'm using UserControl in WPF.

Bastard Programmer from Hell
If you can't read my code, try converting it here[^]

.GetMethods(BindingFlags.Public | BindingFlags.Instance | BindingFlags.DeclaredOnly);