How can I make to stopwatch work?

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C#

WPF

TimeSpan

stopwatch

Hello everybody,
I have a question.
Why is my measured time still 0?? I created a stopwatch´s measuring time in milliseconds. I think I'm making a mistake converting to time format. All this in WPF Application. I apologize in advance for mixing multiple languages.
I have no idea if I am making a mistake in Stop() or somewhere else. Before I wanted to convert time to MessageBox.Show("Tvůj čas je "+ trvani.ToString("mm\:ss\.ff")); so time showed me

Can anyone tell me a mistake just by reading the code?

I'll be glad for any advice

Thank you

What I have tried:

public partial class hra : Window
    {
        Stopwatch mt = new Stopwatch();
        DispatcherTimer posunTimer = new DispatcherTimer();
        decimal trvani = new decimal();
        
        
        public hra()
        {
            InitializeComponent();
            
            posunTimer.Tick += posunTimer_Tick;
            posunTimer.Interval = TimeSpan.FromSeconds(1);
            
        }
        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            mt.Start();
            timer.Value = 0;
            posunTimer.Start();
            Random prvni = new Random();
            Random druhy = new Random();
            int maxprvni = 10;
            int maxdruhy = 10;
            int prvnic = prvni.Next(1, maxprvni);
            int druhyc = druhy.Next(2, maxdruhy);
            int total = (prvnic + druhyc);
            prvnit.Text = prvnic.ToString();
            druhyt.Text = druhyc.ToString();
            vysledek.Text = total.ToString();
        } 

        void posunTimer_Tick(object sender, object e)
        {
            timer.Value += 1;
            if (timer.Value >= timer.Maximum)
            {
                posunTimer.Stop();
                hlaska.Text = "Špatně";
                poch++;
                pocetchyb.Text = poch.ToString();
                if (poch == 4)
                {

                    MessageBox.Show("Konec hry!");
                }
            }
        }

        
        private int poch;

        private void kontrola_Click(object sender, RoutedEventArgs e)
        {
            
            timer.Value += 1;
            posunTimer.Start();
            if (vysledekhrac.Text == vysledek.Text && timer.Value < timer.Maximum)
            {
                posunTimer.Stop();
                hlaska.Text = "Správně";
            }
            else if (timer.Value >= timer.Maximum)

            {
                posunTimer.Stop();
                hlaska.Text = "Špatně";
                poch++;
                pocetchyb.Text = poch.ToString();
                if (poch == 4)
                {
                    mt.Stop();
                    MessageBox.Show("Konec hry!");
                }
            }
            else
            {
                posunTimer.Stop();
                hlaska.Text = "Špatně";
                poch++;
                pocetchyb.Text = poch.ToString();
                if (poch == 4)
                {
                    mt.Stop();
                    MessageBox.Show("Konec hry!");
                }
            }
            Random prvni = new Random();
            Random druhy = new Random();
            int maxprvni = 10;
            int maxdruhy = 10;
            int prvnic = prvni.Next(1, maxprvni);
            int druhyc = druhy.Next(2, maxdruhy);
            int total = (prvnic + druhyc);
            prvnit.Text = prvnic.ToString();
            druhyt.Text = druhyc.ToString();
            vysledek.Text = total.ToString();
            vysledekhrac.Text = String.Empty;
            timer.Value = 0;
            posunTimer.Start();
        }
        string path = @"C:\\Žebříček.txt";
        private void konec_Click(object sender, RoutedEventArgs e)
        {
            
            if (mt.IsRunning==true)
            {
                mt.Stop();
            }
            TimeSpan trvani = new TimeSpan(mt.ElapsedMilliseconds);
            MessageBox.Show("Tvůj čas je "+ trvani.ToString("mm\\:ss\\.ff"));
            if (!File.Exists(path))
            {
                StreamWriter str = File.CreateText(path);
                str.WriteLine(trvani);
            }
            else if (File.Exists(path))
            {
                var str = new StreamWriter(path);
                str.WriteLine(trvani);
            }
                
            this.Close();
        }
    }
}