Unable to see the Date time text on the WPF application screen

Question

I am trying to write a simple MVVM application. The expectation of this sample is to show up the time on the screen. The xaml designer showed the time once in the designer

but when I run the app, I dont see the time on the screen at all. Could you please help me find what the issue is? I am attaching the code here.

MainWindow.xaml

    <Window x:Class="WPFDigitalClock.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WPFDigitalClock"        
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <local:ClockVM/>
    </Window.DataContext>
    <Grid>
        <StackPanel>
            <TextBlock Foreground="White" Background="Black" FontSize="30" TextAlignment="Center" Text="{Binding Path=DisplayTime}"/>
        </StackPanel>
    </Grid>
</Window>

MainWindow.xaml.cs

using System.Windows;

namespace WPFDigitalClock
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();            
        }
    }
}

ClockVM.cs

using System;
using System.ComponentModel;
using System.Windows.Threading;

namespace WPFDigitalClock
{
    class ClockVM : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        private ClockModel clockModel;       

        DispatcherTimer Timer = new DispatcherTimer();
        public string DisplayTime { get; set; }

        public ClockVM()
        {
            clockModel = new ClockModel();
            DisplayTime = "";
            Timer.Interval = new TimeSpan(0, 0, 1);
            Timer.Tick += new EventHandler(Timer_Click);
            Timer.Start();

        }

        private void Timer_Click(object sender, EventArgs e)
        {
            DisplayTime = clockModel.Hour + ":" + clockModel.Minute + ":" + clockModel.Second;
            if (PropertyChanged != null)
            {
                PropertyChanged(sender, new PropertyChangedEventArgs("DisplayTime"));
            }
        }
    }
}

ClockModel.cs

using System;

namespace WPFDigitalClock
{
    class ClockModel
    {
        public string Hour
        {
            get { return DateTime.Now.Hour.ToString(); }            
        }

        public string Minute
        {
            get { return DateTime.Now.Minute.ToString(); }
        }

        public string Second
        {
            get { return DateTime.Now.Second.ToString(); }
        }
    }
}

Answer 1

Cannot reproduce your error. I've just added a counter and it updates perfectly.

int counter = 0;
private void Timer_Tick(object sender, EventArgs e)
{
   DisplayTime = clockModel.Hour + ":" + clockModel.Minute + ":" + 
   clockModel.Second + " Counter:" + counter++.ToString();          
   OnPropertyChanged("DisplayTime");            
 }


 public event PropertyChangedEventHandler PropertyChanged;
 protected virtual void OnPropertyChanged(string propertyName)
 {
     var handler = PropertyChanged;
     if (handler != null)
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
 }

My xaml is:

<Window x:Class="DataGridSelectedItemsWpfApplication.MainWindow"
        <!--The code omitted for the brevity-->
        xmlns:vm="clr-namespace:DataGridSelectedItemsWpfApplication.ViewModel"
        Title="MainWindow" WindowStartupLocation="CenterScreen" Height="350" Width="525">
    <Window.DataContext>
        <vm:MyViewModel/>
    </Window.DataContext>
    <Grid>
        <TextBlock Text="{Binding DisplayTime}"/>        
    </Grid>
</Window>

The model is the same like your model.

This works like this:

Update:

As @KyloRen says correctly about your mistake. Just change from PropertyChanged(SENDER, new PropertyChangedEventArgs(propertyName)); TO PropertyChanged(this, new PropertyChangedEventArgs(propertyName));. It means that you raise an event from ClockVM.

Answer 2

Problem is with how you have Raised the change of property DisplayTime. use belowDispatcherTimer.Tick handler as:

 private void Timer_Click(object sender, EventArgs e)
    {
        DisplayTime = clockModel.Hour + ":" + clockModel.Minute + ":" + clockModel.Second;
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs("DisplayTime"));
        }
    }

You have passed the sender object as argument in PropertyChanged method. It expect owner class of Property DisplayTime in sender argument.