WPF MVVM textBox Text Binding

c#, inotifypropertychanged, mvvm, wpf

Solution

The Problem that you have is that you are changing the wrong Property. Instead of changing the `HelloWorld.Message` Property, you are changing `MainWindowViewModel.HelloWorld` property. Your code will work OK if you change this line:

public void UpdateTime()
{
    helloWorld = new HelloWorld("The time is " + DateTime.Now.ToString("HH:mm:ss"));
}

For this one

public void UpdateTime()
{
    helloWorld.Message = "The time is " + DateTime.Now.ToString("HH:mm:ss");
}

If you want to keep your original code, then you need to implement INotifyPropertyChanged for your ViewModel, and rise the event when you change `helloWorld` object.

Hope this helps

Problem

I am just getting started with MVVM so apologies if I've done something really stupid. I tried writing a very simple test to see if I could remember everything, and for the life of me I can't see why its not working. In my view I have a textBox where its text property is bound to a value in the ViewModel. Then when pressing a button the value should be altered and the textBox update. I can see the value does alter (I have added a MessageBox.Show() line in the buttom press command) however the textBox does not update. I assume that this means I have not properly implemented the `INotifyPropertyChanged` event properly but am unable to see my mistake. Could anyone point me in the right direction? Here is the code: View ``` <Window x:Class="Mvvm.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="MainWindow" Height="350" Width="525"> <StackPanel Orientation="Horizontal" VerticalAlignment="Top"> <TextBox Height="40" Width="200" Text="{Binding helloWorld.Message, UpdateSourceTrigger=PropertyChanged}"/> <Button Command="{Binding UpdateTimeCommand}">Update</Button> </StackPanel> </Window> ``` Behind View ``` public partial class MainWindow : Window { public MainWindow() { InitializeComponent(); DataContext = new ViewModel.MainWindowViewModel(); } } ``` ViewModel ``` namespace Mvvm.ViewModel { internal class MainWindowViewModel { private HelloWorld _helloWorld; /// <summary> /// Creates a new instance of the ViewModel Class /// </summary> public MainWindowViewModel() { _helloWorld = new HelloWorld("The time is " + DateTime.Now.ToString("HH:mm:ss")); UpdateTimeCommand = new Commands.UpdateTimeCommand(this); } /// <summary> /// Gets the HellowWorld instance /// </summary> public HelloWorld helloWorld { get { return _helloWorld; } set { _helloWorld = value; } } /// <summary> /// Updates the time shown in the helloWorld /// </summary> public void UpdateTime() { helloWorld = new HelloWorld("The time is " + DateTime.Now.ToString("HH:mm:ss")); } public ICommand UpdateTimeCommand { get; private set; } } ``` Model ``` namespace Mvvm.Model { class HelloWorld : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; public HelloWorld(string helloWorldMessage) { Message = "Hello World! " + helloWorldMessage; } private string _Message; public string Message { get { return _Message; } set { _Message = value; OnPropertyChanged("Message"); } } private void OnPropertyChanged(string p) { PropertyChangedEventHandler handler = PropertyChanged; if (handler != null) { handler(this, new PropertyChangedEventArgs(p)); } } } } ``` Commands ``` namespace Mvvm.Commands { internal class UpdateTimeCommand : ICommand { private ViewModel.MainWindowViewModel _viewModel; public UpdateTimeCommand(ViewModel.MainWindowViewModel viewModel) { _viewModel = viewModel; } public event EventHandler CanExecuteChanged { add { CommandManager.RequerySuggested += value; } remove { CommandManager.RequerySuggested -= value; } } public bool CanExecute(object parameter) { return true; } public void Execute(object parameter) { _viewModel.UpdateTime(); } } } ``` Sorry for such a long post and it being a spot my mistake post but I've looked at it for so long and I don't know what I'm doing wrong Thanks!

Original source