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Comments on WPF MVVM ListBox not updating

Post

WPF MVVM ListBox not updating

+4
−0

I am trying to combine MVVM in WPF using Microsoft.Toolkit.MVVM. Somethings are working as expected. The text boxes bind to fields and update in both directions. The button command executes and changes the values of the instance and in the box. The ListBox shows the initial values.

However, the ListBox does not show the values after the execute changes the List.

This little demo shows the problem I'm having. Here is the code (install Microsoft.Toolkit.Mvvm).

Demo.cs

public class Demo
{
    public int Value { get; set; } = 2;
    public List<int> AllValues { get; set; } = new List<int> { 1, 2 };
}

DemoViewModel

public class DemoViewModel : ObservableObject
{
    private Demo _demo;

    public DemoViewModel()
    {
        _demo = new Demo();
        IncrementCommand = new RelayCommand(DoIncrement);
    }

    public Demo Demo
    {
        get => _demo;
        set
        {
            SetProperty(ref _demo, value);
        }
    }

    public IRelayCommand IncrementCommand { get; }

    private void DoIncrement()
    {
        _demo.Value++;
        _demo.AllValues.Add(_demo.Value);
        OnPropertyChanged("Demo");
    }
}

MainWindow.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new DemoViewModel();
    }
}

Grid portion of MainWindow.xaml.

<Grid Margin="0,0,0,10">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*" />
    </Grid.ColumnDefinitions>
    <TextBox Name="txtNumber" Text="{Binding Path=Demo.Value}"></TextBox>
    <Button Grid.Row="1" Name="btnIncrement" Command="{Binding IncrementCommand}">Increment</Button>
    <ListBox Grid.Row="2" ItemsSource="{Binding Path=Demo.AllValues}"></ListBox>
</Grid>

What needs fixing so that the ListBox updates after the button is clicked with the new list of values?

Thank you.

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2 comment threads

General (1 comment)
Setting with a new list instead of extending the existing list (2 comments)
General
FoggyFinder‭ wrote over 2 years ago

You already got correct answers but to me it just doesn't look like right architecture. You either get inefficient structure (since you have to copy mutable list over and over again) or model class with an observable property that logically doesn't fit there. I think it'd better to move AllValues property to VM and make Demo class immutable.