Data binding in C# WPF wont reflect in the UI

Question

I would like to know what is wrong with my data binding. Simply, when I click on an element, I must get the X and Y mouse position. It should reflect on the textboxes. BUt my code does not. Here is the XAML code:

<TextBox Name="positionX_txtbox" Width="58" Text="{Binding x,NotifyOnTargetUpdated=True,UpdateSourceTrigger=PropertyChanged}"/>
<TextBox Name="positionY_txtbox" Width="58" Text="{Binding y,NotifyOnTargetUpdated=True,UpdateSourceTrigger=PropertyChanged}"/>

Here is the class that inherits from INotifyPropertyChanged:

public class VideoViewModel : INotifyPropertyChanged {
    private double _x;
    private double _y;

        public double x {
            get { return _x; }
            set {
                _x = value;
                OnPropertyChanged();
            }
        }

        public double y {
            get { return _y; }
            set {
                _y = value;
                OnPropertyChanged();
            }
        }

public event PropertyChangedEventHandler? PropertyChanged;

public void OnPropertyChanged([CallerMemberName] string propertyName = null) { 
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
}

This is the code that runs upon mouse click:

VideoViewModel VideoViewModel = new VideoViewModel();

private void Video_MouseLeftButtonDown(object sender, MouseButtonEventArgs e) {
            Point point = e.GetPosition(null);            
            VideoViewModel.x = point.X;
            VideoViewModel.y = point.Y;
            DataContext = VideoViewModel;
        }

I know this is a beginner level question but I'm fairly new to C# and I'd appreciate any help.

try to set the datacontext earlier while initializing the viewmodel. if that does not work, please provide us your OnPropertyChanged() method and where you initialize your viewmodel — Tommehh, Aug 12 '22 at 05:36
VideoViewModel VideoViewModel = new VideoViewModel(); is just outside of my Video_MouseLeftButtonDown function and is still within the same namespace. Could you elaborate more on 'setting the datacontext earlier'? — sean trees, Aug 12 '22 at 05:57
As for the OnPropertyChanged, I edited my question above to include it. — sean trees, Aug 12 '22 at 06:00
Please note that `NotifyOnTargetUpdated=True` and `UpdateSourceTrigger=PropertyChanged` on the Bindings is totally pointless. The Binding should just look like this: `Text="{Binding x}"`. — Clemens, Aug 12 '22 at 07:10

score 1 · Answer 1 · edited Aug 12 '22 at 07:10

It works after setting the DataContext earlier.

Heres the MainWindow.xaml.cs I made (setting the DataContext after InitializeComponent()):

public MainWindow()
{
    InitializeComponent();
    VideoViewModel = new VideoViewModel();
    DataContext = VideoViewModel;
}

public VideoViewModel VideoViewModel { get; }

private void TextBlock_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    Point point = e.GetPosition(null);
    VideoViewModel.x = point.X;
    VideoViewModel.y = point.Y;
}

Your Binding ist just fine just the timing of setting the DataContext isn't. Here an article about it: https://stackoverflow.com/a/7262322/11000908

My XAML looks like this:

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition></RowDefinition>
        <RowDefinition></RowDefinition>
        <RowDefinition></RowDefinition>
    </Grid.RowDefinitions>
    <TextBox Grid.Row="0" Name="positionX_txtbox" Width="700" Height="50"
             Text="{Binding x}"/>
    <TextBox Grid.Row="1" Name="positionY_txtbox" Width="700" Height="50"
             Text="{Binding y}"/>
    <TextBlock Grid.Row="2" MouseLeftButtonDown="TextBlock_MouseLeftButtonDown" >Press mouse button</TextBlock>
</Grid>

And my VideoViewModel Class like this:

public class VideoViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler? PropertyChanged;

    private double _x;
    private double _y;

    public double x
    {
        get { return _x; }
        set
        {
            _x = value;
            OnPropertyChanged();
        }
    }

    public double y
    {
        get { return _y; }
        set
        {
            _y = value;
            OnPropertyChanged();
        }
    }

    protected void OnPropertyChanged([CallerMemberName] string name = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
    }
}

Btw since you are new to C#. Coding conventions suggest writing properties like your x and y with a capital letter. And in a real project I wouldn't just call them x and y, maybe something like MousePositionX and MousePositionY. But those are details.

There is no reason to make ViewModel a public property of the MainWindow class. It would work as well with the private field. — Clemens, Aug 12 '22 at 07:14

Data binding in C# WPF wont reflect in the UI

1 Answers1