CODEWITHSUNDEEP
c#wpfdata-bindinginotifypropertychangedtextblock
ed askew
ed askew

Reputation: 123

WPF C# PropertyChanged always null

I have played around with this for a while and decided to see if someone can help, I have set in the constructor of StatusInfo the DataContext = this and didn't work. When I write a string to ScreenStatusBarText it does call the OnPropertyChanged method but every time the PropertyChanged value is null. I The status block I have at the bottom of the screen. I have a tab section above this stack panel that has many components that use bindings and work.

Screen Code

<StackPanel Margin="0,1047,0,0">
  <Grid Name="StatusBarItemGrid">
  <TextBlock Name="StatusBarText" Text="may the force be with you"   VerticalAlignment="Center" HorizontalAlignment="Stretch" />
  </Grid>
 </StackPanel>

Data Model:

public partial class StatusInfo :  INotifyPropertyChanged
{

    private string screenStatusBarText;

    public StatusInfo()
    {
        BindScreenStatusBarText();
        screenStatusBarText = "Initialized";
    }

    public string ScreenStatusBarText
    {
        get { return screenStatusBarText; }
        set
        {
            screenStatusBarText = value;
            OnPropertyChanged("StatusBarText");
        }
    }

    private void BindScreenStatusBarText()
    {

        Binding b = new Binding();
        b.Source = screenStatusBarText;
        b.Mode = BindingMode.OneWay;
        b.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
        b.Path = new PropertyPath("StatusBarText");
        MainWindow.mainWindow.StatusBarText.SetBinding(TextBlock.TextProperty, b);

        MainWindow.mainWindow.StatusBarText.DataContext = this;
    }

    public event PropertyChangedEventHandler PropertyChanged;

    void OnPropertyChanged(string propName)
    {
        if (this.PropertyChanged != null)
            this.PropertyChanged(
                this, new PropertyChangedEventArgs(propName));
    } 
}

My main :

public partial class MainWindow : Window
{
    public static StatusInfo status;
    public MainWindow()
    {
        InitializeComponent();
        SourceInitialized += MainWindow_SourceInitialized;
    }

    private void MainWindow_SourceInitialized(object sender, EventArgs e)
    {
        SetUpDisplay();
    }

    private void SetUpDisplay()
    {
            status = new StatusInfo();
    }
}

Upvotes: 0

Views: 4404

Answers (2)

Clemens
Clemens

Reputation: 128013

Set the Binding in XAML instead of code behind:

<TextBlock Text="{Binding ScreenStatusBarText}" />

And use a view model like

public class StatusInfo : INotifyPropertyChanged
{
    private string screenStatusBarText = "Initialized";

    public string ScreenStatusBarText
    {
        get { return screenStatusBarText; }
        set
        {
            screenStatusBarText = value;
            OnPropertyChanged(nameof(ScreenStatusBarText));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this,
            new PropertyChangedEventArgs(propertyName));
    }
}

with an instance of the view model class assigned to the MainWindow's DataContext:

private readonly StatusInfo statusInfo = new StatusInfo();

public MainWindow()
{
    InitializeComponent();
    DataContext = statusInfo;
}

You may now access the view model class at any time later, e.g. in an event handler of an element of MainWindow:

statusInfo.ScreenStatusBarText = "Something";

Upvotes: 0

Derrick Moeller
Derrick Moeller

Reputation: 4950

I think your going to struggle doing your binding in code behind.

Having said that, with regards to why your PropertyChanged value is null. You've simply made a typo, as-is you're notifying subscribers that a property that doesn't exist has changed. One solution to avoid such typos is to use nameof. 

public string ScreenStatusBarText
{
    get { return screenStatusBarText; }
    set
    {
        screenStatusBarText = value;
        OnPropertyChanged(nameof(ScreenStatusBarText));
    }
}

It occurred to me you may also have meant that your event was null. This simply means you don't have any subscribers. See Why is my "Event" always null?.

private void OnPropertyChanged(string propertyName)
{
    var handler = PropertyChanged;

    if (handler != null) // I have a subscriber.
        handler(this, new PropertyChangedEventArgs(propertyName));
}

Upvotes: 0

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