Hey there, let’s break down possible issues and fixes for your problem with the PropertyChanged event being null and your DataContext setup not working as expected.

1. Double-check your INotifyPropertyChanged implementation #

First off, make sure your StatusInfo class properly implements the interface—this is the most common culprit here. It’s easy to miss declaring the event or miswriting the OnPropertyChanged method. Here’s a correct implementation example:

public class StatusInfo : INotifyPropertyChanged
{
    // Don't forget to declare this event!
    public event PropertyChangedEventHandler PropertyChanged;

    private string _screenStatusBarText;
    public string ScreenStatusBarText
    {
        get => _screenStatusBarText;
        set
        {
            if (_screenStatusBarText == value) return; // Avoid unnecessary calls
            _screenStatusBarText = value;
            OnPropertyChanged(nameof(ScreenStatusBarText));
        }
    }

    protected virtual void OnPropertyChanged(string propertyName)
    {
        // Use null-conditional operator to safely invoke the event
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    public StatusInfo()
    {
        DataContext = this; // We'll revisit this in the next point
    }
}

2. Fix your DataContext setup (especially if this is a UserControl) #

Setting DataContext = this in the constructor can be fragile—if the parent control’s DataContext propagates down, it might overwrite your setup. A more reliable approach is to set it directly in XAML using a relative source:

<UserControl x:Class="YourAppNamespace.StatusInfo"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             <!-- This ensures the control binds to its own properties -->
             DataContext="{Binding RelativeSource={RelativeSource Self}}">
    
    <StackPanel>
        <!-- Your status bar elements here -->
        <TextBlock Text="{Binding ScreenStatusBarText}" />
    </StackPanel>
</UserControl>

This way, you avoid conflicts with parent DataContext values and ensure the binding stays tied to your StatusInfo instance.

3. Verify you’re updating the correct instance #

If PropertyChanged is null, that means no subscribers are listening to the event. Double-check:

• Are you updating the same StatusInfo instance that’s bound to your UI? It’s easy to accidentally create a new, unbound instance in your main code and update that instead.
• In your main program, how are you referencing the StatusInfo control? If you’re using a resource or a named element, make sure you’re accessing it correctly (e.g., FindName in code-behind or proper viewmodel wiring).

4. Debug the binding directly #

Add a debug trace to your XAML binding to see if there are any errors:

<TextBlock Text="{Binding ScreenStatusBarText, PresentationTraceSources.TraceLevel=High}" />

Check the Output window in Visual Studio—this will show you exactly where the binding is failing (e.g., missing property, incorrect source).

内容的提问来源于stack exchange，提问作者ed askew