WPF TextBox databinding, works only partially

Question

I have a simple WPF textbox that I want to update once when the window is initialized, and then each time a certain event occurs. I have followed the instructions in many StackOverflow answers, in particular this one: WPF: simple TextBox data binding So I came up with the following.

In xaml:

<TextBox x:Name="txtFeedback" 
         TextWrapping="Wrap" 
         Text="{Binding Path=FeedbackText, Mode=TwoWay,
                        UpdateSourceTrigger=PropertyChanged}" 
         IsReadOnly="True" 
         AcceptsReturn="True"/>

And in the MainWindow:

private string _feedbackText;
public event PropertyChangedEventHandler PropertyChanged;

public string FeedbackText
{
    get
    {
        return _feedbackText;
    }
    set
    {
        _feedbackText = value;
        OnPropertyChanged("FeedbackText");
    }
}

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

public MainWindow()
{
    InitializeComponent();
    this.DataContext = this;
    FeedbackText = "Awaiting start of process...";
}
private void FinishedWorksheet(object sender, EventArgs e)
{
    FeedbackText += "Done another worksheet" ;
}

When the constructor of the form is called, the textbox correctly displays "Awaiting start of process...", but when I call the FinishedWorksheet method (which is called by some other event) the textbox is not updated.

Please note that if I put breakpoints inside OnPropertyChanged I can see that it is called, and in the immediate window I can see the Text property actually changing, but somehow the textbox is not updated.

txtFeedback.Text

"Awaiting start of process...Done another worksheet"

Update

The problem was much simpler, and sorry to everyone for not posting one fundamental part of the code: I was doing everything inside the GUI thread! So of course the GUI could not react until I released the thread! Once I made the external call Asynchronous, it all works fine. Thank you all for your suggestions though, I learned a lot, and I hope others did too.

Answer 1

Do it without information of bidirectional binding

<TextBox x:Name="txtFeedback" HorizontalAlignment="Left" Height="213" Margin="10,196,0,0" TextWrapping="Wrap" Text="{Binding FeedbackText}" VerticalAlignment="Top" Width="772" IsReadOnly="True" AcceptsReturn="True"/>

Change += to =

private void FinishedWorksheet(object sender, EventArgs e)
{
    FeedbackText = "Done another worksheet" ;
}

MainWindow:

public partial class MainWindow : Window, INotifyPropertyChanged
{
    private string _feedbackText;
    public event PropertyChangedEventHandler PropertyChanged;

    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = this;
        FeedbackText = "Awaiting start of process...";
    }

    public string FeedbackText
    {
        get
        {
            return _feedbackText;
        }
        set
        {
            _feedbackText = value;
            OnPropertyChanged("FeedbackText");
        }

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

    private void FinishedWorksheet(object sender, EventArgs e)
    {
        FeedbackText = "Done another worksheet";
    }
}

I do not know how you call FinishedWorksheet. But if this is the method assigned to Button, it should work.

Answer 2

Remove IsReadOnly and binding Enabled in Model instead. I think it solves your problem

<TextBox x:Name="txtFeedback" HorizontalAlignment="Left" Height="213" Margin="10,196,0,0" TextWrapping="Wrap" Text="{Binding Path=FeedbackText}" VerticalAlignment="Top" Width="772" Enabled="{Binding Path=IsEnabled}" AcceptsReturn="True"/>

public bool IsEnabled
{
    get
    {
        return _isEnabled;
    }
    set
    {
        _isEnabled= value;
        OnPropertyChanged("IsEnabled");
    }
}

Answer 3

On controls always use DependencyProperty. Then change the binding too (try to avoid to set the DataContext of a Control to this):

public static readonly DependencyProperty FeedbackTextProperty = DependencyProperty.Register(
  "FeedbackText",
  typeof(string),
  typeof(MainWindow),
  new PropertyMetadata(default(string)));

public string FeedbackText
{
  get => (string) GetValue(MainWindow.FeedbackTextProperty);
  set => SetValue(MainWindow.FeedbackTextProperty, value);
}

Also because the TextBox is for display only consider to use a TextBlock instead:

<TextBlock x:Name="TxtFeedback" 
           TextWrapping="Wrap" 
           Text="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=MainWindow}, Path=FeedbackText}" />

Remarks
The Binding.Mode of the TextBox.Text property is TwoWay by default. A read-only TextBox doesn't require TwoWay binding at all since it can't receive input.
Also always avoid string literals. Instead use nameof({Property}) e.g. OnPropertyChanged(mameof(this.FeedbackText);. This will also make refactoring easier and is safe.