RichTextBox Binding is broken when text is manually modified or cleared

Question

I have a RichTextBox bound to a string.

Using C# I generate a string that writes to it.

But if I want to manually change the text by clicking into t

Answer 1

When you edit the RichTextBox, you alter the elements inside of the FlowDocument element. The element you have a binding on, is probably removed at some point during this editing. Have a look at RichtTextBox.Document.Groups to see what's happening when you edit the RichTextBox.

The default RichTextBox does not really support MVVM/Binding very well. You'd want to have a binding on the Document property, but this is not supported for the default RichTextBox. You could have a look here.

Or extend it yourself, something like this?:

BindableRichTextBox class

public class BindableRichTextBox : RichTextBox
{
    public static readonly DependencyProperty DocumentProperty = DependencyProperty.Register(nameof(Document), typeof(FlowDocument), typeof(BindableRichTextBox), new FrameworkPropertyMetadata(null, OnDocumentChanged));

    public new FlowDocument Document
    {
        get => (FlowDocument)GetValue(DocumentProperty);
        set => SetValue(DocumentProperty, value);
    }

    public static void OnDocumentChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
    {
        var rtb = (RichTextBox)obj;
        rtb.Document = args.NewValue != null ? (FlowDocument)args.NewValue : new FlowDocument();
    }   
}

XAML

Then you can get the string from the FlowDocument.