Why does Run.Text bind two-way by default?

Question

In .NET 4.0, Run.Text is bindable. So I tried to bind it:

<Run Text="{Binding DisplayText}"/> 

But when I ran, I got an error: "A TwoWay or OneWayToSource binding cannot work on the read-only property 'DisplayText' of type 'SomeNamespace.SomeClass'."

My DisplayText property was indeed read-only, but so is a Run -- Runs go in in TextBlocks, which you can't edit. So why would I be getting this error? I dug into PresentationFramework with dotPeek and sure enough:

public static readonly DependencyProperty TextProperty =     DependencyProperty.Register("Text", typeof (string), typeof (Run),     (PropertyMetadata) new FrameworkPropertyMetadata((object) string.Empty,         FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,         new PropertyChangedCallback(Run.OnTextPropertyChanged),         new CoerceValueCallback(Run.CoerceText))); 

The fourth line, plain as day, specifies that Run.Text should bind two-way by default, which makes no sense and seems like a glaring design bug.

Of course, this is easy enough to work around:

<Run Text="{Binding DisplayText, Mode=OneWay}"/> 

But why should I have to work around it? Why does Run bind two-way by default?

Answer 1

Just a guess here:

It might be because Run objects are also used in the RichTextBox control, and I can imagine this control might want to Bind TwoWay by default!