New Post: RichTextBox: Binding doesn't work when Mode=OneWay.

<extToolkit:RichTextBox IsReadOnly="True" Text="{Binding MyText, Mode=OneWay}">
    <extToolkit:RichTextBox.TextFormatter> 
        <extToolkit:PlainTextFormatter />
    </extToolkit:RichTextBox.TextFormatter>
</extToolkit:RichTextBox>

public string MyText { get; private set; }

this.MyTextChangeCommand = new RelayCommand(() => 
    this.MyText = "new value";
    this.RaisePropertyChanged(nameof(this.MyText)); 
}});

<extToolkit:RichTextBox IsReadOnly="True" Text="{Binding MyText}">
    <extToolkit:RichTextBox.TextFormatter> 
        <extToolkit:PlainTextFormatter />
    </extToolkit:RichTextBox.TextFormatter>
</extToolkit:RichTextBox>

public string MyText { get; set; }

MyTextChangeCommand - the same