The StringChanged event is a protected method of the DataProviderBase class and could be accessed only by derived classes.
In order to achieve the desired goal, you could use the DocumentContentChanged event of RadRichTextBox - it will notify you when the users enter, paste or modify the styles of the document content.
Another option, which could help you track only the paste is to subscribe to the CommandExecuting event of the control and check whether the Command is a PasteCommand:
sender, Telerik.Windows.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
Hope this helps.
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