you must use WinForm RichTextBox
(not in UI, just in code), even if you are on WPF, in order to convert RTF to plain text. Use this method in your Copy event.
C# code :
private String ConvertRtfToText()
{
System.Windows.Forms.RichTextBox rtfBox = new System.Windows.Forms.RichTextBox();
rtfBox.Rtf = this.rtfData;
return rtfBox.Text;
}
VB.Net Code :
Private Function ConvertRtfToText() As String
Dim rtfBox As RichTextBox = New RichTextBox()
rtfBox.Rtf = Me.rtfData
Return rtfBox.Text
End Function
source : http://msdn.microsoft.com/en-US/en-en/library/vstudio/cc488002.aspx