Try
 
            Dim sr As StreamReader = New StreamReader("D:\test.rtf")
            Dim line As String
            Do
                line = sr.ReadLine().ToString
                RichTextBox21.AppendText(line)
            Loop Until line Is Nothing
            sr.Close()
        Catch Ex As Exception
            ' Let the user know what went wrong.
            Console.WriteLine("The file could not be read:")
            Console.WriteLine(Ex.Message)
        End Try