Tag Property

if (IsPositionContainedBetween(rtb.CaretPosition, startP, endP))
     {
...

     }

// This method first checks for compatible text container scope, and then checks whether
// a specified position is between two other specified positions.
        bool IsPositionContainedBetween(TextPointer positionToTest, TextPointer start, TextPointer end)
        {
// Note that without this check, an exception will be raised by CompareTo if positionToTest 
// does not point to a position that is in the same text container used by start and end.
//
// This test also implicitely indicates whether start and end share a common text container.
            if (!positionToTest.IsInSameDocument(start) || !positionToTest.IsInSameDocument(end))
                return false;

            return start.CompareTo(positionToTest) <= 0 && positionToTest.CompareTo(end) <= 0;
        }