If you would like to implement CompareTo() method, check this: [IComparable.CompareTo Method (Object) (System)](https://msdn.microsoft.com/en-us/library/system.icomparable.compareto(v=vs.110).aspx). GreaterThan() and LessThan() methods won't work on strings. Why? It's simple. String is not numeric value! To be able to compare lowValueString and highValueString with the set of numbers, you need to convert string values into proper data type. Convertion should be made before you data into Between method.