Extention Methods In .Net
You want to create Method inside any of the system class or your custom class
without creating a new derived type, recompiling, or otherwise modifying the
original type/class.
Here is solution ...
Create extention method.
Extension methods enable you to "add" methods to existing types without
creating a new derived type, recompiling, or otherwise modifying the original
type. Extension methods are a special kind of static method, but they are called
as if they were instance methods on the extended type. For client code written
in C# and Visual Basic, there is no apparent difference between calling an
extension method and the methods that are actually defined in a type.
See below screen shot :

you can try
it with string.
As its not possible to override string class and its methods, the reasonbeing
that String is defined to be a Sealed class. You can extend it using extention
method.
Refer this MSDN artical for more details:
http://msdn.microsoft.com/en-us/library/bb383977.aspx.