What are Extension Method? Orcas / VB 2008 / VB9.0

What are Extension Methods?

Extension methods allow developers to add new methods to the public contract of an existing CLR type, without having to sub-class it or recompile the original type.

Simple Extension Method Example:

Ever wanted to check to see whether a string variable is a valid email address? Today you'd probably implement this by calling a separate class (probably with a static method) to check to see whether the string is valid. For example, something like:


Dim email As String = "itgalary@yahoo.com"

If EmailValidator.IsValid(email) Then

End If


Using the new "extension method" language feature in C# and VB, I can instead add a useful "IsValidEmailAddress()" method onto the string class itself, which returns whether the string instance is a valid string or not. I can then re-write my code to be cleaner and more descriptive like so:


Dim email As String = "itgalary@yahoo.com"

If email.IsValidEmailAddress() Then

End If

How did we add this new IsValidEmailAddress() method to the existing string type? We did it by defining a static class with a static method containing our "IsValidEmailAddress" extension method like below:

<System.Runtime.CompilerServices.Extension()> _
Public Function IsValidEmailAddress(ByVal s As String) as Boolean
Dim pattern As String = "^[a-zA-Z][\w\.-]*[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$"
Dim emailAddressMatch As Match = Regex.Match(s, pattern)

If emailAddressMatch.Success Then
     IsValidEmailAddress = True
Else
     IsValidEmailAddress = False
End If
End Function

Note the first parameter argument of type string. This tells the compiler that this particular Extension Method should be added to objects of type "String".