In my previous article about extension methods I discussed how to implement them only in C#, as I had not yet looked at them in Visual Basic.  Today I decided to finally take a look and was expecting something similar to how it is implemented in C# but what I found was different enough to warrant talking about it again, I believe.  Anyway, let's get to it.

The first big difference I notice is that, in Visual Basic, you must define extension methods in a module.  I'm not sure why this is the case, but it is.  The second difference is that you must mark your extension methods with the extension attribute <Extension()> from the System.Runtime.CompilerServices namespace.  So once you have your module and you've imported the System.Runtime.CompilerServices namespace you're ready to create an extension method.  Here's the Visual Basic version of a sample extension from Scott Guthrie's blog:

Imports System.Text.RegularExpressions
Imports System.Runtime.CompilerServices

Module MyExtensions

    <Extension()> _
    Public Function IsValidEmailAddress(ByVal s As String) As Boolean
        Dim regex As New Regex("^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$")
        Return regex.IsMatch(s)
    End Function

End Module

In this function I've extended the string type.  Using this approach I can now call IsValidEmailAddress on the string variable itself.  I think this example is a perfect demonstration of the ease in which extension methods can be created and used.  A word of caution is in order, though, as it is quite easy to cause some namespace and method name collisions or make your code a little more difficult to debug.  As an example, some of the examples I used when creating the C# post on this topic don't seem to work very well using VB, so use with care. 

If you want more information please read the C# post or visit the MSDN article regarding Extension Methods in Visual Basic.