Cool VB.NET Features – Extensions

Extensions are a great way to call often used methods to tweak a value while shortening code length and improving readability.

If you’ve done much .NET programming, you’ve already used built-in extensions, for example ToLower:

Dim tmpString1 As String = "BLAH"

Dim FinalValue As String = tmpString1.ToLower

But another cool feature of VB.NET is that you can define your own extensions.  You must define your extensions in side a module (not a class).  Here are the example extensions I’ll use for the rest of this article:

Imports System.Runtime.CompilerServices

Module StringExtensions

   <Extension()> _
   Function SafeConvertToLong(ByVal TheString As String, Optional ByVal DefaultValueWhenBlankOrInvalid As Long = 0) As Long

      If TheString = "" Then Return DefaultValueWhenBlankOrInvalid

      Dim ReturnLong As Long

      If Long.TryParse(TheString, ReturnLong) Then
         Return ReturnLong
      End If

      Return DefaultValueWhenBlankOrInvalid

   End Function

   <Extension()> _
   Public Function FirstWord(ByVal TheString As String) As String

      Dim SpacePosition As Integer = InStr(TheString, " ")

      If SpacePosition = 0 Then Return TheString

      Return Left(TheString, SpacePosition).Trim

   End Function

End Module

Extensions can be used via the DOT notation after a variable of the same type as the first parameter to the extension.  For example, since the two example extensions above both have a String value as their first parameter, then they “extend” the String type.

The code below shows some examples of using the “SafeConvertToLong” example String extension:

Dim tmpLong1 As Long
Dim tmpLong2 As Long
Dim tmpLong3 As Long

tmpLong1 = "3832".SafeConvertToLong
tmpLong2 = "blah".SafeConvertToLong
tmpLong3 = "blah".SafeConvertToLong(-1L)

Console.WriteLine("tmpLong1 = " & tmpLong1.ToString)
Console.WriteLine("tmpLong2 = " & tmpLong2.ToString)
Console.WriteLine("tmpLong3 = " & tmpLong3.ToString)

and this code produces the following output:

tmpLong1 = 3832
tmpLong2 = 0
tmpLong3 = -1

Even though you’ve declared a function as an extension, you can use it just like a normal function as well:

Dim tmpLong4 As Long

tmpLong4 = StringExtensions.SafeConvertToLong("75")
Console.WriteLine("tmpLong4 = " & tmpLong4.ToString)

Will produce this output:

tmpLong4 = 75

Extensions get even cooler when you stack multiples together and even use your own extensions along with the built-in ones:

Dim tmpString2 As String

tmpString2 = "THIS IS A TEST".FirstWord.ToLower
Console.WriteLine("tmpString1 = " & tmpString2)

Which produces this output:

tmpString1 = this

Conclusion

Extensions are a great way to implement highly reusable code that slightly transforms a value.  Used correctly, they can reduce the size of your code and make it much more readable.