Extension

Extension method syntax is available in VB.NET. With it, we call a method in a separate Module as though it is defined on the Class.

This sometimes leads to clearer syntax. We use the Extension attribute and implement an extension method in a VB.NET program.

Example

This program introduces an extension method called LastChar. This method is called upon a String instance. It is called like a String method, but it is not defined on the String type.

Here LastChar() returns the final char in a string. It first checks that the string is not Nothing, and has one or more chars.

Tip In the Extensions module, we use the Extension() syntax. The parameter String is the instance the extension method is called upon.

Info You can have more parameters on an extension method. The first one is always the instance the method is called upon.

Important The System.Runtime.CompilerServices namespace must be imported. This provides access to the Extension attribute.

Imports System.Runtime.CompilerServices

Module Module1

    Sub Main()
        ' Test extension on a string.
        Dim v As String = "ABC"
        Console.WriteLine(v.LastChar())

        ' Test nothing value.
        v = Nothing
        Console.WriteLine(v.LastChar())

        ' Test empty string.
        v = ""
        Console.WriteLine(v.LastChar())
    End Sub

End Module

Module Extensions

    <Extension()>
    Public Function LastChar(ByVal value As String) As Char
        ' If string is not nothing, and has at least one char,
        ' ... return last char.
        ' Otherwise return a question mark.
        If Not value = Nothing Then
            If value.Length >= 1 Then
                Return value(value.Length - 1)
            End If
        End If
        Return "?"c
    End Function

End ModuleC
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Discussion

When should you use extension methods in VB.NET programs? If a small and useful method is used often, it may be helpful to turn it into an extension method.

And I often use extension methods to test characters and strings. For example, I use a "IsUpperOrDigit" extension method.

Info No such method exists in the .NET Framework, but the implementation is obvious.

Warning I usually end up with too many extension methods. It is preferable to limit the number, and have strict requirements.

An extension method changes the syntax for calling a method. It makes a non-Class method look like a Class method. This sometimes can yield simpler programs that are easier to read.