How to: Write an Extension Method (Visual Basic)

Extension methods enable you to add methods to an existing class. The extension method can be called as if it were an instance of that class.

To define an extension method

1. Open a new or existing Visual Basic application in Visual Studio.

2. At the top of the file in which you want to define an extension method, include the following import statement:

Imports System.Runtime.CompilerServices

3. Within a module in your new or existing application, begin the method definition with the <Extension> attribute:

<Extension()>


Note that the Extension attribute can only be applied to a method (a Sub or Function procedure) in a Visual Basic Module. If you apply it to a method in a Class or a Structure, the Visual Basic compiler generates error BC36551, "Extension methods can be defined only in modules."

4. Declare your method in the ordinary way, except that the type of the first parameter must be the data type you want to extend.

<Extension()>
Public Sub SubName(para1 As ExtendedType, <other parameters>)
' < Body of the method >
End Sub


Example

The following example declares an extension method in module StringExtensions. A second module, Module1, imports StringExtensions and calls the method. The extension method must be in scope when it is called. Extension method PrintAndPunctuate extends the String class with a method that displays the string instance followed by a string of punctuation symbols sent in as a parameter.

' Declarations will typically be in a separate module.
Imports System.Runtime.CompilerServices

Module StringExtensions
<Extension()>
Public Sub PrintAndPunctuate(aString As String, punc As String)
Console.WriteLine(aString & punc)
End Sub

End Module

' Import the module that holds the extension method you want to use,
' and call it.

Imports ConsoleApplication2.StringExtensions

Module Module1

Sub Main()
Dim example = "Hello"
example.PrintAndPunctuate("?")
example.PrintAndPunctuate("!!!!")
End Sub

End Module


Notice that the method is defined with two parameters and called with only one. The first parameter, aString, in the method definition is bound to example, the instance of String that calls the method. The output of the example is as follows:

Hello?
Hello!!!!