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'Extension' attribute can be applied only to 'Module', 'Sub', or 'Function' declarations

The only way to extend a data type in Visual Basic is to define an extension method inside a standard module. The extension method can be a Sub procedure or a Function procedure. All extension methods must be marked with the extension attribute, <Extension()>, from the System.Runtime.CompilerServices namespace. Optionally, a module that contains an extension method may be marked in the same way. No other use of the extension attribute is valid.

Error ID: BC36550

To correct this error

  • Remove the extension attribute.

  • Redesign your extension as a method, defined in an enclosing module.

The following example defines a Print method for the String data type.

Imports StringUtility
Imports System.Runtime.CompilerServices
Namespace StringUtility
    <Extension()> 
    Module StringExtensions
        <Extension()> 
        Public Sub Print (ByVal str As String)
            Console.WriteLine(str)
        End Sub
    End Module
End Namespace
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