Type System.Runtime.CompilerServices.Extension is not defined.

这是我的样本

<System.Runtime.CompilerServices.Extension()> _
     Public Sub test()

End Sub

我哪里错了？

编辑〜直接从MSDN文章here,同样的错误

Imports System.Runtime.CompilerServices

Module StringExtensions
     _
  Public Sub Print(ByVal aString As String)
        Console.WriteLine(aString)
    End Sub

End Module

我在我的项目中使用Visual Studio 2008和3.5 Framework.

Solution ~ The project was on 2.0 Framework. Changed to 3.5 and it works.