VB.NET:无法在System.Object实例上使用Extension方法

Imports System.Runtime.CompilerServices

Module ExtensionMethods
   <Extension()>
    Public Sub MyExtensionMethod(ByVal obj As Object)
        ' 在这里编写扩展方法的代码
    End Sub
End Module

Imports System.Runtime.CompilerServices

Module ExtensionMethods
   <Extension()>
    Public Sub MyExtensionMethod(Of T As IMyInterface)(ByVal obj As T)
        ' 在这里编写扩展方法的代码
    End Sub
End Module

Imports System.Runtime.CompilerServices

Module ExtensionMethods
   <Extension()>
    Public Sub MyExtensionMethod(ByVal obj As String)
        ' 在这里编写扩展方法的代码
    End Sub
End Module

Module MainModule
    Sub Main()
        Dim obj As Object = "Hello, World!"
        Dim str As String = CType(obj, String)
        str.MyExtensionMethod()
    End Sub
End Module