为什么在同一个扩展方法中使用一个扩展方法不起作用？

Question

我得到了一个扩展方法，它为我提供了实例中每个属性的值。对于标量值，它工作正常。但是对于 Collections 有一个问题。这是我的代码：

<Extension()>
Public Function ToXml(Of T)(ByVal source As T) As XmlDocument
    Dim oXmlDocument As New XmlDocument
    oXmlDocument.AppendChild(oXmlDocument.CreateXmlDeclaration("1.0", "utf-8", Nothing))
    oXmlDocument.AppendChild(oXmlDocument.CreateElement(XmlConvert.EncodeName(source.GetType.ToString)))

    For Each Item As System.Reflection.FieldInfo In source.GetType.GetFields
        Dim oElement As XmlElement = oXmlDocument.CreateElement(XmlConvert.EncodeName(Item.MemberType.ToString))
        oElement.Attributes.Append(oXmlDocument.CreateAttribute("Name")).Value = Item.Name
        oElement.Attributes.Append(oXmlDocument.CreateAttribute("Value")).Value = Item.GetValue(source)

        oXmlDocument.DocumentElement.AppendChild(oElement)
    Next

    For Each Item As System.Reflection.PropertyInfo In source.GetType.GetProperties
        Dim oElement As XmlElement = oXmlDocument.CreateElement(XmlConvert.EncodeName(Item.MemberType.ToString))
        oElement.Attributes.Append(oXmlDocument.CreateAttribute("Name")).Value = Item.Name

        If (Not (TryCast(Item.GetValue(source, Nothing), ICollection) Is Nothing)) Then
            For Each SubItem As Object In CType(Item.GetValue(source, Nothing), ICollection)
                For Each Node As XmlNode In SubItem.ToXml().DocumentElement.SelectNodes("node()")
                    oElement.AppendChild(oElement.OwnerDocument.ImportNode(Node, True))
                Next
            Next
        Else
            oElement.Attributes.Append(oXmlDocument.CreateAttribute("Value")).Value = If(Not (Item.GetValue(source, Nothing) Is Nothing), Item.GetValue(source, Nothing).ToString, "Nothing")
        End If

        oXmlDocument.DocumentElement.AppendChild(oElement)
    Next

    Return oXmlDocument
End Function

行

For Each Node As XmlNode In SubItem.ToXml().DocumentElement.SelectNodes("node()")

抛出错误 Public member 'ToXml' on type 'MyClass' not found.

但如果我这样做

Dim instance As new MyClass
instance.ToXml()

这也行。循环中我的错在哪里？

提前感谢您的回复。

Answer 1

在 VB.NET 中，为了向后兼容，扩展方法不适用于声明为 Object 的变量。

尝试：

Dim instance As Object = new MyClass()
instance.ToXml()

会失败。

所以你不能在 SubItem 上调用 ToXml 因为 SubItem 的类型是 Object.

但是，您可以像常规方法一样调用 ToXml：

For Each Node As XmlNode In ToXml(SubItem).DocumentElement.SelectNodes("node()")

为什么在同一个扩展方法中使用一个扩展方法不起作用？

Why does not work the use of an extension method in the same extension method?

vb.net

reflection

extension-methods

dynamic-programming

generic-programming