如何通过RealProxy透明代理将对象作为返回值返回？

Question

我正在开发一个系统，其中我计划使用RealProxy对象来实现对一组对象的方法调用的拦截，处理该调用，然后返回适当的结果。

这只适用于简单的返回类型，比如字符串或int，但我似乎不能从RealProxy.Invoke方法返回对象。

一切正常。我没有收到错误，但返回的值始终是NOTHING，而不是object。

我已经尽可能地编写了最小的示例代码，并将其包含在下面。

本质上，只需调用RPtest并单步执行即可。该代码创建一个简单的对象RPTestA，其中包含一个字符串字段和一个对象值字段，然后检索字符串Dim x= c.Name，该字符串运行良好，然后尝试检索该对象

Dim r = c.SubObj

它总是不返回任何内容。

但是，在FieldGetter例程中，此代码：

'---- the field is an OBJECT type field  
Dim mc = New MethodCallMessageWrapper(Msg)  

'---- create the object  
Dim o = Activator.CreateInstance(t)  
'---- and construct the return message with that object  
Dim r = New ReturnMessage(o, mc.Args, mc.Args.Length, mc.LogicalCallContext, mc)  
Return r  

将ReturnMessage的ReturnValue字段设置为由上面的Activator.CreateInstance(t)调用创建的对象，似乎可以很好地工作。

我怀疑这是某种序列化的东西，但我不知所措。

您应该能够直接运行此代码，但只需将其粘贴到新的VB.net项目中即可。

'----------------------------------------------------------------------------
Imports System.Security.Permissions
Imports System.Diagnostics
Imports System.Reflection
Imports System.Runtime.CompilerServices
Imports System.Runtime.Serialization
Imports System.Runtime.Remoting
Imports System.Runtime.Remoting.Activation
Imports System.Runtime.Remoting.Messaging
Imports System.Runtime.Remoting.Proxies


Public Module RPTest
    Public Sub RPTest()
        '---- create a new object that is automatically proxied
        '     See the RPProxyAttribute for details
        Dim c = New RPTestA

        Dim x = c.Name
        'x is returned as a string value just fine
        Dim r = c.SubObj
        '********* PROBLEM IS HERE, r ends up nothing
    End Sub
End Module


'ROOT test object
Public Class RPTestA
    Inherits RPBase

    Public Name As String = "Test Name"
    Public SubObj As RPTestB

End Class


'SUB OBJECT which should be returned as a field value from the root object above
Public Class RPTestB
    Inherits RPBase

    Public SubProperty As String = "SubObj Test Property"
End Class


''' <summary>
''' Base proxyable object class
''' </summary>
''' <remarks></remarks>
<RPProxy()> _
Public MustInherit Class RPBase
    Inherits ContextBoundObject

End Class


<PermissionSet(SecurityAction.Demand, Name:="FullTrust")> _
Public Class RPProxy
    Inherits RealProxy

    Private m_target As MarshalByRefObject


    Public Sub New()
        m_target = DirectCast(Activator.CreateInstance(GetType(ConfigRP)), MarshalByRefObject)
        Dim myObjRef = RemotingServices.Marshal(m_target)
    End Sub

    Public Sub New(ByVal classToProxy As Type)
        MyBase.New(classToProxy)
    End Sub


    Public Sub New(ByVal ClassToProxy As Type, ByVal targetObject As MarshalByRefObject)
        m_target = targetObject
        Dim myObjRef = RemotingServices.Marshal(m_target)
    End Sub


    Public Overrides Function Invoke(ByVal msg As IMessage) As IMessage
        Dim returnMsg As IMethodReturnMessage = Nothing

        If TypeOf msg Is IConstructionCallMessage Then
            '---- handle constructor calls
            Dim ConstructionCallMessage = DirectCast(msg, IConstructionCallMessage)
            returnMsg = InitializeServerObject(ConstructionCallMessage)
            Me.m_target = Me.GetUnwrappedServer()
            SetStubData(Me, Me.m_target)
            Return returnMsg

        ElseIf TypeOf msg Is IMethodCallMessage Then
            '---- handle all other method calls
            Dim methodCallMessage = DirectCast(msg, IMethodCallMessage)

            '---- before message processing
            preprocess(methodCallMessage)

            '---- execute the method call
            Dim rawReturnMessage = RemotingServices.ExecuteMessage(Me.m_target, methodCallMessage)

            '---- and postprocess
            returnMsg = postprocess(methodCallMessage, rawReturnMessage)

        Else
            Throw New NotSupportedException()
        End If

        Return returnMsg
    End Function


    'Called BEFORE the actual method is invoked
    Private Sub PreProcess(ByVal msg As IMessage)
        Console.WriteLine("before method call...")
    End Sub


    'Called AFTER the actual method is invoked
    Private Function PostProcess(ByVal Msg As IMethodCallMessage, ByVal msgReturn As ReturnMessage) As ReturnMessage
        Dim r As ReturnMessage
        If Msg.MethodName = "FieldGetter" Then
            r = FieldGetter(Msg, msgReturn)
        ElseIf Msg.MethodName = "FieldSetter" Then
            'na
            r = msgReturn
        ElseIf Msg.MethodName.StartsWith("get_") Then
            'na
            r = msgReturn
        ElseIf Msg.MethodName.StartsWith("set_") Then
            'na
            r = msgReturn
        Else
            r = msgReturn
        End If
        Return r
    End Function


    Private Function FieldGetter(ByVal Msg As IMethodCallMessage, ByVal msgReturn As IMethodReturnMessage) As IMethodReturnMessage
        Dim t = Me.Target.GetType

        '---- This retrieves the type of the field that the getter should retrieve
        t = t.GetField(Msg.InArgs(1), BindingFlags.Instance Or BindingFlags.Public).FieldType

        If t.Name = "String" Then
            '---- just return what the object returned as a result of ExecuteMessage
            Return msgReturn

        ElseIf t.BaseType.Equals(GetType(RPBase)) Then
            '---- the field is an OBJECT type field
            Dim mc = New MethodCallMessageWrapper(Msg)
            '---- create the object
            Dim o = Activator.CreateInstance(t)
            '---- and construct the return message with that object
            Dim r = New ReturnMessage(o, mc.Args, mc.Args.Length, mc.LogicalCallContext, mc)
            Return r

        Else
            Return msgReturn
        End If
    End Function


    Public Property Target() As Object
        Get
            Return Me.m_target
        End Get
        Set(ByVal value As Object)
            Me.m_target = value
        End Set
    End Property
End Class


<AttributeUsage(AttributeTargets.Class)> _
<SecurityPermissionAttribute(SecurityAction.Demand, Flags:=SecurityPermissionFlag.Infrastructure)> _
Public Class RPProxyAttribute
    Inherits ProxyAttribute


    Public Overrides Function CreateInstance(ByVal Type As Type) As MarshalByRefObject
        Dim proxy = New RPProxy(Type)
        Dim transparentProxy = DirectCast(proxy.GetTransparentProxy(), MarshalByRefObject)
        Return transparentProxy
    End Function
End Class

Answer 1

好吧，事实证明这是一个非常简单的修复方法，一旦你越过了可怕的ReturnMessage构造函数，那就会产生误导！

非常感谢我的一位老同事，Rich Quackenbush，他花了几分钟时间来查看这篇文章。有时候，只见树木不见森林！

总之，在FieldGetter中，我是这样做的

ElseIf t.BaseType.Equals(GetType(RPBase)) Then
        '---- the field is an OBJECT type field
        Dim mc = New MethodCallMessageWrapper(Msg)
        '---- create the object
        Dim o = Activator.CreateInstance(t)
        '---- and construct the return message with that object
        Dim r = New ReturnMessage(o, mc.Args, mc.Args.Length, mc.LogicalCallContext, mc)
        Return r

看起来完全合理，新创建的对象被传递到名为ReturnValue的ReturnMessage构造函数参数中。

但不是。实际上，您必须创建一个对象数组，并将其作为该数组中的3元素传递，如下所示：

ElseIf t.BaseType.Equals(GetType(RPBase)) Then
        '---- the field is an OBJECT type field
        Dim mc = New MethodCallMessageWrapper(Msg)            '---- create the object
        Dim o = Activator.CreateInstance(t)
        '---- and construct the return message with that object
        Dim r = New ReturnMessage(Nothing, New Object() {Nothing, Nothing, o}, 3, mc.LogicalCallContext, mc)
        Return r

事实证明，这是因为FieldGetter函数是被代理“调用”和拦截的函数，它的签名是

FieldGetter(StringtypeName,StringfieldName,Object&val)

对于为该调用构造ReturnMessage的目的，这意味着它根本没有返回值，而是返回值作为该列表中的第三个参数返回。

因为我实际上并没有调用真正的FieldGetter函数，所以前两个参数(类型名和字段名)是无关紧要的，但是第三个参数是放置返回值的合适位置。

事后看来，这总是显而易见的！

非常感谢Rich。