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For the remoting mechanism to work correctly, the client application must have access to the metadata of the remotable object; otherwise, the .NET runtime can neither resolve the New operator nor invoke the object s members. The simplest way to let the client access the object s metadata is to deploy the component DLL on the client sys tem. This approach is fine for our initial experiments but is clearly contrary to the prin ciples of distributing programming. Providing clients with a copy of the object s DLL makes deployment more difficult and opens up a security hole, as malicious clients can decompile and reverse-engineer the component. I ll tackle this problem in the section Metadata Deployment, later in this chapter, but for now let s agree on deploying the DLL on both the server and the client machine. This unrealistic assumption will greatly simplify the explanation that follows. .NET remoting differs from both COM+ and Web services architectures in one important detail. When a client calls a serviced component in a server application, the COM+ infra structure can intercept the call and instantiate the object inside the DllHost.exe execut able. A similar thing happens with Web services in this case, the aspnet_isapi.dll filter intercepts the request and creates the object on behalf of the client. In a sense, you could say that DllHost.exe is the host application for COM+ compo nents and IIS is the host application for Web services components. If COM+ and IIS are up and running, they can serve incoming requests for serviced components and Web service components, respectively. Alas, the .NET remoting infrastructure doesn t offer a similar service and doesn t come with a predefined host application. You have to create your own host application and ensure that this application is running when the request from the client arrives; otherwise, the request fails.
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The main goal of the host application is to inform the .NET remoting subsystem that the component in the DLL can be accessed by remote clients and to provide an address space in which the DLL can be loaded. From that point on, the .NET runtime will respond to requests from clients without any intervention by the host application. Even if the host application doesn t play any active role after it registers the remote object, the application must be running for this mechanism to work; otherwise, the .NET runt ime would have no AppDomain in which the DLL can run. The host can be any managed application. Real-world remoting systems typically use either a Windows NT service or IIS as the host application. Both these solutions have a great advantage over other types of programs: they can run even if no user has logged onto the system, which is a requirement if the distributed application must run in unattended mode. For our experiments, however, we ll adopt a simpler approach and use a console application as the host application. It is therefore essential that you run the console application on the server machine before the client requests the object; otherwise, the request will fail.
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Create a new Class library project, name it RemoteComponents, rename the Class1.vb file as Calculator.vb, and type this code:
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Public Class Calculator Inherits MarshalByRefObject This variable holds the total number of calls. Dim Counter As Integer = 0 Return the sum of two numbers
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Function Add(ByVal n1 As Double, ByVal n2 As Double) As Double
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Add = n1 + n2 Display a diagnostic message on the console window. Counter += 1 Console.WriteLine( Call #{0} : {1} + {2} = {3}", Counter, n1, n2, Add) End Function Return information the process that hosts this object. Function GetProcessID() As String Return String.Format( Process #{0} on {1}", _ Process.GetCurrentProcess.Id, Environment.MachineName) End Function End Class
Admittedly, except for the Inherits keyword that makes this an MBR class, this code isn t very interesting. The Add method displays a diagnostic message that contains the value of the Counter member variable, a piece of information useful for understanding what s going on behind the scenes. Both methods are instance methods: like serviced components, .NET remoting doesn t support remote invocation of shared methods. (Calls to shared methods seem to work correctly, but some debugging proves that they are run in the local assembly, not the remote assembly.)
Part VII:
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