Advanced Topics in .NET framework

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If you now run the host application and then the client application, you ll see that repeated calls to the Add method are all labeled as Call #1, as shown in Figure 32-3. This happens because single-call SAOs are created and destroyed at each method invo cation; therefore, their member variables are reset each time.
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Figure 32-3
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Private members of a single-call SAO are initialized at each method invocation.
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Registering a Singleton Server-Activated Object
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Registering a singleton SAO requires the same steps as registering single-call SAOs, both in the host and in the client application. The only difference is the last argument that the host application passes to the RegisterWellKnownServiceType method:
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(In the host application.)
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ChannelServices.RegisterChannel(New TcpChannel(50000))
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Register the object as a singleton SAO.
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RemotingConfiguration.RegisterWellKnownServiceType(GetType(Calculator), _
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calculator.rem", WellKnownObjectMode.Singleton)
The client application works the same way, regardless of whether it accesses a singlecall or a singleton SAO:
(In the client application.)
Dim url As String = tcp://localhost:50000/calculator.rem"
RemotingConfiguration.RegisterWellKnownClientType(GetType(Calculator), url)
You must run two or more instances of the client application to see how a singleton SAO differs from a single-call SAO. All the various clients receive a reference to the same singleton object, thus you ll get the result shown in Figure 32-2 even if the method is called by different clients. Instead of registering an SAO and then instantiating the proxy with a regular New keyword, the client application can use the Activator.GetObject method, as in this code snippet:
(In the client application.)
Dim calc As Calculator = DirectCast(Activator.GetObject( _
GetType(Calculator), url), RemoteComponent.Calculator)
This technique makes it clear that the object is instantiated remotely and is especially useful when the client application needs to instantiate the object on a specific remote
32:
Remoting
system. Additionally, this technique can greatly simplify the deployment of the distrib uted application, as I ll explain in the section Shared Interfaces with SAOs, later in this chapter.
Publishing a Singleton Server-Activated Object
Instead of registering a singleton SAO and then letting the .NET remoting instantiate a singleton of that type when a client request arrives, you can use the RemotingSer vices.Marshal shared method to publish a specific instance of an object as a singleton SAO. (This is the technique used in the first example in this chapter.) In this case, the server doesn t register the class; rather, it publishes a specific instance that it has cre ated previously:
(In the host application.) ChannelServices.RegisterChannel(New TcpChannel(50000)) Dim calc As New Calculator RemotingServices.Marshal(calc, calculator.rem )
The client is oblivious to the fact that it is accessing a published or registered object, so it accesses the remote instance as any SAO, either by registering it and then using New or by getting a reference with the RemotingServices.Connect method, as shown by the following equivalent declarations:
1. The registration method. RemotingConfiguration.RegisterWellKnownClientType(GetType(Calculator), url) Dim calc As New Calculator 2. The publishing method (no registration is necessary). Dim calc As Calculator = DirectCast(RemotingServices.Connect( _ GetType(Calculator), url), Calculator)
Publishing a specific object instance has two advantages over the registration tech niques described earlier. First, the host application can use parameterized constructors when creating the object. Second, the host application can publish different objects at different times, as in this code:
This code assumes that the Calculator class has a constructor
that takes one integer value.
Dim calc0 As New Calculator(0)
Dim calc1 As New Calculator(1)
RemotingServices.Marshal(calc0, calculator.rem )
Client applications will connect to calc0.
Disconnect calc0 and publish calc1.
RemotingServices.Disconnect(calc0)
RemotingServices.Marshal(calc1, calculator.rem )
Client applications will connect to calc1.
Disconnect calc1.
RemotingServices.Disconnect(calc1)
Clients receive an exception if they attempt to create an object now.
Part VII:
Advanced Topics
Registering a Client-Activated Object
The host application registers a CAO by means of the RegisterActivatedServiceType shared method of the RemotingConfiguration class. An important difference from SAOs is that you don t specify any URI when registering a client-activated object:
(In the host application.) ChannelServices.RegisterChannel(New TcpChannel(50000)) RemotingConfiguration.ApplicationName = Calculator" RemotingConfiguration.RegisterActivatedServiceType(GetType(Calculator))
The client application registers a CAO with the RegisterActivatedClientType method. The string you pass to the second argument of this method is the concatenation of the URL of the remote machine and the application name:
(In the client application.)
Dim url As String = tcp://localhost:50000/Calculator"
RemotingConfiguration.RegisterActivatedClientType(GetType(Calculator), url)
Instead of registering the object on the client and then instantiating it with a New keyword, the client application can explicitly create an instance of a CAO with the Activa tor.CreateInstance method:
(In the client application.) Dim attrs() As Object = {New System.Runtime.Remoting.Activation.UrlAttribute( _ tcp://localhost:50000/Calculator )} Dim calc As Calculator = DirectCast(Activator.CreateInstance( _ GetType(Calculator), Nothing, attrs), Calculator)
You see the difference between CAOs and singleton SAOs by launching two or more instances of the client application. Each application receives a reference to a different remote object; intermixed method calls from different clients produce a result similar to what you see in Figure 32-4.
Figure 32-4 Client application receives references to distinct instances of a CAO, and each instance maintains its own member variables. Here we have three calls from client A, two calls from client B, and then two more calls from client A.
32:
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