C# 4 and .NET 4

emote objects, Clients, and servers ❘ OC197
activation
A new context is created if an instance of a class that’s created needs a context different from the calling
context. The attribute classes that are associated with the target class are asked if all the properties of
the current context are acceptable. If any of these properties are unacceptable, the runtime asks for all
property classes associated with the attribute class and creates a new context. The runtime then asks
the property classes for the sinks they want to install. A property class can implement one of the
IContributeXXXSink interfaces to contribute sink objects. Several of these interfaces are available to go with
the variety of sinks.
attributes and Properties
The properties of a context are defined with context attributes. A context attribute class is primarily an
attribute — you can find out more about attributes in Chapter 14, “Reflection.” Context attribute classes
must implement the interface IContextAttribute. A custom context attribute class can derive from the
class ContextAttribute, because this class already has a default implementation of this interface.
The .NET Framework includes two context attribute classes: System.Runtime.Remoting.Contexts.
SynchronizationAttribute and System.Runtime.Remoting.Activation.UrlAttribute.
The Synchronization attribute defines synchronization requirements; it specifies the synchronization
property that is needed by the object. With this attribute you can specify that multiple threads cannot access
the object concurrently, but the thread accessing the object can change.
With the constructor of this attribute, you can set one of four values:
➤
➤
➤
NOT_SUPPORTED defines that the class should not be instantiated in a context where the
synchronization is set.
REQUIRED specifies that a synchronization context is required.
REQUIRES_NEW always creates a new context.
➤
SUPPORTED means that it doesn’t matter what context you get; the object can live in it.
Communication between Contexts
How does the communication between contexts happen The client uses a proxy instead of the real object.
The proxy creates a message that is transferred to a channel, and sinks can intercept. Does this sound
familiar It ought to. The same mechanism is used for communication across different application domains
or different systems. A TCP or HTTP channel is not required for the communication across contexts, but
a channel is used here too. CrossContextChannel can use the same virtual memory in both the client and
server sides of the channel, and formatters are not required for crossing contexts.
remoTe objeCTs, ClienTs,
and serVers
Before stepping into the details of the .NET Remoting
architecture, this section looks briefly at a remote
object and a very small, simple server and client
application that uses this remote object. After that the
required steps and options are discussed in more detail.
Figure 54-3 shows the major .NET Remoting classes
in the client and server application. The remote object
that will be implemented is called Hello. HelloServer
is the main class of the application on the server, and
HelloClient is used for the client.
HelloClient
+Main()
figure 54-3
MarshalByRefObject
Hello
+Greeting()
HelloServer
+Main()
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