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CHAPTER 7 IN-DEPTH .NET REMOTING
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Console.WriteLine("Done"); Console.ReadLine(); }
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Note The CallContext is scoped on the level of a thread. If your application has multiple threads, you will
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have independently operating call context objects.
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On the server side, you can access the CallContext s data by calling GetData() and casting the resulting object to the corresponding type as shown in Listing 7-9. Listing 7-9. Accessing the CallContext on the Server Side class CustomerManager: MarshalByRefObject, IRemoteCustomerManager { public Customer GetCustomer(int id) { LogSettings ls = CallContext.GetData("log_settings") as LogSettings; if (ls!= null && ls.EnableLog) { // simulate write to a logfile Console.WriteLine("LOG: Loading Customer " + id); } Customer cust = new Customer(); return cust; } } If you run the application without specifying a command-line switch, you will receive the server-side output in Figure 7-9. If you however specify /enablelog when starting the client, the server-side output will look like what appears in Figure 7-10.
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Figure 7-9. Server-side output when client has been started without switches
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CHAPTER 7 IN-DEPTH .NET REMOTING
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Figure 7-10. Server-side output when client has been started with /enablelog
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Note You can set the command-line switches that should be used during debugging in Visual Studio .NET
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by right-clicking your project and navigating to Configuration Properties Debugging. The field you are looking for is called Command Line Arguments .
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As the call context is valid for more than one method call, you will have to manually remove elements that you do not need anymore (otherwise they will travel back and forth with every method call). You can do this by using CallContext.FreeNamedDataSlot().
Best Practices
In your application code, you should normally avoid directly accessing the call context, as it ties your business code to the .NET Remoting framework. I usually recommend using certain strongly typed wrapper classes like the ones shown in Listing 7-10 instead. This will allow you to later (with future versions of the .NET Framework) move your implementation to a different protocol without any dependencies to the .NET Remoting framework. In this case, you will only need to change the wrapper class, which provides for a clean separation between your business logic and the transport layer. You can define such a wrapper similar to the one in Listing 7-10. Listing 7-10. A Wrapper for Log Settings public class LogSettingContext { public static bool EnableLog { get { LogSettings ls = CallContext.GetData("log_settings") as LogSettings; if (ls!= null) { return ls.EnableLog; } else { return false; }
CHAPTER 7 IN-DEPTH .NET REMOTING
} set { LogSettings ls = new LogSettings(); ls.EnableLog = value; CallContext.SetData("log_settings", ls); } } } You can then set the call context flag on the client side like this: if (args.Length > 0 && args[0].ToLower() == "/enablelog") { LogSettingContext.EnableLog=true; } and read it on the server side with code similar to the following: if (LogSettingContext.EnableLog) { // simulate write to a logfile Console.WriteLine("LOG: Loading Customer " + id); }
Security and the Call Context
Please note that you should never use the CallContext to transfer the thread s current principal (i.e., information about the currently logged-in user) between client and server. This would be insecure, and a malicious client application could easily send whatever information it would like.
Remoting Events
With the remoting of events, you are now reaching an area of .NET Remoting where the intuitive way of solving a problem might not be the correct one.
Serious Warning Before I show you how to work with remoting events, let me please state that these A events are mostly useful for cross-application communication on a single machine. An example of such a scenario is to have a Windows service that does some background processing and that communicates with a Systray application/icon or with a conventional Windows Forms application. Remoting events can be used in LAN-based environments with a limited number of listeners. They will, however, not scale up to support hundreds of receivers or to support WAN-based environments. This is not directly a fault of the .NET Remoting system, but of the way the underlying TCP connections are used. In the next chapter, I ve detailed the reasons for this and present some non-remoting-based solutions for asynchronous notifications in different environments.
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