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This is the purpose of the metadata exchange entry we saw earlier when we looked at the configuration Visual Studio generated for our WCF service. Metadata exchange is just a fancy way of saying that a service provides a way for a client to discover the contract and related information about the service. The Add Service Reference dialog uses this information to configure a client application to communicate with the service, and to provide it with a copy of the contract. To see the results of this, we ll finish with this dialog. In the Namespace text box near the bottom, we ll type ChatService Visual Studio will put the contract and any other types relating to this service into this namespace. When we click OK a Service References item appears in the project in the Solution Explorer, and it will contain an entry called ChatService. (Now that we ve done this, we can stop the service host console window we ran earlier.) Visual Studio generates some code when adding a service reference. By default, it hides this, but we can take a look at it. At the top of the Solution Explorer, there s a toolbar, and if you hover your mouse pointer over the buttons you ll find that one has a tool tip of Show All Files. This button toggles each time you click it. When it s pressed in, the ChatService service reference can be expanded, as Figure 13-7 shows.

The most interesting file in here is Reference.cs, inside the Reference.svcmap item. Inside this file, near the top, there s a copy of IChatService the contract we wrote earlier:

[System.CodeDom.Compiler.GeneratedCodeAttribute("System.ServiceModel", "4.0.0.0")] [System.ServiceModel.ServiceContractAttribute( ConfigurationName="ChatService.IChatService"] public interface IChatService { [System.ServiceModel.OperationContractAttribute( Action="http://tempuri.org/IChatService/PostNote", ReplyAction="http://tempuri.org/IChatService/PostNoteResponse")] void PostNote(string from, string note);

}

Figure 11-3. The three forms of the catch clause The general catch clause can accept any exception, but cannot determine the type of exception that caused it. This allows general processing and cleanup for whatever exception might occur. The specific catch clause form takes the name of an exception class as a parameter. It matches exceptions of the specified class or exception classes derived from it. The specific catch clause with ID form gives you the most information about the exception. It matches exceptions of the specified class, or exception classes derived from it. It also gives you an exception instance ID, called the exception variable, which is a reference to the exception object created by the CLR. You can access the exception variable s properties within the block of the catch clause to get specific information about the exception raised. For example, the following code handles exceptions of type IndexOutOfRangeException. When one occurs, a reference to the actual exception object is passed into the code with parameter name e. The three WriteLine statements each read a string field from the exception object. Exception type Exception variable catch ( IndexOutOfRangeException e ) { Accessing the exception variable Console.WriteLine( "Message: {0}", e.Message ); Console.WriteLine( "Source: {0}", e.Source ); Console.WriteLine( "Stack: {0}", e.StackTrace );

It looks a little more complex than the original, because Visual Studio has annotated it with various attributes, but it s simply being explicit about the values that WCF fills in by default. Aside from these extra details, you can see that it is essentially a copy of the original contract.

You might wonder why we jumped through all these hoops rather than just copying IChatService from the service project to the client. In fact, that would have worked, and we could even have written a separate DLL project to define the contract interface and shared that DLL across the two projects. As you ll see shortly, Visual Studio generated a few other useful things for us as part of this Add Service Reference process, but as it happens, sharing the contract definition directly is sometimes a perfectly reasonable thing to do you re not obliged to use metadata exchange. Of course, you won t always own the code at both ends. If you need to connect to a service on the Internet provided by someone else, metadata exchange becomes more important it provides a way to get hold of a contract you didn t write. And since the metadata exchange mechanisms are standards-based, this can work even when the service is not written in .NET.

Metadata exchange is not universally supported. In practice, contract discovery can happen in all sorts of ways, including (and we re not making this up) being faxed a printout showing samples of the messages the service expects to send and receive. If you re getting the contract through that kind of informal channel, you ll need to write an interface (by hand) in your client program to represent the service contract.

The process of metadata import also highlights an important point about service evolution. You might modify the ChatService after the ChatClient has added its reference. If these modifications involve changing the contract, it s clear that there s a problem: the client s copy of the contract is out of date. You might think that sharing the interface directly through a common DLL would be a good way to avoid this problem, but it might only make the problem harder to see: what if you ve already deployed a version of the client If you then modify the contract the modified code might run fine on your machine, but if you deploy an update to the service with this changed contract any copies of the old client out there will now be in trouble because they re still working with an old copy of the contract. Explicitly going through the metadata exchange

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