Creating a WCF Project in C#.NET

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Creating a WCF Project
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We ll start with the server for our chat application. If you want to build your own copy of the project as you read, open Visual Studio s New Project dialog (Ctrl-Shift-N) and in the template list on the left, select Visual C# WCF. Choose the WCF Service Library project template. Call the project ChatServerLibrary. Ensure that the Create directory for solution checkbox is checked, and call the solution WcfChat. This project will produce a DLL as its output, because the WCF Service Library project template doesn t commit to hosting the WCF service in any particular container application. WCF can run inside IIS, a Windows Service, a console application, or indeed pretty much any .NET application. If you want to use a particular kind of host, you can just create the relevant type of project for example, instead of creating a WCF Service Library, you could create an ASP.NET web application project if you wanted to host your WCF service in there. (You can add a WCF service as a new item to an existing web project, so you don t need a WCF-specific project type.) But there are a couple of benefits to this library-based template: as you ll see shortly, it provides an easy way to do simple manual testing of the service. Also, it means you can host the service in multiple different host applications, which can be useful for automated testing you can test the service without having to deploy it into its intended environment. Visual Studio will have added a single service to the project, called Service1. This contains some example code that does things we don t need in our chat application, so we ll ignore that. (Feel free to delete them if you re building your own version as you read this.) We ll add a new WCF Service item to the project with the Add New Item dialog, called ChatService. Visual Studio adds two files to the project: ChatService.cs and IChatService.cs. This reflects the fact that WCF makes a distinction between the code that implements a service, and the contract for that service.
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WCF Contracts
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When two systems communicate over a network, they need to agree on what information is to be sent back and forth. WCF formalizes this with what it calls contracts. So the IChatService interface added by the wizard represents a service contract. The service contract defines the operations the service offers. As Example 13-1 shows, the interface is marked with a ServiceContract attribute to make it clear that it s a contract definition.
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[ServiceContract] public interface IChatService { [OperationContract] void DoWork(); }
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Each method in the interface that defines an operation offered by the service must be marked with an OperationContract. You might have thought that it would be enough that the interface is marked as ServiceContract why do we also need to annotate each method WCF requires you to be explicit so that it s always obvious when you re defining some aspect of your system that will be visible across the network. A method call to a local object is a quite different kind of operation than using a remote service the performance and reliability characteristics are poles apart so it s important for such boundaries to be clearly visible in the code.
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Although we re defining a method for each operation, ultimately the contract defines what messages can go in and out of the service. To invoke an operation, a client will need to send a message to the server over the network. When you add a method marked with OperationCon tract to an interface marked with ServiceContract, you are really defining the logical structure of the message that will be sent to invoke that operation, and also of the message that will be sent back to the client when the operation is complete. WCF lets you represent these message formats as method signatures because it s a convenient abstraction for developers. WCF supports other ways of defining message formats you can write a contract in WSDL, the Web Service Definition Language, and then generate types from that. This approach is beyond the scope of this book.
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Our service is designed to let people chat, so it will need to provide clients with a way to send a short bit of text, which we ll refer to as a note. (A more obvious name would be message, but that would introduce ambiguity WCF sends messages to and from the server for every operation, so to call one of the pieces of information that crops up
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in certain messages a message would be confusing.) To keep things simple, we ll just have one big chat room where everyone can see every note; we re not going to support private conversations. To support sending notes, we ll get rid of the DoWork method provided by Visual Studio, and replace it with the code in Example 13-2.
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[OperationContract] void PostNote(string from, string note);
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If you attempt to build your project in Visual Studio, you ll get a compiler error:
error CS0535: 'ChatServerLibrary.ChatService' does not implement interface member 'ChatServerLibrary.IChatService.PostNote(string, string)'
Remember that Visual Studio added two files: IChatService.cs (the contract) and ChatService.cs (the service implementation). The compiler is pointing out to us that our service implementation no longer conforms to the contract for the service. So in ChatService.cs, we need to replace the DoWork method with this code:
public void PostNote(string from, string note) { Debug.WriteLine("{0}: {1}", from, note); }
For this to compile, you ll need to add a using System.Diagnostics; directive to the top of your file.
There s an obvious security question with this service: how do we know that the note comes from the person it claims to come from The answer is that we don t identification is a complex topic, with many possible solutions. The appropriate choice of solution would depend on the context in which the application will be used on a corporate network, integrated Windows security might be best, but that wouldn t work for a public-facing Internet application. The way to solve these problems is currently an area of debate, and could easily fill a chapter. Since this example just illustrates the basic mechanics of WCF, we are using the na ve trust model for identity: users can claim to be whoever they want to be, and our application will believe them.
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