10: Exception Handling in Visual Studio .NET

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10: Exception Handling
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describe the structures used to convey fault information from a service to a caller attributes can be used to decorate a service One or more operation This indicates to WCF that a service s WSDL de nition should include the details of fault-related information potentially thrown by the operation This means that proxy-generating tools will be able to create strongly typed representations of the classes you use to convey fault information Having well-de ned proxies for the fault information will help developers create robust and reliable client applications that utilize your services Note that because the details are contained in standard WSDL format, any type of tool can generate platform-speci c proxies for your faults, not just NET applications Let s create a data structure to convey details of an error condition back class as shown in Listing 109 to a calling application Create a
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Listing 109 Creating a for Use from a
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Creating and Consuming Strongly Typed Faults
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Defining a FaultContract After you have one or more data contracts that you want to use to convey attribute to your exception details to your callers, add the operation, specifying the name of the associated data contract(s) operation in Listing 1010 has been For example, the based on the extended to potentially raise a data contract we de ned earlier
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Listing 1010 Extending an Operation Definition with a
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As mentioned earlier, one-way operations do not return messages to callers, so there is no direct mechanism for returning faults Because will be thrown faults are not returned, an at service load time if you decorate any one-way operations with a
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10: Exception Handling
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Throwing a with a Defined Having indicated to WCF that the operation may throw an data contract, we exception that serializes data within the need only add the logic to populate that data contract and raise the exception Listing 1011 demonstrates this by extending the code from Listing 107
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Listing 1011 Throwing a with a
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Implementing Client Fault Handlers
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Fault Contract Strategies A variety of strategies exist for describing your services with fault contracts You could de ne a shared core library of contracts that are used by all your company s services Fault contracts may be speci c to a single service or application, including details speci c to that system You might decide to create a contract associated speci cally with individual problems potentially encountered by your services We suggest that each time you create WCF service projects, you consider -based approach to be your default excepa comprehensive for tion policy Careful decoration of your operations with expected exceptions will enable creation of client applications with strongly typed representations of the exceptions that may be thrown by your services Whatever strategy you adopt, ensure that you consistently follow that strategy so that clients developed against your services will have improved maintainability and usability
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Implementing Client Fault Handlers
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If you have declared your service s operations using the attribute to de ne the data contracts included with any thrown faults, your service s WSDL description will include details of those data contracts This will enable you or others to create client-side handlers that have full access to the details and types of those contracts Extending the earlier example of a basic client-side service invocation with exception handling, Listing 1012 demonstrates how the strongly based on the data contract could be typed structured
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Listing 1012 Processing Untyped
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10: Exception Handling
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Listing 1012 continued
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Exposing the details of via the service s WSDL allows a proxy generation tool to create strongly typed properties for each of the s members A client exception handler can access property of the details of the embedded data contract by using the instance In Listing 1012, the user is given the trackthe property of the typed exceping ID by accessing the tion instance Remember that, as with all NET exception code, the ordering of the regions is signi cant The is the is more most speci c type and so should be rst Because (the type from which it derives), it should be next, speci c than , which should be last followed by Full implementation details have been omitted from the example, but could include a variety of options from logging exception details, to informing the user and aborting, to retrying the service call
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