Using Faults with Untyped Messages

When using a typed contract, incoming messages on the server are shredded on your behalf to be turned into method calls and parameters. Ordinarily, the particular method call selected for an application messages will have the same parameterized contract as the message. This allows the transformation between messages and parameters to be made with a high degree of fidelity. However, operations also permit fault responses in addition to the normal application response.

The parameterized fault contract is going to look nothing like the standard application contract. Therefore, there's really no transformation that will take you from the fault message to the same parameterized contract in a way that makes any sense. The response that comes back from the server is unrepresentable using the standard data structure that the application is expecting to receive for an application response. This is why with a typed contract fault messages have to be expressed as an exceptional condition. Exceptions tend to transform the message with a much lower fidelity to the original content.

With an untyped contract, incoming messages on the server are not shredded on your behalf but rather preserved in their entirety. You can think about this as performing the transformation between messages and parameters with perfect fidelity since the parameter is equal to the message. Similarly, any message response, whether it's a fault response or a normal application response, is also going to be representable with perfect fidelity. Both types of responses have the same format with an untyped contract so the application can handle them equally well. This is why with an untyped contract fault messages are preserved as messages. One of the major reasons for using untyped contracts is to have great fidelity with the wire. It wouldn't make sense to force the application to lose that fidelity for a certain class of messages.

If you choose to in your application though, you can still run the same exception machinery. For details, read some of the past articles on creating and consuming faults.

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Comments

• Anonymous
  August 15, 2008
  How do I write a contract for a wrapped message in the default namespace? I've written a quick sample