WCF Extensibility – Message Formatters

Message formatters are the component which do the translation between CLR operations and the WCF Message object – their role is to convert all the operation parameters and return values (possibly via serialization) into a Message on output, and deconstruct the message into parameter and return values on input. Anytime a new format is to be used (be it a new serialization format for currently supported CLR types, or supporting a new CLR type altogether), a message formatter is the interface you’d implement to enable this scenario. For example, even though the type System.IO.Stream is not serializable (it’s even abstract), WCF allows it to be used as the input or return values for methods (for example, in the WCF REST Raw programming model) and a formatter deals with converting it into messages. Like the message inspectors, there are two versions, one for the server side (IDispatchMessageFormatter), and one for the client side (IClientMessageFormatter).

Among the extensibility points listed in this series, the message formatters are the first kind to be required in the WCF pipeline – a service does not need to have any service / endpoint / contract / operation behavior, nor any message / parameter inspector. If the user doesn’t add any of those, the client / service will just work (some behaviors are automatically added to the operations when one adds an endpoint, but that’s an implementation detail for WCF – they aren’t strictly necessary). Formatters, on the other hand, are required (to bridge the gap between the message world and the operations using CLR types). If we don’t add any behaviors to the service / endpoint / contract / operation that sets a formatter, WCF will add one formatter to the operation (usually via the DataContractSerializerOperationBehavior, which is added by default in all operation contracts).

Public implementations in WCF

None. As with most of the runtime extensibility points, there are no public implementations of either the dispatch or the client message formatter. There are a lot of internal implementations, though, such as a formatter which converts operation parameters into a message (both using the DataContractSerializer and the XmlSerializer), formatters which map parameters to the HTTP URI (used in REST operations with UriTemplate), among others.

Interface declaration

public interface IDispatchMessageFormatter

{

    void DeserializeRequest(Message message, object[] parameters);

    Message SerializeReply(MessageVersion messageVersion, object[] parameters, object result);

}

 

public interface IClientMessageFormatter

{

    Message SerializeRequest(MessageVersion messageVersion, object[] parameters);

    object DeserializeReply(Message message, object[] parameters);

}

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