When data arrives at a receiving endpoint, the WCF runtime attempts to deserialize the data into an instance of a common language runtime (CLR) type. The type that is instantiated for deserialization is chosen by first inspecting the incoming message to determine the data contract to which the contents of the message conform. The deserialization engine then attempts to find a CLR type that implements a data contract compatible with the message contents. The set of candidate types that the deserialization engine allows for during this process is referred to as the deserializer's set of "known types."

One way to let the deserialization engine know about a type is by using the KnownTypeAttribute. The attribute cannot be applied to individual data members, only to whole data contract types.

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Primitive types, as well as certain types treated as primitives (for example, DateTime and XmlElement) are always "known" and never have to be added through this mechanism. However, arrays of primitive types have to be added explicitly. Most collections are considered equivalent to arrays.

[http://msdn.microsoft.com/en-us/library/ms730167.aspx]