Programming WCF Services, 3rd Edition

Delegates and Data Contracts

All delegate definitions are compiled into serializable classes, so in theory your data contract types can contain delegates as member variables:

[DataContract]
class MyDataContract
{
   [DataMember]
   public EventHandler MyEvent;
}

Or even as events (note the use of the field qualifier):

[DataContract]
class MyDataContract
{
   [field:DataMember]
   public event EventHandler MyEvent;
}

In practice, however, when the data contract refers to a custom delegate, the imported data contract will contain an invalid delegate definition. While you could manually fix that definition, the bigger problem is that when you serialize an object that has a delegate member variable, the internal invocation list of the delegates is serialized, too. In most cases, this is not the desired effect with services and clients, because the exact structure of the list is local to the client or the service and should not be shared across the service boundary. In addition, there are no guarantees that the target objects in the internal list are serializable or are valid data contracts. Consequently, sometimes the serialization will work, and sometimes it will fail.

The simplest way to avoid this pitfall is not to apply the DataMember attribute on delegates. If the data contract is a serializable type, you need to explicitly exclude the delegate from the data contract:

[Serializable]
public class MyDataContract
{
   [NonSerialized]
   public EventHandler MyEvent;
}