In C#, when you define an event in your class, e.g.:
the event handlers will be serialized along with other properties, etc.
This is because under the hood, the compiler translates your event into the following, as can be seen through JetBrain’s dotPeek decompiler:
Since the generated EventHandler is not marked with the [NonSerialized] attribute there’s no way for the serializer to know that it should be ignored during the serialization process.
Solution 1 : implement event explicitly
One solution to this is to implement the event explicitly, so you end up with something along the line of:
Solution 2 : use [field:NonSerialized]
A much better solution would be to use the [NonSerialized] attribute on the event, and more specifically, you want to apply it to the private EventHandler instance that the compiler generates for you. Applying the [NonSerialized] attribute on the event generates a compiler error telling you that it’s only valid on field declarations:
So instead, if you prefix the attribute with the ‘field’ keyword everything will work as you’d expect:
Closing thoughts…
On a related note, you may have noticed that my event declaration assigns an empty handler to the event, this is to remove the need for null checking before invoking an event (which as you can imagine requires even more work to make thread-safe). There are other techniques (see @BlkRabbitCoder’s post on raising events safely and efficiently here for more details) to deal with this common problem.
In the case of a class that has to be serialized, this technique won’t work if the event handlers are not serialized as part of the object, because by the time it’s deserialized the event handler will be null.
So instead, courtesy of @BlkRabbitCoder, here’re two simple extension methods that encapsulates the code necessary to safely (prevents NullReferenceException) invoke event handlers in a thread-safe manner:
Further reading
Here are some more blog posts that go in to more detail about some intricacies with events in .Net:
Jon Skeet – Delegates and Events
Eric Lippert – Events and Races
Safely and Efficiently raising Events
