Sahil has started a discussion on a change to the BinaryFormatter behavior in .NET 2.0.

Serialization is just the process of taking complex data and converting it into a single byte stream. Motivation is a separate issue :)

There are certainly different reasons to "serialize" an object.

One for what I do in CSLA .NET, which is to truly clone an object graph (not just an object, but a whole graph), either in memory or across the network. This is a valid reason, and has a set of constraints that a serializer must meet to be useful. This is what the BinaryFormatter is all about today.

Another is to easily convert some or all of an object’s data into actual data. To externalize the object’s state. This is what the XmlSerializer is all about today. The purpose isn’t to replicate a .NET type, it is to convert that type into a simple data representation.

Note that in both cases the formatter/serializer attempts to serialize the entire object graph, not just a single object. This is because the “state” of an object is really the state of the object graph. That implies that all references from your object to any other objects are followed, because they collectively constitute the object graph. If you want to “prune” the graph, you mark references such that the formatter/serializer doesn’t follow them.

In the case of the BinaryFormatter, it does this by working with each object’s fields, and it follows references to other objects. An event inside an object is just another field (though the actual backing field is typically hidden). This backing field is just a delegate reference, which is just a type of object reference – and thus that object reference is followed like any other.

In the case of the XmlSerializer, only the public fields and read/write properties are examined. But still, if one of them is a reference the serializer attempts to follow that reference. The event/delegate issue doesn’t exist here only because the backing field for events isn’t a public field. Make it a public field and you’ll have issues.

In .NET 1.x the BinaryFormatter attempts to follow all references unless the field is marked as [NonSerializable]. In C# the field: target provided (what I consider to be) a hack to apply this attribute to the backing field. It also has a better approach, which is to use a block structure to declare the event so you get to manually declare the backing field and can apply the attribute yourself.

The trick is that the default behavior is for the backing field to be serializable, and things like Windows Forms or other nonserializable objects might subscribe to the object’s events. When the BinaryFormatter follows the references to those objects it throws an exception (as it should).

I made a big stink about this when I wrote my VB Business Objects book though, because I discovered this issue and there was no obvious solution. This is because VB.NET has neither the Field: target hack, nor the block structure declaration for events. I was forced to implement a C# base class to safely declare my events.

I spent a lot of time talking to both the VB and CLR (later Indigo) teams about this issue.

The VB team provided a solution by supporting the block structure for declaring events, thus allowing us to have control over how the backing field is serialized. This is a very nice solution as I see it.

I am not familiar with the change Sahil is seeing in .NET 2.0. But that is probably because I’ve been using the block structure event declarations as shown in my C# and VB 2005 links above, so I’m manually ensuring that only serializable event handlers are being traced by the BinaryFormatter.

But I should point out that the C# code in the example above is for .NET 1.1 and works today just as it does in .NET 2.0.