I recently ran into an odd and annoying error when working with the DataContractSerializer class for a WP7 project. I thought I’d share it to save others who might encounter it the same annoyance I had. So I had an instance of  ObservableCollection<T> that I was trying to serialize (with T being a class I wrote for the project) and whenever it would hit the code to save it, it would give me:

The data contract type 'ProjectName.MyMagicItemsClass' is not serializable because it is not public. Making the type public will fix this error. Alternatively, you can make it internal, and use the InternalsVisibleToAttribute attribute on your assembly in order to enable serialization of internal members - see documentation for more details. Be aware that doing so has certain security implications.

This, of course, was malarkey. I was trying to write an instance of MyAwesomeClass that looked like this:

[DataContract]

public class MyAwesomeClass

{

    [DataMember]

    public ObservableCollection<MyMagicItemsClass> GreatItems { get; set; }

    [DataMember]

    public ObservableCollection<MyMagicItemsClass> SuperbItems { get; set; }

    public MyAwesomeClass

    {

        GreatItems = new ObservableCollection<MyMagicItemsClass>();

        SuperbItems = new ObservableCollection<MyMagicItemsClass>();

    }

}

That’s all well and fine. And MyMagicItemsClass was also public with a parameterless public constructor. It too had DataContractAttribute applied to it and it had DataMemberAttribute applied to all the properties and fields I wanted to serialize. Everything should be cool, but it’s not because I keep getting that “not public” exception. I could tell you about all the things I tried (generating a List<T> on the fly to make sure it wasn’t ObservableCollection<T>, trying to serialize the the Collections directly, moving it all to a separate library project, etc.), but I want to keep this short.

In the end, I remembered my the “Debug->Exceptions…” VS menu option that brings up the list of exception-related circumstances under which the Visual Studio debugger will break. I checked the “Thrown” checkbox for “Common Language Runtime Exceptions”, started the project under the debugger, and voilà: the true problem revealed itself. Some of my properties had fairly elaborate setters whose logic I wanted to ignore. So for some of them, I applied an IgnoreDataMember attribute to them and applied the DataMember attribute to the underlying fields instead. All of which, in line with good programming practices, were private.

Well, it just so happens that WP7 apps run in a “partial trust” environment and outside of “full trust”-land, DataContractSerializer refuses to serialize or deserialize non-public members. Of course that exception was swallowed up internally by .NET so all I ever saw was that bizarre message about things that I knew for certain were public being “not public”. I changed all the private fields I was serializing to public and everything worked just fine. In hindsight it all makes perfect sense. The serializer uses reflection to build up its graph of the object in order to write it out. In partial trust, you don’t want people using reflection to get at non-public members of an object since there are potential security problems with allowing that (you could break out of the sandbox pretty quickly by reflecting and calling the appropriate methods and cause some havoc by reflecting and setting the appropriate fields in certain circumstances. The fact that you cannot reflect your own assembly seems a bit heavy-handed, but then again I’m not a compiler writer or a framework designer and I have no idea what sorts of difficulties would go into allowing that from a compilation standpoint or what sorts of security problems allowing that could present (if any).

So, lesson learned. If you get an incomprehensible exception message, turn on break on all thrown exceptions and try running it again (it might take a couple of tries, depending) and see what pops out. Chances are you’ll find the buried exception that actually explains what was going on. And if you’re getting a weird exception when trying to use DataContractSerializer complaining about public types not being public, chances are you’re trying to serialize a private or protected field/property.