How can I read data from a serialized object whose definition isn't in my code base any more?

Question

Say I have the following class:

[Serializable]
public class A
{
    public string B { get; set; }
}

and the following method was used to serialize it:

public void Serialize()
{
    BinaryFormatter b = new BinaryFormatter();
    b.Serialize(new FileStream(@"C:\Temp\out.dat", FileMode.Create), new A());
}

If at some point, someone came along and modified the class definition to contain an extra property (or remove one):

[Serializable]
public class A // Same class
{
    public string B { get; set; }
    public string C { get; set; } // New property
}

then the following will break:

public void Deserialize()
{
    BinaryFormatter b = new BinaryFormatter();
    A deserialized = (A)b.Deserialize(new FileStream(@"C:\Temp\out.dat", FileMode.Open));
}

because the serialized version of the class does not match the class definition of the new class.

I really dislike the idea of serialization as a persistence mechanism because it's so fragile. I would have dealt with this much earlier if I had been involved in the decision.

Is there any way to get my serialized data into a form I can read it without reverting all of my current class definitions to their serialized state?

Even if it's hacky "to the max", I was hoping I could do this, because I would hopefully only do it once (or until I could figure out how to fix the root of the problem).

edit

There are dozens of these classes that have been serialized and then modified in my system already. It is not feasible to use version control to see exactly when and how each individual class changed.

I'm currently trying to figure out a way I can retain "old A"'s settings before the user tries to deserialize the object to the "new A" format and I have to 1) try, 2) catch, 3) swallow the error, and finally 4) recreate A with default values (based on the new object definition).

Would this answer help? http://stackoverflow.com/questions/3648858/conditional-c-binary-serialization — Richard Morgan, Jun 15 '11 at 15:16
How would you handle cases which are more complicated than a missing property in an old serialized chunk? Like a property with a changed type? Sounds like you're either going to have to resort to key/value-based data (which can have any definition) or you should have a strategy to upgrade all old serialized data when your definitions change. Even if you could deserialize an old chunk of data, if several important properties are missing, what use is that deserialized object to you? — bzlm, Jun 15 '11 at 15:26
A `BinaryFormatter` is probably not an ideal choice if you want to be flexible in handling old data. This makes the deserialization process more dependent on the class definitions than, say, an XML formatter or a JSON formatter would. — bzlm, Jun 15 '11 at 15:28
@bzlm, I agree that BinaryFormatter is the square peg in this round-hole problem, but I'm just trying to do damage control and convert data that's already been BinaryFormatter-serialized into a more fault tolerant form. — Michael, Jun 15 '11 at 15:39
So the hypothetical "If at some point, someone came along and modified the class" has already occurred? Then you should update the question with more specifics. In the question's current form, it's hard to give solid advice. — bzlm, Jun 15 '11 at 21:04

score 2 · Answer 1 · answered Jun 15 '11 at 15:21

I believe that you can decorate newly added members with the OptionalFieldAttribute (System.Runtime.Serialization namespace), which will allow you to deserialize instances that were serialized before that member was added.

[Serializable]
public class A // Same class
{
    public string B { get; set; }
    [OptionalField] public string C { get; set; } // New property
}

How can I read data from a serialized object whose definition isn't in my code base any more?

1 Answers1