Question

I have a class that I have serialized to disk on local users machines. I need to refactor this class and I changed all objects (anything except enums, string, numbers) to interfaces. Underneath it is still the same concrete class. my concern is breaking existing users persistance

From:

public class Foo
{
     public double Count;
     public State MyState;
}

To

public class IFoo
{
     public double Count;
     public IState MyState;
}

but now I am getting errors from the serialization code that says “can’t serialize because its an interface”

the error states:

“There was an error reflecting type ‘Foo’.”
“Cannot serialize member ‘Foo.My’ of type ‘IState’, see inner exception for more details.”

what is the best way around this?

Answer 1

You cannot serialize interfaces because the amount of types that can implement the interface is infinite and the serializer does not know what concrete type it is.

class A : IFoo {}
class B : IFoo {}
class C : IFoo {}
//snip//

IFoo f = new A();
     f = new B();
     f = new C();

You must specify if you are serializing A,B or C.

Another way to think of it is when deserializing to IFoo, how would you know which to create … A, B or C .. etc?