BinaryFormatter: Serializing an object's base class

[Mark Salsbery]

Ennis Ray Lynch, Jr. wrote:

Objects have to have public, empty constructors to be serialization using the built in methods

Not true. No constructor is used in deserialization AFAIK.

Ennis Ray Lynch, Jr. wrote:

Base classes, in general, can't be serialized if they can't be instantiated.

Huh? :)

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Mark Salsbery]

dogAsAnimal is still a Dog object, regardless of the "cast". You can see that in the debugger. "as" doesn't create a new object of a different type. Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

[mav northwind]

Well, the debugger shows "Animal {Dog}" as type for dogAsAnimal. a yields "Animal" and d yields "Dog", so I expected dogAsAnimal to be an Animal-type reference to an object that is Dog. It's the same with a direct cast. I cannot access the Dog-only properties of dogAsAnimal, but the BinaryFormatter seems to see through the cast and serialize the deepest inheritance.

Regards, mav -- Black holes are the places where God divided by 0...

[Ennis Ray Lynch Jr]

For standard XML serializaiton the default constructor needs to be public. For binary serializaiton you need to have the serialization constructor defined.

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[mav northwind]

:) Well, things tend do become simplified when stripping the context. I have a system of several client applications and a server that are communicating via remoting. They all have a notion of Animal. Animal has a method public static void ToFile(Animal a, string file) (and the corresponding FromFile()). For a new client, a Dog class had to be created, but since this class is only used with this client it doesn't make sense to put it into the library defining Animal. Then the client wanted to serialize the Animal-part of one of the Dog objects using the static Animal.ToFile() method, but the binary formatter threw an exception because Dog wasn't marked as serializable. The most intuitive method (for me) to solve this problem would be to find a way to tell the BinaryFormatter that what he gets really really is just an Animal, but I don't know how. Other solutions would be to use aggregation instead of inheritance, but that would require some rewriting of existing code. Or I could add a copy constuctor to Animal and actally create a new object, but that's not really the idea - I don't want to serialize a copy of a Dog, I only want to serialize the Animal-parts of the Dog...

Regards, mav -- Black holes are the places where God divided by 0...

[Mark Salsbery]

Ennis Ray Lynch, Jr. wrote:

For standard XML serializaiton the default constructor needs to be public

What if there's no default constructor?

Ennis Ray Lynch, Jr. wrote:

For binary serializaiton you need to have the serialization constructor defined.

Not true. That's only for ISerializable-derived classes. ISerializable isn't required for binary serialization.

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Mark Salsbery]

What if you make the Dog class [Serializable], mark all its members [NonSerialized], and use a SerializationBinder[^] for deserialization? I'm not sure if it will work or not :)

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Ennis Ray Lynch Jr]

I will believe you on the second one, however, a default constructor is defined by default unless you create either a non-default constructor or a default constructor with visibility other than public. See: http://support.microsoft.com/kb/330592[^]

public class Foo{

}

and

public class Foo{
public Foo(){}
}

Both contain default constructors whereas

public class Foo{
public Foo(int a){}
}

and

public class Foo{
private Foo(){}
}

Do not.

Need software developed? Offering C# development all over the United States, ERL GLOBAL, Inc is the only call you will have to make.
Happiness in intelligent people is the rarest thing I know. -- Ernest Hemingway
Most of this sig is for Google, not ego.

[Mark Salsbery]

Ennis Ray Lynch, Jr. wrote:

a default constructor is defined by default unless you create either a non-default constructor or a default constructor with visibility other than public.

Got it, thanks :) Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Mark Salsbery]

Also, the constructor is only called when using XmlSerializer. No constructor is called with binary/SOAP deserialization. Cheers, Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Ennis Ray Lynch Jr]

I didn't know that, I just assumed a constructor would be called.

Need software developed? Offering C# development all over the United States, ERL GLOBAL, Inc is the only call you will have to make.
Happiness in intelligent people is the rarest thing I know. -- Ernest Hemingway
Most of this sig is for Google, not ego.

[mav northwind]

Thanks for the suggestion, I'll take a look. Jeez, I didn't expect it to get so complicated... ;P

Regards, mav -- Black holes are the places where God divided by 0...

[Mark Salsbery]

I was trying to convince myself more than you I think :) I knew I read it somewhere once, and testing confirmed it. Its obscurely "documented" here[^]. I appreciate the discussion, thanks! Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Mark Salsbery]

I was bored, so I tried it. Works great :)

\[Serializable\]
public class Animal
{
	public string str
	{
		get;
		set;
	}
}

\[Serializable\]
public class Dog : Animal
{
	\[NonSerialized\]  `//<-- Optional!`
	public Int32 A;

	\[NonSerialized\]  `//<-- Optional!`
	public Int32 B;

	public Dog()
	{
		A = 5;
		B = 10;
	}
}

sealed class DogToAnimalDeserializationBinder : SerializationBinder
{
	public override Type BindToType(string assemblyName, string typeName)
	{
		Type typeToDeserialize = null;

		String assemVer1 = Assembly.GetExecutingAssembly().FullName;
		String typeVer1 = "Dog";

		if (assemblyName == assemVer1 && typeName == typeVer1)
		{
			typeName = "Animal";
		}

		// The following line of code returns the type.
		typeToDeserialize = Type.GetType(String.Format("{0}, {1}",
			typeName, assemblyName));

		return typeToDeserialize;
	}
}

...

Dog dog = new Dog();
dog.A = 3;
dog.B = 5;
dog.str = "Animal String";

MemoryStream MemStream = new MemoryStream();

// Serialize a Dog object
BinaryFormatter Serializer = new BinaryFormatter();
Serializer.Serialize(MemStream, dog);

MemStream.Seek(0, SeekOrigin.Begin);

// Deserialize as an Animal object
Serializer.Binder = new DogToAnimalDeserializationBinder();
Animal animal = (Animal)Serializer.Deserialize(MemStream);

Mark Salsbery Microsoft MVP - Visual C++ :java:

[Mark Salsbery]

Wow - my code was overcomplicated. The Binder there is completely unnecessary :rolleyes:

mav.northwind wrote:

The other side of the line (where the object is supposed to be deserialized) doesn't know about Dogs, it only knows about Animals (i.e. there's only the assembly defining Animal available, not the one defining Dog, so trying to deserialize this object will throw a TypeLoadException).

I just noticed that post. The only way that's going to work is to serialize an Animal object in the first place, as you already know. The deserializing end could use a custom binder to deserialize a Dog, but that would still require that end to have the assembly implementing the Dog class. Basically I helped get you back where you started, sorry! :)

Mark Salsbery Microsoft MVP - Visual C++ :java:

[mav northwind]

Hmmm, I've played around with the SerializationBinder a bit, but didn't get a useable result either. If I always return typeof(Animal) as target type in BindToType, then the deserializing end doesn't require knowledge about Dog, but unfortunately it doesn't work because then even Animal's properties aren't deserialized if I have a Dog object in my serialization stream. :(

Regards, mav -- Black holes are the places where God divided by 0...

[mav northwind]

Thanks for the link, it was quite instructive, although I couldn't achieve useable results so far with a SerializationBinder... Nevertheless: thanks for taking your time.

Regards, mav -- Black holes are the places where God divided by 0...

[Mark Salsbery]

Right. The only way I can see to do it, if the deserialization end has no access to the Dog class (or an equivalent class), is to add a method to Dog that returns an Animal object and serialize that object. It's really unconventional anyway....it generally doesn't make sense to only serialize the base class portion of an object. But if you have a special need I suppose you need a special solution. :) Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

[mav northwind]

I think I got it! After your and Giorgi's hints concerning the SerializationBinder, I looked at BinaryFormatter's other properties and found SurrogateSelector and subsequently ISerializationSurrogate. Using these two interfaces I was able to control the serialization process in a way that only Animal's properties are being serialized and during deserialization a real Animal object is being created. This should do the trick... Thanks for taking your time!

Regards, mav -- Black holes are the places where God divided by 0...

[Mark Salsbery]

Very cool! Thanks for the update! Cheers, Mark

Mark Salsbery Microsoft MVP - Visual C++ :java: