Deserializing derived type using DataContractSerializer when derived type is not known beforehand

Question

In Assembly A:

[DataContract]    
        public class Base
        {
            [DataMember]
            public string SomeText { get; set; }
        }

In Assembly B:

internal class Helper
        {
           internal static Base Deserialize(string serializedInstanceOfTypeBase)
    {
                   DataContractSerializer serializer = new DataContractSerializer(typeof (Base));
                    XmlReader reader = XmlReader.Create(new StringReader(serializedInstanceOfTypeBase));
                    return (Base)serializer.ReadObject(reader);
    }
    }

In Assembly C:

[DataContract]    
        public class Derived : Base
        {
            [DataMember]
            public string SomeMoreText { get; set; }
        }

If I serialize an instance of type Derived and pass it to Helper.Deserialize() method, it fails with the SerializationException:

Error in line 1 position 2. Expecting element 'Base' from namespace 'http://schemas.datacontract.org/2004/07'.. Encountered 'Element' with name 'Derived', namespace 'http://schemas.datacontract.org/2004/07'.

How can I get rid of this issue?

I am aware of the KnownType attribute, but at the time of coding in assembly A and B, I am absolutely not aware of its derived types. So I cannot use that solution.

Design of my product is more complex which I cannot post here entirely. Helper.Desrialize() method just gets a string argument. There is no way (at present at least) for assembly A or B, to know about derived types of Base class, even at runtime.

Assembly B references assembly A. But A & B cannot reference assembly C.

I am using C# 4.0. It's ok if the solution you provide is NOT using DataContractSerializer.

Conspicuous

I don't know about a out-of-the-box solution. I know in my experience when I had a wide variety of types, I would actually double-serialize my content with type information. That is, I would have a SerializedObject which would contain full type information (assembly name, full class name), along with its serialized XML string. My data transfer layer would strongly type itself against SerializedObject, grab a runtime reference to the type (say via Type.GetType), then deserialize the XML data accordingly via an XmlSerializer. – Ellington

You mean 'Base' class is in 'Assembly A' and 'Derived' class is in 'Assembly C'? If this is so then I think this is the problem. The Data Contracts should be defined in same assembly. The DataContracts should not be splitted across assemblies. – Mccreary

Answer 1

It looks like you need both:

• Pass a collection of KnownTypes into your deserializer (as mentioned above)

• Tell the deserializer not to check the type (use verifyObjectNameparameter):

  return (Base)serializer.ReadObject(reader, false);
  

see details here: http://msdn.microsoft.com/en-us/library/ms573850%28v=vs.110%29.aspx

Iconoclast

Answer 2

Do you know the types at runtime ? If so, a simple approach might be just:

List<Type> knownTypes = ...; // now that you know what to expect
DataContractSerializer serializer = new DataContractSerializer(
    typeof(Base), knownTypes);

Tumefy

Thanks for your answer. It's very helpful but I have added more details to the question. I should have added those details before. Note that Helper class is in different assembly and it's not public. Also, Deserialize method just gets any string value. – Conspicuous