How to serialize a derived class as its base class

Question

I have a derived class that adds only methods to a base class. How can serialize the derived class so that it matches the serialization of the base class? i.e. The serialized xml of the derived class should look like:

<BaseClass>
  ...
</BaseClass>

e.g. The following will throw an InvalidOperationException "The type DerivedClass was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically."

Class BaseClass {}

Class DerivedClass : BaseClass {}

DerivedClass derived = new DerivedClass();

StreamWriter stream = new StreamWriter("output file path");
XmlSerializer serializer = new XmlSerializer(GetType(BaseClass));
serializer(stream, derived);

Answer 1

I haven't tried it but can you not just cast it ?

serializer(stream, (BaseClass)derived);

Edit

Also, if you want to have a single XmlSerialiser that can cope with multiple derived classes & the base class then you need to specify all of the types in the XmlSerialiser constructor.

  XmlSerializer serializer = new XmlSerializer(typeof(BaseClass), new Type[] {typeof(DerivedClass)});

Then it will happily serialize multiple types. However you will also have to use the solution mentioned above to get the output xml to match between classes.

Answer 2

You'll have to pass GetType(DerivedClass) to the serializer constructor, it must match the type of the object you serialize. You can use the <XmlRoot> attribute to rename to the root element. This example code worked as intended:

using System;
using System.Xml.Serialization;
using System.IO;

class Program {
  static void Main(string[] args) {
    var obj = new DerivedClass();
    obj.Prop = 42;
    var xs = new XmlSerializer(typeof(DerivedClass));
    var sw = new StringWriter();
    xs.Serialize(sw, obj);
    Console.WriteLine(sw.ToString());

    var sr = new StringReader(sw.ToString());
    var obj2 = (BaseClass)xs.Deserialize(sr);
    Console.ReadLine();
  }
}

public class BaseClass {
  public int Prop { get; set; }
}
[XmlRoot("BaseClass")]
public class DerivedClass : BaseClass { }

Answer 3

After a little bit of googling ;) I can say that you'll need some more code and implement IXmlSerializable for your base class, declaring all interface methods as virtual and overriding them on your derived class. Here's a sample topic and similar problem resolved by interface mentioned.