.NET Interface Serialization Issue

Interface can not be serialized or de-serialized. That's because interface is just a contract not detailed implementation. However, in some cases, we do want to expose interface so objects are not restricted by a specific type.

Look at a simple example:

    public interface IPriceable
    {
        double GetPrice();
    }
    [Serializable]
    public class ProductA : IPriceable
    {
        public double GetPrice()
        {
            return 123.345;
        }
    }

    [Serializable]
    public class ProductB : IPriceable 
    {
        public double GetPrice()
        {
            return 234.567;
        }
    }

    [Serializable]
    public class PromotionPackage
    {
        public List<IPriceable> Items = new List<IPriceable>();
        double GetPromotePrice()
        {
            //return PromotionCaculator.GetPrice(_items);
            return 567.89; ;
        }
    }

The PromotionPackage contains a list of IPriceable objects (ProductA, ProductB etc.). Everything is okay except that an exception will be thrown when we serialize the PromotionPackage object:

{"Cannot serialize member PromotionPackage.Items of type IPriceable because it is an interface."}

A workaround would be wrapping the interface to an abstract class:

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using System.Xml;
using System.Xml.Serialization;

public class Program
{
    public interface IPriceable
    {
        double GetPrice();
    }

    [XmlInclude(typeof(ProductA))]
    [XmlInclude(typeof(ProductB))]
    public abstract class ProductBase : IPriceable
    {
        public abstract double GetPrice();
    }

    [Serializable]
    public class ProductA : ProductBase
    {
        public override double GetPrice()
        {
            return 123.345;
        }
    }

    [Serializable]
    public class ProductB : ProductBase
    {
        public override double GetPrice()
        {
            return 234.567;
        }
    }

    [Serializable]
    public class PromotionPackage
    {
        public List<ProductBase> Items = new List<ProductBase>();
        double GetPromotePrice()
        {
            //return PromotionCaculator.GetPrice(_items);
            return 567.89; ;
        }
    }

    static void Main(string[] args)
    {
        PromotionPackage package = new PromotionPackage();
        package.Items.Add(new ProductA());
        package.Items.Add(new ProductB());

        string xmlString = string.Empty;
        XmlSerializer serializer = new XmlSerializer(typeof(PromotionPackage));

        // Serialize
        using (StringWriter sw = new StringWriter())
        {
            serializer.Serialize(sw, package);
            xmlString = sw.ToString();
        }

        // De-serialize
        PromotionPackage newPackage = null;
        using (StringReader sr = new StringReader(xmlString))
        {
            XmlTextReader reader = new XmlTextReader(sr);
            newPackage = (PromotionPackage)serializer.Deserialize(reader);
        }

        Console.WriteLine("Done.");
        Console.Read();
    }
}