Question

I am converting a project so I can use Moq (Mock) testing.

I have a class which has a List of interfaces. I understand interfaces cannot be serialized. So I am trying to work around it.

I have created a console to test, however, after trying numerous methods from other SO posts ( Serializing a List hold an interface to XML, XmlSerializer serialize generic List of interface); I feel I need to ask for help.

The sample code below wraps the List in its own class so as to be able to implement ISerializable interface. However, I have the following summarized error:

Cannot serialize member MyConcreteCollection.ChildList of type IChild because 
it is an interface.

The issue is in this code

public void WriteXml(System.Xml.XmlWriter writer)
{
    //-- HAVE also tried IChild child in ChildList
    foreach (ConcreteChild child in ChildList)
    {
        XmlSerializer s = new XmlSerializer(child .GetType());
        s.Serialize(writer, child );
    }
}

Here is the full code list. This is working and has fix implemented by user Jay

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

[Serializable]
public class ConcreteParent
{
    public MyConcreteCollection MyConcreteCollection { get; set; }
}

[Serializable]
public class MyConcreteCollection : IXmlSerializable //(Was ISerializable, as fixed by user Jay)
{
    public List<IChild> ChildList { get; set; }

    public System.Xml.Schema.XmlSchema GetSchema() { return null; }
    public void ReadXml(System.Xml.XmlReader reader) { }
    public void GetObjectData(SerializationInfo info, StreamingContext context) { }

    public void WriteXml(System.Xml.XmlWriter writer)
    {
        foreach (ConcreteChild child in ChildList)
        {
            XmlSerializer s = new XmlSerializer(child.GetType());
            s.Serialize(writer, child);
        }
    }

}

public interface IChild { }
[Serializable] public class ConcreteChild : IChild { }


public static class SerializerHelper<T>
{
    public static string Serialize(T myobject)
    {
        XmlSerializer xmlSerializer = new XmlSerializer(typeof(T));
        StringWriter stringWriter = new StringWriter();
        xmlSerializer.Serialize(stringWriter, myobject);
        string xml = stringWriter.ToString();

        XmlDocument xmlDoc = new XmlDocument();
        xmlDoc.LoadXml(xml);
        StringWriter sw = new StringWriter();
        XmlTextWriter xw = new XmlTextWriter(sw);
        xmlDoc.WriteTo(xw);

        return sw.ToString();
    }
}

class Program
{
    static void Main(string[] args)
    {
        try
        {
            ConcreteParent concreteParent = new ConcreteParent();
            concreteParent.MyConcreteCollection = new MyConcreteCollection();
            concreteParent.MyConcreteCollection.ChildList = new List<IChild>();
            concreteParent.MyConcreteCollection.ChildList.Add(new ConcreteChild());

            string xml = SerializerHelper<ConcreteParent>.Serialize(concreteParent);
            Console.WriteLine(xml);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.ToString());
        }
        Console.ReadKey();
    }
}

Answer 1

You have MyConcreteCollection implementing ISerializable. As shown in the first post to which you linked, you need instead to implement IXmlSerializable.