Xml Serialize Interface Typed Members

If you try to serialize a class that has a public member of an interface type to xml file using the regular .net XmlSerializer, you will get an error: “Cannot serialize member {m} of type {t} because it is an interface.” ({m} and {t} are placeholders). 

In this post I suggest a workaround to this issue.

Consider the following scenario:

You have an object model like this:

 1: public class Car

   2: {

   3:     public string Model { get; set; }

   4:     public int Year { get; set; }

   5:     public IEngine Engine { get; set; }

   6: }

   7:  

   8: public interface IEngine

   9: {

  10:     void Work();

  11: }

  12:  

  13: public class ElectricEngine : IEngine

  14: {

  15:     int batteryPrecentageLeft;

  16:     public int BatteryPrecentageLeft

  17:     {

  18:         get { return this.batteryPrecentageLeft; }

  19:         set { this.batteryPrecentageLeft = value; }

  20:     }

  21:  

  22:     void IEngine.Work() { }

  23: }

If you would try to serialize myCar using the regular XmlSerializer, you will get the error above.

There are many reasons why you shouldn’t want to serialize an interface typed member. I will not dive into this debate right now. I assume that you have considered this before, and are just looking for a workaround.

So first lets understand what we are trying to achieve. 

In the example above, I expect the serialized output of myCar to be something like this:

   1: <Car xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

   2:   <Model>Ford Focus</Model>

   3:   <Year>2011</Year>

   4:   <Engine>

   5:     <ElectricEngine FullAssemblyQualifiedTypeName="Common.ElectricEngine, Common, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null">

   6:       <BatteryPrecentageLeft>70</BatteryPrecentageLeft>

   7:     </ElectricEngine>

   8:   </Engine>

   9: </Car>

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