Serializing objects which implement IEnumerable<>, using XmlSerializer

Question

Hi,

I'm trying to serialize an object which contains an object which implements IEnumerable<>. The last object also contains an object which implements IEnumerable<>. The objects strucutre is as followed:

[Serializable]
 public class A
 {
   public B _b {get; set; }
 }

 [Serializable]
 public class B : IEnumerable<C>
 {
    public List<C> c_items {get; set;}
    public void Add (C obj){}
 }

 [Serializable]
 public class C : IEnumerable<D>
 {
   public List<D>  d_items {get; set;}
   public void Add (D obj){}
 }

 [Serializable]
 public class D
 {
    public int Key {get; set;}
    public int Value {get; set;}
 }

In the code line:

A obj = new A();
XmlSerializer serializer = new XmlSerializer(obj.GetType());

I'm getting the exception: unable to generate temporary class (result=1), error CS0030: Cannot convert type 'C' to type 'D'...

When I remove the IEnumerable implementation from one of B or C, the serialization works.

What as I getting wrong?

Any help would be appreciated!