Question

I would like to return an ExpandoObject from a WebMethod, like this:

[WebMethod]
public ExpandoObject TestMethod(int val)
{
     dynamic item = new ExpandoObject();

     item.Value = val;
     item.SomeOtherStuff = "SomeOtherStuff";

     DynamicallyAddMoreFields(item);

     return item;
}

When I try to do it, I get this error:

To be XML serializable, types which inherit from IEnumerable must have an implementation of Add(System.Object)

And I can’t extend the ExpandoObject class, since it’s sealed.

Is it possible to do this some other way?

Answer 1

You can create your own serializable version of the ExpandoObject class by inheriting from DynamicObject and implementing ISerializable.

The ExpandoObject is basically a dictionary storing the names of the dynamically attached properties together their with values:

[Serializable]
public class SerializableExpandoObject : DynamicObject, ISerializable
{
    private Dictionary<string, object> properties = new Dictionary<string, object>()

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        return properties.TryGetValue(binder.Name, out result);
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        properties.Add(binder.Name, value);
        return true;
    }

    ....
}

From here you can implement any serialization format you need to support, including IXmlSerializable.