Question

I have a DynamicObjectand I want it to be castable to IDictionary, exactly the same way as ExpandoObject. For example, casting an ExpandoObject to IDictionary is perfectly valid:

dynamic ex = new ExpandoObject ();
ex.Field = "Foobar";
IDictionary<string, object> dict = ex as IDictionary<string, object>;
Console.WriteLine (dict["Field"]);

Now I try to implement this into my own DynamicObject:

public class MyDynamicObject : DynamicObject
{
    public Dictionary<string, object> members = new Dictionary<string, object> ();

    public override bool TryGetMember (GetMemberBinder binder, out object result)
    {
        if (members.ContainsKey (binder.Name)) {
            result = members[binder.Name];
            return true;
        }
        else {
            result = null;
            return false;
        }
    }
    public override bool TrySetMember (SetMemberBinder binder, object value)
    {
        this.members.Add (binder.Name, value);
        return true;
    }
    public static implicit operator IDictionary<string, object> (MyDynamicObject mydo) 
    {
        return (IDictionary<string, object>) mydo.members;
    }
}

But the compiler will fail on the public static implicit operator IDictionary<string, object>line, giving the error: “Cannot convert to or from an interface type”. If I change the implicit operator to Dictionary, I can cast MyDynamicObject to Dictionary without any Problems, but not IDictionary.

How does ExpandoObject does this?

Answer 1

ExpandoObject simply implements IDictionary<string, object>.

If you want to have the same behaviour, you also need to implement this interface and delegate all calls to your members field.

Something like this:

public class MyDynamicObject : DynamicObject, IDictionary<string, object>
{
    public Dictionary<string, object> members = new Dictionary<string, object>();

    public IEnumerator<KeyValuePair<string, object>> GetEnumerator()
    {
        return members.GetEnumerator();
    }

    public void Add(KeyValuePair<string, object> item)
    {
        members.Add(item.Key, item.Value);
    }

    // ...
}