为什么我不能索引到 ExpandoObject？

Question

Something caught me by surprise when looking into C# dynamics today (I've never used them much, but lately I've been experimenting with the Nancy web framework). I found that I couldn't do this:

dynamic expando = new ExpandoObject();

expando.name = "John";

Console.WriteLine(expando["name"]);

The last line throws an exception:

Cannot apply indexing with [] to an expression of type 'System.Dynamic.ExpandoObject'

I understand the error message, but I don't understand why this is happening. I have looked at the documentation for ExpandoObject and it explicitly implements IDictionary<,> and thus has a this.[index] method (MSDN ). Why can't I call it?

Of course, there's nothing to stop me from downcasting the ExpandoObject to a dictionary manually and then indexing into it, but that kind of defies the point; it also doesn't explain how the Expando was able to hide the method of one of its interfaces.

What's going on here?

Answer 1

how the Expando was able to hide the method of one of its interfaces.

Because as you correctly found out in the documentation, the indexer is an explicit interface implementation . From Explicit Interface Implementation Tutorial :

A class that implements an interface can explicitly implement a member of that interface. When a member is explicitly implemented, it cannot be accessed through a class instance, but only through an instance of the interface.

This means you'll have to cast the reference to the interface to access it:

((IDictionary<String, Object>)expando)["name"]

Answer 2

Use this factory class to create ExpandoObjects! Then use HasProperty("prop name") or GetValue("prop name")

void Main()
{
    dynamic _obj = ExpandoObjectFactory.Create();
    if (_obj.HasProperty("Foo") == false)
    {
        _obj.Foo = "Foo";
    }
    Console.WriteLine(_obj); // Foo;
    object bar = _obj.GetValue("Bar");
    Console.WriteLine(bar); // null
}

public static class ExpandoObjectFactory
{
    public static ExpandoObject Create()
    {
        dynamic expandoObject = new ExpandoObject();
        expandoObject.HasProperty = new Func<string, bool>((string name) => ((IDictionary<string, object>)expandoObject).ContainsKey(name));
        expandoObject.GetValue = new Func<string, object>(delegate (string name)
        {
            ((IDictionary<string, object>)expandoObject).TryGetValue(name, out object value);
            return value;
        });
        return expandoObject;
    }
}