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ExpandoObject in C# 4.0 Inside Look - 'Attaching' Members (Properties, Methods etc) To Objects At Runtime

About .NET 4.0 Series - I'll be covering various aspects of .NET 4.0 and related technologies in these posts

In my previous post and related article, I gave a quick introduction towards creating and using your own custom dynamic types, inheriting from the DynamicObject class. .NET framework 4.0 has a cool new ExpandoObject class, in System.Dynamic namespace. What makes this special is, it allows you to create an object with members that can be dynamically added and removed at run time.

To clarify the point, have a look at this example.

  
            dynamic obj = new ExpandoObject();
            obj.Value = 10;
            var action = new Action<string >((line) => Console.WriteLine(line));
            obj.WriteNow = action;
            obj.WriteNow(obj.Value.ToString());

And you'll get 10 as output in your console. As you can see, the property 'Value' and method 'WriteNow' is attached at runtime, when the invocation happens.

The objective of this post is to examine how the ExpandoObject itself is implemented. To make this clear, let us create a MyExpando class, a minimal implementation of ExpandoObject.

We are inheriting MyExpando class from the DynamicObject, and these are the major methods we are overriding.

  • TrySetMember- Provides the implementation of setting a member.
  • TryGetMember-Provides the implementation of getting a member.
  • TryInvokeMember- Provides the implementation of calling a member.
  • GetDynamicMemberNames- Returns the enumeration of all dynamic member names.

And Here is our minimal MyExpando class, inherited from DynamicObject, and overrides the above methods.
  public class MyExpando : DynamicObject
    {

        private Dictionary < string, object > _members = new Dictionary < string, object > ();


        /// 
        /// When a new property is set, add the property name and value to the dictionary
        ///      
        public override bool TrySetMember(SetMemberBinder binder, object value)
        {
            if (!_members.ContainsKey(binder.Name))
                _members.Add(binder.Name, value);
            else
                _members[binder.Name] = value;

            return true;
        }

        /// 
        /// When user accesses something, return the value if we have it
        ///       
        public override bool TryGetMember(GetMemberBinder binder, out object result)
        {
            if (_members.ContainsKey(binder.Name))
            {
                result = _members[binder.Name];
                return true;
            }
            else
            {
                return base.TryGetMember(binder, out result);
            }
        }

        /// 
        /// If a property value is a delegate, invoke it
        ///      
        public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result)
        {
            if (_members.ContainsKey(binder.Name) && _members[binder.Name] is Delegate)
            {
                result = (_members[binder.Name] as Delegate).DynamicInvoke(args);
                return true;
            }
            else
            {
                return base.TryInvokeMember(binder, args, out result);
            }
        }


        /// 
        /// Return all dynamic member names
        /// 
        /// 
        public override IEnumerable&ltstring> GetDynamicMemberNames()
        {
            return _members.Keys;
        }
    }
When ever a property set operation happens, the runtime will invoke TrySetMember, and there, we are pushing the property name and value to a dictionary. Similarly, when ever a get operation happens (TryGetMember), we are simply returning the value object if available in the dictionary. Also, when a method call is made (TryInvokeMember), if the value type is a delegate, we just invoke the same by passing the arguments we received. Pretty simple, huh?

Well, that is it. Now, let us try the above scenario with our MyExpando object

           dynamic obj = new MyExpando();
            obj.Value = 10;
            var action = new Action((line) => Console.WriteLine(line));
            obj.WriteNow = action;
            obj.WriteNow(obj.Value.ToString());

Run the application, and guess what you get!!

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