What are the true benefits of ExpandoObject?

Question

The ExpandoObject class being added to .NET 4 allows you to arbitrarily set properties onto an object at runtime.

Are there any advantages to this over using a Dictionary<string, object>, or really even a Hashtable? As far as I can tell, this is nothing but a hash table that you can access with slightly more succinct syntax.

For example, why is this:

dynamic obj = new ExpandoObject(); obj.MyInt = 3; obj.MyString = "Foo"; Console.WriteLine(obj.MyString); 

Really better, or substantially different, than:

var obj = new Dictionary<string, object>(); obj["MyInt"] = 3; obj["MyString"] = "Foo";  Console.WriteLine(obj["MyString"]); 

What real advantages are gained by using ExpandoObject instead of just using an arbitrary dictionary type, other than not being obvious that you're using a type that's going to be determined at runtime.

Answer 1

Since I wrote the MSDN article you are referring to, I guess I have to answer this one.

First, I anticipated this question and that's why I wrote a blog post that shows a more or less real use case for ExpandoObject: Dynamic in C# 4.0: Introducing the ExpandoObject.

Shortly, ExpandoObject can help you create complex hierarchical objects. For example, imagine that you have a dictionary within a dictionary:

Dictionary<String, object> dict = new Dictionary<string, object>(); Dictionary<String, object> address = new Dictionary<string,object>(); dict["Address"] = address; address["State"] = "WA"; Console.WriteLine(((Dictionary<string,object>)dict["Address"])["State"]); 

The deeper the hierarchy, the uglier the code. With ExpandoObject, it stays elegant and readable.

dynamic expando = new ExpandoObject(); expando.Address = new ExpandoObject(); expando.Address.State = "WA"; Console.WriteLine(expando.Address.State); 

Second, as was already pointed out, ExpandoObject implements INotifyPropertyChanged interface which gives you more control over properties than a dictionary.

Finally, you can add events to ExpandoObject like here:

class Program {    static void Main(string[] args)    {        dynamic d = new ExpandoObject();         // Initialize the event to null (meaning no handlers)        d.MyEvent = null;         // Add some handlers        d.MyEvent += new EventHandler(OnMyEvent);        d.MyEvent += new EventHandler(OnMyEvent2);         // Fire the event        EventHandler e = d.MyEvent;         e?.Invoke(d, new EventArgs());    }     static void OnMyEvent(object sender, EventArgs e)    {        Console.WriteLine("OnMyEvent fired by: {0}", sender);    }     static void OnMyEvent2(object sender, EventArgs e)    {        Console.WriteLine("OnMyEvent2 fired by: {0}", sender);    } } 

Also, keep in mind that nothing is preventing you from accepting event arguments in a dynamic way. In other words, instead of using EventHandler, you can use EventHandler<dynamic> which would cause the second argument of the handler to be dynamic.