C# 4.0 Dynamic vs Expando… where do they fit?

Question

I am trying to learn all the new goodies that come with C# 4.0. I am failing to understand the differences between the DynamicObject and ExpandoObject types. It seems like DynamicObject is used e.g. when you want to access variables from Python scripts and ExpandoObject when talking with COM/Office objects. Am I right? What is the difference in their use?

Answer 1

Expando is a dynamic type to which members can be added (or removed) at runtime. dynamic is designed to allow .NET to interoperate with types when interfacing with dynamic typing languages such as Python and JavaScript.

So, if you need to handle a dynamic type: use dynamic and if you need to handle dynamic data such as XML or JSON: use ExpandoObject

The declaration of an expando shows the relationship between dynamic and the expando:

dynamic expando = new ExpandoObject();

And the ability to add a new property:

expando.SomeNewStringVal = "Hello World!";

That last line of code creates a brand new string property in the expando object called SomeNewStringVal. The string type is inferred from the assignment.

So an expando is a dynamic data type that can represent dynamically changing data. That's it in a nutshell. Here's a deeper look at dynamic and expando.

Complete example:

using System;
using System.Dynamic;

class Program
{
    static void Main(string[] args)
    {
        dynamic expando = new ExpandoObject();
        expando.SomeNewStringVal = "Hello Brave New Whirrled!";
        Console.WriteLine(expando.SomeNewStringVal);

        // more expando coolness/weirdness:
        var p = expando as IDictionary<String, object>;
        p["A"] = "New val 1";
        p["B"] = "New val 2";

        Console.WriteLine(expando.A);
        Console.WriteLine(expando.B);
    }
}