That Wild and Crazy Dynamic Type

I had a mind-blowing weekend playing with the ExpandoObject in C# 4.0.  When an article appeared on MSDN about it prior to the release of Visual Studio 2010 Beta 2, it looked interesting but not particularly useful.  The ExpandoObject appeared to be merely a property bag that was useful in limited circumstances in which the program flow is aware of the properties being added.  The ExpandoObject is, in essence, a property bag (and also a method bag?) that pretends to be a real type.  With the Beta 1, I tried to bind a WPF Window to it and was quickly disappointed to find that WPF just treated it as a dictionary (which is what it really is) rather than as the typed class it is pretending to be.

With the Beta 2, however, this changed.  The WPF binding stack had apparently been modified to recognize the ExpandoObject and to treat it on its own terms.  If I have a binding on a TextBox in WPF to an arbitrary property called, for instance, “Name”, all I have to do is add that string to the ExpandoObject’s internal dictionary and the value I added will show up in my TextBox.  Even better, the ExpandoObject automatically supports INotifyPropertyChanged on that new pseudo-property.

           dynamic Employee = new ExpandoObject();
           Employee.Name = "Jim Henson";

This small change makes the ExpandoObject suddenly very useful, especially in situations where we need a glue layer between servers and clients that are indifferent to static typing such as an object layer between a REST service and WPF.

Phil Haack has a playful post about this aspect of programming with the dynamic type on his blog: http://haacked.com/archive/2009/08/26/method-missing-csharp-4.aspx .  He prefaces it with this:

Warning: What I’m about to show you is quite possibly an abuse of the C# language. Then again, maybe it’s not. 😉 You’ve been warned.

There’s a lot of truth in this.  On the one hand, dynamic types goes deeply against the grain of anyone who has programmed in C# for the past several years.  I’m not sure how this feels to VB developers who always have had the option to turn Option Strict off in their code.  It gives me the heebie-jeebies, though.

At the same time, it looks extremely fun.  Maybe even a little subversive. I want to start finding applications for this new code feature for working within C# rather than simply using it for interop with supported dynamic languages.

The ExpandoObject does some rather cool things not initially obvious.  For one thing, I can add methods to it as well as properties.  This can be done by passing a delegate to it that references a lambda statement, for instance:

        dynamic obj = new ExpandoObject();

        obj.DoSomething = new Func<string>(() => 
            { Console.WriteLine(DateTime.Now.ToLongTimeString()); return "Hello"; });

        obj.DoSomethingWithParams = new Func<string, bool>((x) => 
            { Console.WriteLine(DateTime.Now.ToLongTimeString());  
                Console.WriteLine(x); 
                return true; 
            });
        Console.WriteLine(obj.DoSomething());
        Console.WriteLine(obj.DoSomethingWithParams("something different"));

Another thing it can do is load properties dynamically (loading a dynamic type dynamically, yeah!) by simply casting it back to the dictionary it really is internally.  Here’s a sample of dynamically loading up an ExpandoObject based on an XML Document:

           XDocument root = XDocument.Load("Employee.xml");
            //convert xml document into an ExpandoObject
           dynamic Employee = new ExpandoObject();

           var EmployeeDictionary = Employee as IDictionary<string, object>;
           foreach (XElement el in root.Elements())
            {
                    EmployeeDictionary.Add(el.Name.LocalName, el.Value);
            }

I now have an object that I can bind to the DataContext of a WPF Window.  I could, of course, have just bound the original XMLDocument to my WPF, but then I wouldn’t have nice extras like INotifyPropertyChanged that I normally expect to have at my disposal in WPF development.

The Dynamic Type is a complicated beast however.  Just reading over Bill Wagner’s post on implementing a Method Bag fills me with anxiety: http://msdn.microsoft.com/en-us/library/ee658247.aspx

That post was nothing, however, compared to this gem I found on Nikhil Kothari’s blog from over a year ago!  http://www.nikhilk.net/CSharp-Dynamic-Programming-REST-Services.aspx

Mr. Kothari builds a class that can point to multiple (and, thanks to the Dynamic Type, interchangeable) REST services and generates properties based on the XML or JSON that comes back.  It does much more than this, also, I think, but Nikhil Kothari is so far beyond me that I have trouble understanding everything that is happening in his code.

The source code, by the way, doesn’t actually compile as it is currently written – but then he did this over a year ago, so…

In any case, walking through and coming to terms with what he has created is certainly one of my goals for November.  It is an implementation that is a game-changer for C# development, and raises Dynamic Types from an oddity to something that will actually solve architectural bottlenecks … at least, I think it does.  As I said, I don’t actually understand it, yet.

3 thoughts on “That Wild and Crazy Dynamic Type”

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