The Basics

[TestMethod()]
public void BasicDynamicTest()
{
    dynamic f = Activator.CreateInstance(Type.GetType("System.Text.StringBuilder"));
    f.Append("Hello");
    f.Append(" World!");
    Assert.AreEqual("Hello World!", f.ToString());
}

The type of f is is determined at runtime.  You might be wondering why you’d want to do something like the above.  You might start thinking about evaluating expressions at runtime, a la scripting or dynamic languages like Python or Ruby.  How about something even more clever, like defining the structure of an object at runtime?

DynamicObject

To do so, you need to create a class that inherits from the DynamicObject class and then you can do things like override the TryGetMember method so you can create your own methods on the class at runtime:

// First, make sure MyXML inherits from DynamicObject
class MyXML : DynamicObject
{
	private XDocument _xml = new XDocument();

	// Simple constructor
	public MyXML(string x)
	{
		this._xml = XDocument.Parse(x);
	}

	// Let's dynamically create our own methods
	public override bool TryGetMember(GetMemberBinder binder, out object result)
	{
		string nodeName = binder.Name;
		result = _xml.Element("top").Element(nodeName).Value;
		return true;
	}
}

[TestMethod()]
public void MyXMLConstructorTest()
{
	dynamic xml = new MyXML(@"
		<top>
			<node1>Alpha</node1>
			<node2>Beta</node2>
		</top>");
	Assert.AreEqual("Alpha", xml.node1);
	Assert.AreEqual("Beta", xml.node2);
}

Drawbacks?

While you can basically do this with Reflection, dynamic code is much easier to deal with. You lose Intellisense on dynamic types, compile-time checking, and performance. The DLR (Dynamic Language Runtime) does the runtime interpreting for you. As the author of the book that I’m reading states, you generally wouldn’t want to use dynamic code except where you’d otherwise be using reflection; C# by nature is statically-typed so most of the time, you’re better served by writing static code.

Incidentally, the C#/.NET 4.0 book I’m reading (1 of 3) is Introducing .NET 4.0 With Visual Studio 2010 by Alex Mackey. I haven’t gotten through enough of it to have a firm opinion but I picked it up because I liked the sharp and tight focus on things that are new in 4.0 as well as the focus on MVC 2 and jQuery (there looks to be some nice interop there).

I wouldn’t call ideas worthless, per se, but it’s clear that ideas alone are a hollow sort of currency. Success is rarely determined by the quality of your ideas. But it is frequently determined by the quality of your execution. So instead of worrying about whether the Next Big Idea you’re all working on is sufficiently brilliant, worry about how well you’re executing.

If you give a good idea to a mediocre group, they’ll screw it up. If you give a mediocre idea to a good group, they’ll fix it. Or they’ll throw it away and come up with something else.