C# 4 and .NET 4

304 ❘ ChaPTer 12 dynAmic lAnGuAGe extensiOns
What about adding a method That can be done easily. You can use the same WroxDynamicObject and add
a GetTomorrowDate method on it. It takes a DateTime object and returns a date string that is the next day.
Here’s the code:
dynamic wroxDyn = new WroxDynamicObject();
Func GetTomorrow = today => today.AddDays(1).ToShortDateString();
wroxDyn.GetTomorrowDate = GetTomorrow;
Console.WriteLine("Tomorrow is {0}", wroxDyn.GetTomorrowDate(DateTime.Now));
You create the delegate GetTomorrow using Func. The method the delegate represents is the
call to AddDays. One day is added to the Date that is passed in, and a string of that date is returned. The
delegate is then set to GetTomorrowDate on the wroxDyn object. The last line calls the new method, passing
in today’s date.
Once again the dynamic magic happens and you have an object with a valid method.
expandoobject
ExpandoObject works similar to the WroxDynamicObject created in the previous section. The difference is
that you don’t have to override any methods, as shown in the following code example:
static void DoExpando()
{
dynamic expObj = new ExpandoObject();
expObj.FirstName = "Daffy";
expObj.LastName = "Duck";
Console.WriteLine(expObj.FirstName + " " + expObj.LastName);
Func GetTomorrow = today => today.AddDays(1).ToShortDateString();
expObj.GetTomorrowDate = GetTomorrow;
Console.WriteLine("Tomorrow is {0}", expObj.GetTomorrowDate(DateTime.Now));
}
expObj.Friends = new List();
expObj.Friends.Add(new Person() { FirstName = "Bob", LastName = "Jones" });
expObj.Friends.Add(new Person() { FirstName = "Robert", LastName = "Jones" });
expObj.Friends.Add(new Person() { FirstName = "Bobby", LastName = "Jones" });
foreach (Person friend in expObj.Friends)
{
Console.WriteLine(friend.FirstName + " " + friend.LastName);
}
Notice that this code is almost identical to what you did earlier. You add a FirstName and LastName
property, add a GetTomorrow function, and do one additional thing—add a collection of Person objects as
a property of the object.
At first glance, it may seem that this is no different than using the dynamic type. Well, there are a couple
of subtle differences that are important. First, you can’t just create an empty dynamic typed object. The
dynamic type has to have something assigned to it. For example, the following code won’t work:
dynamic dynObj;
dynObj.FirstName = "Joe";
As seen in the previous example, this is possible with ExpandoObject.
Second, because the dynamic type has to be assigned to, it will report back the type of what was assigned
to it if you do a GetType call. So if you say assign an int, it will report back that it is an int. This won’t
happen with ExpandoObject or an object derived from DynamicObject.
If you have to control the addition and access of properties in your dynamic object, then deriving from
DynamicObject is your best choice. With DynamicObject, you can use several methods to override and
control exactly how the object interacts with the runtime. For other cases, using the dynamic type or the
ExpandoObject may be just what you need.
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