C# 4 and .NET 4

anonymous Types ❘ 79
As noted earlier, date is represented by the class System.DateTime. The following code uses a System.
DateTime constructor that takes three parameters (the year, month, and day of the month — you can find
details of this and other DateTime constructors in the MSDN documentation):
public class Document
{
public readonly DateTime CreationDate;
public Document()
{
// Read in creation date from file. Assume result is 1 Jan 2002
// but in general this can be different for different instances
// of the class
CreationDate = new DateTime(2002, 1, 1);
}
}
CreationDate and MaxDocuments in the previous code snippet are treated like any other field, except that
because they are read-only, they cannot be assigned outside the constructors:
void SomeMethod()
{
MaxDocuments = 10;
}
// compilation error here. MaxDocuments is readonly
It’s also worth noting that you don’t have to assign a value to a readonly field in a constructor. If you don’t
do so, it will be left with the default value for its particular data type or whatever value you initialized it to
at its declaration. That applies to both static and instance readonly fields.
anonymous TyPes
Chapter 2, “Core C#” discussed the var keyword in reference to implicitly typed variables. When used
with the new keyword, anonymous types can be created. An anonymous type is simply a nameless class that
inherits from object. The definition of the class is inferred from the initializer, just as with implicitly typed
variables.
If you need an object that contains a person’s first, middle, and last name, the declaration would look
like this:
var captain = new {FirstName = "James", MiddleName = "T", LastName = "Kirk"};
This would produce an object with FirstName, MiddleName, and LastName properties. If you were to
create another object that looked like this:
var doctor = new {FirstName = "Leonard", MiddleName = "", LastName = "McCoy"};
the types of captain and doctor are the same. You could set captain = doctor, for example.
If the values that are being set come from another object, then the initializer can be abbreviated. If you
already have a class that contains the properties FirstName, MiddleName, and LastName and you have
an instance of that class with the instance name person, then the captain object could be initialized
like this:
var captain = new {person.FirstName, person.MiddleName, person.LastName};
The property names from the person object would be projected to the new object named captain. So the
object named captain would have the FirstName, MiddleName, and LastName properties.
The actual type name of these new objects is unknown. The compiler “makes up” a name for the type,
but only the compiler will ever be able to make use of it. So you can’t and shouldn’t plan on using any type
reflection on the new objects because you will not get consistent results.
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