C# 4 and .NET 4

inTroduCTion
optional and named Parameters
Optional parameters and named parameters have been in Visual Basic for some time but have not been
available to C# until the .NET 4 release. Optional parameters allow you to provide default values for some
of the parameters of your methods and allow for a type of overloading by the consumer, even if there is only
a single method in place to deal with all the variants. Here’s an example:
public void CreateUser(string firstname, string lastname,
bool isAdmin, bool isTrialUser)
{
}
If you wanted to overload this and have default values for the two bool objects, then you could easily have a
few more methods that populate these values for the consumer and then make a call to the master method to
actually create the user. Now with optional parameters, you are able to do something like this:
public void CreateUser(string firstname, string lastname,
bool isAdmin = false, bool isTrialUser = true)
{
}
Looking over this bit of code, the parameters firstname and lastname do not have a default value set,
while isAdmin and isTrailUser do have default values set. As a consumer of something like this, you are
now able to do some of the following:
myClass.CreateUser("Bill", "Evjen");
myClass.CreateUser("Bill", "Evjen", true);
myClass.CreateUser("Bill", "Evjen”, true, false);
myClass.CreateUser("Bill", "Evjen", isTrailUser: false);
The last example makes use of named parameters, which are also a new feature for C# in this release
of the .NET Framework. Named parameters will potentially change the way you write your code. This new
feature will allow you to make your code easier to read and understand. As an example of this in action,
take a look at the File.Copy() method of the System.IO namespace. Typically, it would be constructed
similarly to this:
File.Copy(@"C:\myTestFile.txt", @"C:\myOtherFile.txt", true);
In this case, this simple method is working with three parameters, but what are the actual items being
passed into the Copy() method Unless you know this method backward and forward, it is hard to tell what
is going on by just glancing at this method. Using named parameters, you are able to use the parameter
name in the code prior to the value being provided, as in the following example:
File.Copy(sourceFileName: @"C:\myTestFile.txt",
destFileName: @"C:\myOtherFile.txt", overwrite: true);
Now with the named parameters in place, you can more easily read and understand what is going on with
this line of code. Using named parameters makes no difference to the resulting compilation; they are only
used in the coding of the application.
Covariance and Contravariance
Covariance and contravariance were included in prior versions of the .NET Framework, but they have been
extended in .NET 4 to perform even better when working with generics, delegates, and more. In the prior
versions of .NET, you were able to use contravariance with objects and arrays, but, for instance, you were
unable to use contravariance with generic interfaces. In .NET 4, you are able to do this.
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