C# 5.0 Programmer's Reference

Chapter 6 ❘ 637
The following code shows the King class.
class King : Piece
{
public override bool CanMoveTo(int row, int column)
{
return true;
}
}
9. The following statement defines the ManagersFromEmployeesDelegate type.
delegate Manager[] ManagersFromEmployeesDelegate(Employee[] employees);
The following code shows the Promote method that matches the delegate type.
private Manager[] Promote(Employee[] employees)
{
return null;
}
The following statement creates a variable that holds a reference to the Promote method.
ManagersFromEmployeesDelegate del = Promote;
10. Covariance lets a method return a more derived type than the delegate. In this example,
the delegate returns Manager[], so the new method should return the more derived type
Executive[].
Contravariance lets a method take parameters that are a less derived type than those taken
by the delegate. In this example, the delegate takes an Employee[] as a parameter, so the
new method should take the less derived parameter type Person[].
The following code shows the new version of the Promote method.
private Executive[] Promote2(Person[] people)
{
return null;
}
The following statement creates a variable that holds a reference to the new version of the
Promote method.
ManagersFromEmployeesDelegate del2 = Promote2;
11. The ColorizeImage program, which is available for download on this book’s website, does
this. Download the program to see how it works. In one test on my dual-core computer, processing
an image took roughly 1.78 seconds synchronously and 0.92 seconds asynchronously.
Because the computer has two cores, you might expect the asynchronous version to take
one-half the time used by the synchronous version, but there is some overhead in setting
up and coordinating the threads. The result is still an impressive reduction in time, however,
and would be even greater on a computer with more cores.
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