C# 5.0 Programmer's Reference

Inheritance ❘ 261
Ann: Person.HideMe
Bob: Employee.HideMe
Bob: Person.HideMe
The object person invokes the Employee class’s version of IsVirtual (the third line of output) but it
invokes the Person class’s version of HideMe (the last line of output).
TIP Hiding and overriding are important but confusing concepts. If the difference
isn’t clear, you should reread this section.
There are two other keywords that affect overriding: abstract and sealed.
abstract
An abstract method is one that doesn’t have a method body. It defines the name, parameters, and
return type of the method but doesn’t provide an implementation. (This is similar to the way interfaces
define method signatures but don’t provide an implementation.)
If you give a class an abstract method, there’s a sort of placeholder in the class for the method. Because
the placeholder is empty, you cannot create an instance of the class because it is incomplete. For that
reason, if a class contains an abstract method, you must also mark the class as abstract.
Note that the class could define other non-abstract properties, methods, and events. (Some people
call a non-abstract method, class, or other item concrete.) If it contains even one abstract method,
then the class must be abstract.
Absolutely Abstract
You can also mark a class abstract to prevent someone from creating an instance
of it even if it doesn’t contain any abstract methods.
An abstract method is also considered virtual, so a child class can override it to give it a method
body. Then the child class can be concrete and you can create instances of it.
For an example, consider the following code.
public abstract class Report
{
public abstract void GenerateReport();
public abstract void DistributeReport();
}
public abstract class PersonnelReport : Report
{
public override void DistributeReport()
{
// Code to distribute the report to the personnel department...
}
}
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