Classes and Inheritance - Illustrated C# 2005 - page 165

Abstract Members
An abstract member is a function member that is designed to be overridden. An abstract mem-
ber has the following characteristics:

It is marked with the abstract modifier. ￿

￿ It does not have an implementation code block. The code blocks of abstract members

are replaced with semicolons.

For example, the following code from inside a class declaration declares two abstract
members—an abstract method called PrintStuff and an abstract property called MyProperty .
Notice the semicolons in place of the implementation blocks.

Keyword Semicolon in place of implementation
↓ ↓
abstract public void PrintStuff(string s);

abstract public int MyProperty
{
get; ← Semicolon in place of implementation
set; ← Semicolon in place of implementation
}

Other important facts about abstract members are the following:

￿ Abstract methods, although they must be overridden by a corresponding method in a

derived class, cannot use the virtual modifier in addition to the abstract modifier.

￿ As with virtual methods, the implementation of an abstract method in a derived class

must specify the override modifier.

￿ Abstract members can be declared only in abstract classes , which you will look at in the

next section.

Table 7-3 compares and contrasts virtual members and abstract members.

Table 7-3. Comparing Virtual and Abstract Members

Virtual Member Abstract Member

virtual abstract Keyword

Implementation body Has an implementation body No implementation body—

semicolon instead

Overridden in a derived class Can be overridden—using

override

Must be overridden—using
override

Types of members Methods

Properties
Events
Indexers

Methods
Properties
Events
Indexers