When an instance method declaration includes an abstract
modifier, the method is said to be an abstract method. An abstract method is implicitly also a virtual method.
An abstract declaration introduces a new virtual method but does not provide an implementation of the method. Instead, non-abstract derived classes are required to provide their own implementation by overriding the method. Because an abstract method provides no actual implementation, the method-body of an abstract method simply consists of a semicolon.
Abstract method declarations are only permitted in abstract classes (§10.1.1.1).
It is an error for an abstract method declaration to include any one of the static
, virtual
, or override
modifiers.
In the example
public abstract class Shape { public abstract void Paint(Graphics g, Rectangle r); } public class Ellipse: Shape { public override void Paint(Graphics g, Rectangle r) { g.drawEllipse(r); } } public class Box: Shape { public override void Paint(Graphics g, Rectangle r) { g.drawRect(r); } }
the Shape
class defines the abstract notion of a geometrical shape object that can paint itself. The Paint
method is abstract because there is no meaningful default implementation. The Ellipse
and Box
classes are concrete Shape
implementations. Because theses classes are non-abstract, they are required to override the Paint
method and provide an actual implementation.
It is an error for a base-access (§7.5.8) to reference an abstract method. In the example
class A { public abstract void F(); } class B: A { public override void F() { base.F(); // Error, base.F is abstract } }
an error is reported for the base.F()
invocation because it references an abstract method.