Abstract and Sealed Classes and Class Members
The abstract keyword enables you to create classes and class members that are incomplete and must be implemented in a derived class.
The sealed keyword enables you to prevent the inheritance of a class or certain class members that were previously marked virtual.
Abstract Classes and Class Members
public abstract class A
{
// Class members here.
}
An abstract class cannot be instantiated. The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share.
Abstract classes may also define abstract methods. This is accomplished by adding the keyword abstract before the return type of the method. For example:
public abstract class A
{
public abstract void DoWork(int i);
}
Abstract methods have no implementation, so the method definition is followed by a semicolon instead of a normal method block.
Derived classes of the abstract class must implement all abstract methods. When an abstract class inherits a virtual method from a base class, the abstract class can override the virtual method with an abstract method. For example:
// compile with: -target:library
public class D
{
public virtual void DoWork(int i)
{
// Original implementation.
}
}
public abstract class E : D
{
public abstract override void DoWork(int i);
}
public class F : E
{
public override void DoWork(int i)
{
// New implementation.
}
}
Sealed Classes and Class Members
public sealed class D
{
// Class members here.
}