C # abstract class, sealed class and class members

1. Abstract classes and class members

Use the abstract keyword to create an incomplete class and class member that must be implemented in a derived class.

For example:

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. For example, a class library can define an abstract class that is a parameter to its multiple functions and require the programmer to use the library to provide its own class implementation by creating a derived class.

Abstract classes can also define abstract methods. method is to add the keyword abstract to the front of the return type of the method.

For example:

public abstract class A
{
　　　 public abstract void DoWork(int i);
}

Abstract methods are not implemented, so the method definition is followed by a semicolon rather than a regular method block. Derived classes of an abstract class must implement all abstract methods. When an abstract class inherits a virtual method from a base class, an 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.
　　　 }
}