C # differences between abstract classes and interfaces

I. abstract class:
Abstract classes are special classes, but cannot be instantiated (you can use a derived class to instantiate a base class object). In addition, they have other features of the class; the important thing is that abstract classes can include Abstract METHODS (of course, they can have common methods), which are not supported by common classes. Abstract METHODS can only be declared in abstract classes and do not contain any implementations. The Derived classes must overwrite (override) them.

Ii. interface:
The Interface contains the prototype of a group of methods. The interface is of reference type, similar to a class. There are three similarities with the abstract class:
1. It cannot be instantiated. (See the msdn explicit interface implementation)
2. contains an unimplemented method statement;
3. the method that the derived class must implement the interface;

In addition, interfaces have the following features: In addition to methods, interfaces can also contain attributes, indexers, and events, and these members are defined as common. It cannot contain any other Members, such as constants, fields, constructors, destructor, and static members. A class can directly inherit multiple interfaces, but can only inherit one class (including abstract classes ).

Iii. Differences between abstract classes and interfaces:
1. class is the abstraction of objects. abstract classes can be understood as classes as objects. abstract classes are called abstract classes. the interface is just a behavior specification or provision. Microsoft's custom interface always carries the able field behind it to prove that it represents a class "I can do it... ". Abstract classes are more defined in a series of closely related classes, while interfaces are mostly classes with loose relationships but all implement certain functions.
2. The interface basically does not have any specific characteristics of inheritance. It only promises the methods that can be called;
3. A class can implement several interfaces at a time, but only one parent class can be extended.
4. interfaces can be used to support callback, but inheritance does not.
5. the abstract class cannot be sealed.
6. The specific methods implemented by abstract classes are virtual by default, but the interface methods in the class implementing interfaces are non-virtual by default. Of course, you can also declare them as virtual.
7. Similar to a non-abstract class, an abstract class must provide its own implementation for all the members of the interface listed in the base class list of this class. However, the abstract class is allowed to map interface methods to abstract methods.
8. abstract classes implement a principle in OOP that separates mutable from immutable. Abstract classes and interfaces are defined as immutable classes, while variable class classes are implemented.
9. A good interface definition should be specific and functional, rather than multi-functional, otherwise it will cause interface pollution. If a class only implements a function of this interface and has to implement other methods in the interface, it is called interface pollution.
10. Avoid using inheritance to implement the build function, but use black box multiplexing, that is, object combination. As the hierarchy of inheritance increases, the most direct consequence is that when you call a class in this group, you must load all of them into the stack! The consequences can be imagined (based on the stack principle ). At the same time, some interested friends can note that Microsoft often uses the object combination method when building a class. For example, in Asp.net, the page class has server request and other attributes, but in fact they are all objects of a certain class. This object of the page class is used to call the methods and attributes of other classes. This is a very basic design principle.
11. If an abstract class implements an interface, you can map the methods in the interface to the abstract class as an abstract method without having to implement it. Instead, you can implement the methods in the subclass of the abstract class.

Iv. Use of abstract classes and interfaces:
1. If you want to create multiple versions of a component, create an abstract class. Abstract classes provide simple methods to control component versions.
2. If the created function is used across a wide range of different objects, the interface is used. If you want to design small and concise functional blocks, use interfaces.
3. If you want to design a large functional unit, use the abstract class. If you want to provide general implemented functions among all the implementations of the component, use the abstract class.
4. abstract classes are mainly used for closely related objects. interfaces are suitable for providing general functions for irrelevant classes.

The following is a metaphor I have seen on the Internet:
1. airplanes fly and birds fly. They all inherit the same interface "fly", but f22 belongs to the aircraft abstract class and pigeon belongs to the bird abstract class.
2. just like all doors (abstract class), I can't give you a door (I can't instantiate it), but I can give you a specific door or wooden door (polymorphism ); it can only be a door. You cannot say it is a window (single inheritance); a door can have a lock (Interface) or a doorbell (multiple implementations ). A door (abstract class) defines what you are and an interface (LOCK) specifies what you can do (one interface is best to do only one thing, you cannot require the lock to make sound (interface pollution )).