C＃中对象实例化左侧的抽象类

Question

I have two questions regarding the use of an abstract class on left side of object instantiation.

AbstractClass MyClass = new ConcreteClass();

1 - What is this called? (when you declare the abstract class on the left.)

eg

Car MyNiceCar = new NiceCar();

I know this relates to Polymorphism, but I'm specifically asking how to describe/verbalize the scenario when declare the abstract class on the left.

2 - And why do it? ie Why would you do:

Car MyNiceCar = new NiceCar();

And not:

NiceCar MyNiceCar = new NiceCar();

?

Would the answer to question 2 possibly be so that i can do the following?

Car MyNiceCar = new NiceCar();

.
. //do some logic to decide if I can have a nicer car.
.

MyNiceCar = new EvenNicerCar();

Answer 1

1) You're creating a base class reference to your Derived class.
Edit:
Other words for BaseClass: SuperClass, ParentClass
Other words for DerivedClass: SubClass, ChildClass Don't ask why there are so many words for each. It's kinda a Spaces vs Tabs type thing.

2) You do it so that you can use virtual functions/properties that you know all the derived classes will have. You want to do something that a Car can do, and you don't care if its a CrapCar , NiceCar or SuperNiceCar

Car car = new MyNiceCar();
car.honk(); //meep!
car = new SuperNiceCar();
car.honk(); //beep beep!

However, you can't go the other way around.

SuperNiceCar may support UseTurbo() , but MyNiceCar does not.

But you don't care, you just want the car to honk, so you cast it as a Car , because you know all Car s can honk.

See also

Answer 2

1 - What is this called?

This is a type of polymorphism that's called "inclusion polymorphism". It allows you to create restrictions on the type of variable that can be used in your code, while still allowing some type flexibility. Java and C#, for instance, have generics that allow various types and references to use the same code. However, these often run into run-time issues that could otherwise be caught by a static code analyzer. It's generally considered to be less risky to rely on the static analyzer (assuming your IDE has a good one) than to wait and find out your code has bugs in it after your release.

2 - And Why do it?

In addition to the reason I gave above, one of the most common applications of this is the polymorphic array.

Consider the following:

// Both classes contain an Accelerate() function
NiceCar MyNiceCar = new NiceCar();
EvenNicerCar MyEvenNicerCar = new EvenNicerCar();

// We're forced to use these methods separately
MyNiceCar.Accelerate();
MyEvenNicerCar.Accelerate();

////////////////////////////////////////////////////////////////////////////////////////////////////

// We can make this code much smaller and less prone to copy/paste errors using polymorphism!
Car[] MyPolymorphicArray = new Car[] { new NiceCar(), new EvenNicerCar() };
foreach(c in MyPolymorphicArray) { c.Accelerate(); }

Using this type of syntax, you can see that I'm able to make the code smaller, more-manageable, type-safe, and our static code analyzer will complain if we get it wrong, rather than waiting until runtime to find out. When you use this type of architecture, make sure you require the methods intended to be used by the instances to either be in an interface or in the parent class of that object in order to help avoid implementation errors. Additionally, take note that if you do this, the Accelerate() method may only need to be written in the parent class, which helps make the code even smaller.

Answer 3

Following on from two excellent answers from @Mars and @chris-mauer which both answered Question 2 of my post perfectly. And allowed me to study this concept further.

I want to include that the answer to Question 1 of my post as follows. I asked:

AbstractClass MyClass = new ConcreteClass();

1 - What is this called? (when you declare the abstract class on the left.)

From the article below, I believe the correct answer is:

Upcasting

For more on the overall concept, which is Inclusion Polymorphism, I studied the following article: http://www.msdotnet.co.in/2014/05/how-to-implement-polymorphism-concepts.html