在 C# 中，对于基础 class 的虚拟/抽象方法，在派生 class 中，覆盖修饰符是强制性的

Question

public class Shapes
{
  public virtual void Color()
  {
    Console.WriteLine("Shapes");
  }
}

public class Square : Shapes
{
  public void Color()
  {
    Console.WriteLine("Square");
  }
}

public class Test
{ 
  public static void main()
  {
    Shapes s1 = new Shapes();
    s1.Color();
    Shapes s2 = new Square();
    s2.Color();
  }
}

I want to know if the base class has defined some methods virtual, then is it mandatory to override them in derive class? Similarly, If the base class method is abstract, we need to implement the method in derive class, but is override modifier is mandatory? What if override modifier is omitted.

Answer 1

I want to know if the base class has defined some methods virtual, then is it mandatory to override them in derive class?

Definitely not. It is up to the derived class to decide whether or not to override the virtual method. Note that some classes will recommend that derived classes override a particular virtual method in some cases, but the compiler will not enforce that.

If the base class method is abstract, we need to implement the method in derive class, but is override modifier is mandatory?

Yes, you need to use the override keyword, otherwise the method will be hidden by the definition in the derived class.

If you actually run your code, you will see "Shapes" printed twice. That's because the Color() method in the Squares class isn't declared using the override keyword. As such, it hides the base class method. This means it will only be accessible through a variable of type Squares :

Sqares s3 = new Squares();
s3.Color();

This will print "Square", since you're calling the method on a variable of the correct type.

Answer 2

Take a look at this link , it may be useful.

A copy in that page:

To make a method virtual, the virtual modifier has to be used in the method declaration of the base class. The derived class can then override the base virtual method by using the override keyword or hide the virtual method in the base class by using the new keyword. If neither the override keyword nor the new keyword is specified, the compiler will issue a warning and the method in the derived class will hide the method in the base class

Answer 3

As you can see from the example code you posted, the output will be

Shapes
Shapes

That is because by omitting the override keyword you are by default hiding the base class method - that means the method cannot be used in a polymorphic fashion anymore - even though s2 is an instance of Square since the object reference used to call the method is Shapes , the Shapes method is used.

If the method is marked with override on the other hand it will be called even on object references of type Shape since the most derived Color() method in the inheritance tree for the given instance is used.

Answer 4

It is completely based on your logic in first part of your question, In Square class which is derived from Shapes class you declare Color method which will hide its parent virtual Color method, you don't need to duplicate same method in child class when it exists in its parent, except when you want to hide parent class's method. In abstract class if you have a virtual method, you have to implement or override them in derived classes, in you sample those Shapes's instances will invoke virtual Color method in Shapes class.