在C＃中实例化泛型类

Question

In the piece of code below, the output is;

Ernie Bert Elmo

Why is the last output is Elmo? Shouldn't it be Ernie? Because i instantiate dog.Creature object with new Cat(); . i thought that the Name property of Cat class overrides the Name property of Creature class.

class Class1
{
    public static void Main(string[] args)
    {
        var dog = new Dog();
        var cat = new Cat();
        dog.Creature = new Cat();

        Console.WriteLine(cat.Name); //outputs Ernie
        Console.WriteLine(dog.Name); //outputs Bert 
        Console.WriteLine(dog.Creature.Name); //outputs Elmo, why not Ernie?
        Console.Read();
    }
}
public class Animal<T> where T : Creature
{
    public T Creature { get; set; }
    private string _name = "Oscar";
    public string Name { get { return _name; } set { _name = value; } }
}
public class Creature
{
    private string _name = "Elmo";
    public string Name { get { return _name; } set { _name = value; } }
}
public class Cat : Creature
{
    private string _name = "Ernie";
    public string Name { get { return _name; } set { _name = value; } }
}
public class Dog : Animal<Creature>
{
    private string _name = "Bert";
    public string Name { get { return _name; } set { _name = value; } }
}

Answer 1

Firstly, this has nothing to do with generics. You'll get exactly the same behaviour if you write:

Creature cat = new Cat();
Console.WriteLine(cat.Name);

Don't forget that the compile-time type of Dog.Creature is Creature .

i thought that the Name property of Cat class overrides the Name property of Creature class.

No, because it's not a virtual property and you haven't used override . You should have a compile-time warning explicitly saying that you're hiding the member, not overriding it:

Test.cs(30,19): warning CS0108: 'Cat.Name' hides inherited member
        'Creature.Name'. Use the new keyword if hiding was intended.

If you change Cat and Creature to be like this, it will work:

public class Creature
{
    private string _name = "Elmo";
    public virtual string Name { get { return _name; } set { _name = value; } }
}

public class Cat : Creature
{
    private string _name = "Ernie";
    public override string Name { get { return _name; } set { _name = value; } }
}

... but personally I'd try to avoid this anyway. Why not just use the setter in Creature ? Why would you want there to be two _name fields in a Cat object? It's not like they're meant to serve different purposes, is it?

It's not clear what you're trying to achieve, but I'd almost certainly give the Creature class a constructor taking the name as a parameter. I'd probably make the property read-only if at all possible, too.

Answer 2

I thought that the Name property of Cat class overrides the Name property of Creature class

No, it hides it. You have two different properties with the same name. If you want to override it, you need to make the base property virtual and use the override keyword on the inherited class:

public class Creature
{
    private string _name = "Elmo";
    public virtual string Name { get { return _name; } set { _name = value; } }
}
public class Cat : Creature
{
    private string _name = "Ernie";
    public override string Name { get { return _name; } set { _name = value; } }
}

As to why you are seeing "Elmo" instead of "Ernie" - it's because Dog is an Animal<Creature> , so the property Dog.Creature is of type Creature . Even though you pass in a Cat , since the Name property is not overridden , Creature.Name is called. If you had overrode Name , then Cat.Name would be called instead.

Answer 3

A good practice is to read the warnings the IDE gives you.

warning CS0108: 'Program.Cat.Name' hides inherited member 'Program.Creature.Name'. Use the new keyword if hiding was intended.
warning CS0108: 'Program.Dog.Name' hides inherited member 'Program.Animal<Program.Creature>.Name'. Use the new keyword if hiding was intended.