C＃错误CS0266：无法将类型“狗”隐式转换为“ Rottweiler”

Question

I was trying the following code from a book when the error occured. I am geussing it is generated because of wrong use of as keyword. Please, help me fix this error. This code is an example for subclassing. This code generates two errors (cs0266). The error generating lines are in the Main method and are marked with a comment above the line.

class Program
{
    static void Main(string[] args)
    {
        // CS0266: Cannot implicitly convert type 'Dog' to 'Rottweiler
        Rottweiler butch = new Rottweiler("Butch") as Dog;

        // CS0266: Cannot implicitly convert type 'Dog' to 'Spaniel
        Spaniel mac = new Spaniel("Mac", "yips") as Dog;

        butch.Bark();
        mac.Bark();
        butch.DoYourThing();
        mac.DoYourThing();
    }
}
class Dog
{
    protected string _name;
    protected string _sound;

    public Dog(string name)
    {
        _name = name; _sound = "barks";
    }

    public Dog(string name, string sound)
    {
        _name = name;
        _sound = sound;
    }

    public void Bark()
    {
        Console.WriteLine("{0} {1} at you", _name, _sound);
    }

    public virtual void DoYourThing()
    {
    }
}

class Rottweiler : Dog
{
    public Rottweiler(string name) : base(name) { }
    public Rottweiler(string name, string sound) : base(name, sound) { }

    public override void DoYourThing()
    {
        Console.WriteLine("{0} snarls at you, in a very menacing fashion!", _name);
    }
}
class Spaniel : Dog
{
    public Spaniel(string name) : base(name) { }
    public Spaniel(string name, string sound) : base(name, sound) { }

    public override void DoYourThing()
    {
        Console.WriteLine("{0} drools all over you, then licks you into submission!", _name);
    }

}

Answer 1

While you can cast Spaniel to Dog , you can't do it the other way round. So this code:

Spaniel mac = new Spaniel("Mac", "yips") as Dog;

Is casting to Dog and then attempting to store that in a Spaniel variable. You can however do this:

Dog mac = new Spaniel("Mac", "yips") as Dog;

Also as mentioned by @leppie, the as Dog cast is not needed as there exists an implicit cast from a derived class to it's base:

Dog mac = new Spaniel("Mac", "yips");

Answer 2

Well, here: Rottweiler butch = new Rottweiler("Butch") as Dog; you're creating instance of Rottweiler and casting it to the Dog .

Still ok for now, but then you're assigning instance of Dog to the variable of type Rottweiler - but this is impossible.

Since Rottweiler is inherited from Dog , every Rottweiler is Dog , but not every Dog is Rottweiler - thus this implicit cast during assignment can't be done.

So either remove cast to Dog

Rottweiler butch = new Rottweiler("Butch");

or change type of butch to Dog :

Dog butch = new Rottweiler("Butch");

Notice in this case you also don't need explicit cast ... as Dog , it will be done implicitly with assignment;