CODEWITHSUNDEEP
c#.netlogic
ElizaG
ElizaG

Reputation: 39

How to solve this c# syntax puzzle?

Can someone give me the solution for the following code puzzle? A solution was given but could be more than one solution.

// this code compiles
IAnimal animal= new Dog();
animal.Eat();

// this code doesn't compile
Dog animal = new Dog();
animal.Eat();

How should the class definition look like so that the above code blocks, in separate scopes, compile(first one) and don't compile(second one) ?

Upvotes: 2

Views: 188

Answers (4)

Ivan Stoev
Ivan Stoev

Reputation: 205629

The obvious solution (IAnimal being interface and Dog being a class implementing that interface explicitly) is already provided in other answers.

Here are some other ways I can think of.

All they are based on IAnimal being a (poorly named) class like this

public class IAnimal
{
    public void Eat() { }
}

(A) Solution based on class defining an implicit conversion operator (note that defining a conversion from class to interface is prohibited, that's why it's important IAnimal to be a class/struct)

public class Dog
{
    public static implicit operator IAnimal(Dog dog) { return new IAnimal(); }
}

causes the following compile error in the second snippet:

Error CS1061: 'Dog' does not contain a definition for 'Eat' and no extension method 'Eat' accepting a first argument of type 'Dog' could be found (are you missing a using directive or an assembly reference?)

(B) Solutions based on base method hiding. Although in C# we can't hide the base method by providing in a derived class another method with different signature (because it will be treated as overload), we can do that by using a field, property or event with incompatible signature.

For instance

public class Dog : IAnimal
{
    new public event Action Eat;
}

generates

Error CS0070: The event 'Dog.Eat' can only appear on the left hand side of += or -= (except when used from within the type 'Dog')

or

public class Dog : IAnimal
{
    new public Action<int> Eat;
}

generates

Error CS7036: There is no argument given that corresponds to the required formal parameter 'obj' of 'Action'

Upvotes: 2

rory.ap
rory.ap

Reputation: 35280

The error on the second line is caused because the class Dog explicitly implements the IAnimal interface like this:

class Dog : IAnimal
{
    void IAnimal.Eat()
    {
        throw new NotImplementedException();
    }
}

In this case, you get the compile error on your second line because you cannot call Eat unless you're working with an instance of IAnimal because it's essentially a private method in Dog.

To fix the error, you either need to cast your instance back to an IAnimal like I showed you above, or you can implicitly implement the interface like this instead:

class Dog : IAnimal
{
    public void Eat()
    {
        throw new NotImplementedException();
    }
}

Upvotes: 3

Domysee
Domysee

Reputation: 12846

You can achieve this by using explicit interface implementation: 

class Dog : IAnimal {
    void IAnimal.Eat(){
        //do stuff
    }
}

As opposed to implicit interface implementation:

class Dog : IAnimal {
    public void Eat(){
        //do stuff
    }
}

Upvotes: 4

serhiyb
serhiyb

Reputation: 4833

public interface IAnimal
{
    void Eat();
}

public class Dog : IAnimal
{
    void IAnimal.Eat() { }
}



IAnimal animal = new Dog();
animal.Eat(); // <---- OK

Dog animal2 = new Dog();
animal2.Eat(); // <---- COMPILE ERROR

Upvotes: 3

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