CODEWITHSUNDEEP
c#inheritanceinterface
Aaron Thomas
Aaron Thomas

Reputation: 5281

Interface hides parent class member

When a child class inherits from both a parent class and an interface, why can the child class not access a parent class method?

Consider the following, where I can't access the DoSomething() method.

class Program
{
    static void Main(string[] args)
    {
        IMyInterface myClass = null;
        myClass = new ChildClass();
        // this returns error
        myClass.DoSomething();
    }
}

internal class ParentClass
{
    public ParentClass() { }

    public void DoSomething() { }
}

internal class ChildClass : ParentClass, IMyInterface
{
    public string MyProperty { get; set; }

    public ChildClass() : base() { }
}

internal interface IMyInterface
{
    string MyProperty { get; set; }
}

I've looked through SO discussions here and here, but they seem to focus on how a member is hidden using new, override, and virtual keywords... sorry, but I can't figure out how that applies to this situation. Also, I've browsed through the MSDN API reference on interfaces here and here, with no luck.

Upvotes: 0

Views: 255

Answers (5)

MutantNinjaCodeMonkey
MutantNinjaCodeMonkey

Reputation: 1249

The best way to understand this is to know the difference between an interface and a parent/child class.

An interface is a contract that can exist on any class regardless of it's inheritance chain. You could put that interface on a class that doesn't inherit from ParentClass, and all that class has to fulfill is what is in your interface (in your case, the MyProperty property). If you added DoSomething() to the interface, this class would then also be required to have that method.

A subclass (child class) inheriting from a parent class has established a relationship. The parent class shares it's non-private methods/properties/member subset with it's child class. Therefore, you can cast a child class to it's parent class and retain accessibility to those properties.

Upvotes: 0

David
David

Reputation: 218818

Because the type of myClass is IMyInterface:

IMyInterface myClass = null;

and IMyInterface doesn't have a DoSomething() method:

internal interface IMyInterface
{
    string MyProperty { get; set; }
}

However, with polymorphism the type cal also be ParentClass or ChildClass. So you can use the method by morphing the type:

(myClass as ChildClass).DoSomething();

As with any time you cast or morph the type of an object, be careful for nulls. If the type can't be converted then myClass as ChildClass would be null, so the above would result in a NullReferenceException.

Upvotes: 0

Katana314
Katana314

Reputation: 8610

The problem here is specifically to do with the declaration of the variable in your Main method.

    IMyInterface myClass = null;
    myClass = new ChildClass();
    // this returns error
    myClass.DoSomething();

Taking lines in isolation, we can reduce it to just this.

    IMyInterface myClass = null;

    // BLAH BLAH myClass gets initialized somehow, we don't know/care how.

    myClass.DoSomething();

So at that point, we only know we have an initialized object of interface IMyInterface. In other words, taking that line alone, we don't know that it's a ChildClass. The only known method of IMyInterface is MyProperty, so that's the only thing we know is available to us.

You could fix this by declaring myClass specifically as a ChildClass instance. You can even return this variable in a method that expects to return an IMyInterface type.

Upvotes: 4

Renan Araújo
Renan Araújo

Reputation: 3631

DoSomething() is from ParentClass, and you are using an IMyInterface reference.

To use this method, you need to do a cast:

((ChildClass) myClass).DoSomething();

or

((ParentClass) myClass).DoSomething();

Upvotes: 1

Timothy Shields
Timothy Shields

Reputation: 79441

This does not work for a very straightforward reason:
IMyInterface does not have a DoSomething method.

If you modify your interface as follows, your code will work.

internal interface IMyInterface
{
    string MyProperty { get; set; }
    void DoSomething();
}

Upvotes: 2

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