Is it possible to access the other members of an interface type?

Question

I'm curious if this is possible, and if not, what are the reasons behind it if any, and how would one handle this programming scenario?

Let's say i have this interface:

public interface IBook
{
    void BookClient();
    void CancelClient();
}

and i have this class that implements the above interface:

public class Photographer : IBook
{
     public void BookClient()
     {
        // do something
     }

     public void CancelClient()
     {
        // do something
     }

     // non-interface methods
     public void SaveClients()
     {
        // do something
     }

     public void DeleteClients()
     {
        // do something
     }
} 

Now if I assign this class to an interface type somewhere in my code such as:

IBook photo;
photo = new Photographer();

Is it possible to do this:

// non-interface member from the Photographer class
photo.SaveClients();

Can someone straighten me out on this issue and perhaps point me in the right direction regarding this. Thanks.

Answer 1

Yes, it's possible, but you have to cast your photo into Photographer first:

// non-interface member from the Photographer class
((Photographer)photo).SaveClients();

It's not possible with just photo.SaveClients() syntax, because you can easily create another class:

class TestClass : IBook
{
    public void BookClient()
    {
       // do something
    }
     public void CancelClient()
    {
       // do something
    }
}

And use it like that:

IBook photo;
photo = new Photographer();
photo = new TestClass();

// what should happen here?
photo.SaveClients();

So as long as you use the variable as an interface implementation, you can only access member declared in that interface. However, the object is still a class instance, so you can use other class members, but you have to explicitly cast to that type first.

Answer 2

Interface types can only reference interface members. You're attempting to invoke a member of the class which is not part of the interface.

You can try (can't test right now):

IBook photo;
photo = new Photographer();

(photo as Photographer).SaveClients();
// or
((Photographer)photo).SaveClients();

Answer 3

Yes, it's possible, using reflection. Also you can cast to Photographer and then call SaveClients() .

In my opinion a good solution is defining each group of actions you need to call in an interface, then, cast to that interface, and call the method you need:

public interface IBook
{
    void BookClient();
    void CancelClient();
}

public interface IClient
{
    void SaveClients();
}

And then use as:

IBook photo = new Photographer();
// now cast photo object as a IClient
IClient client = photo as IClient;
if (client != null)
{
    client.SaveClients();
}