Question

A simple interview question.
How do interfaces help with code reusablity?

Answer 1

An interface decouples the consumer from the implementation details of a class. This helps enables reusability because the class implementing an interface can change, without needing to change the code consuming the implementation.

That’s very confusing, maybe an example helps

public interface IUserAuthentication
{
   public bool Authenticate(string name, string password);
}

Now I will write the consumer, it doesn’t care how authentication is performed, it simply knows that it can authenticate users.

public class Consumer
{
  private IUserAutentication _auth;
  public Consumer(IUserAuthentication auth) 
  {
   _auth = auth; 
  }

  public void Working(string username, string password) 
  {
    if (!_auth.Authenticate(username, password)) 
    {
       throw new Exception("error!");
    }
  }
}

The code above will work regardless of the implementation of the IUserAuthentication service. This is one way to reuse code.

Now I can implement the IUserAuthentication interface

public class AuthenticateJasons : IUserAuthentication
{
  public bool Authenticate(string username, string password) 
  {
    return username == "Jason";
  }
}

public class AuthenticateNoone: IUserAuthentication
{
  public bool Authenticate(string username, string password) 
  {
    return false;
  }
}

The point is that these implementations are irrelevant as far as the consumer is concerned. Also, this question is not related to ASP.NET the web framework. This is really a language/platform/framework agnostic question. The answer is the same regardless of the language you choose to implement with.