Instantiating class with interfaces in c#

Question

Hi this might be trivial but I am trying to understand the class instantiation using interface. So below is my code:

public interface IRepository
{
    string GetMemberDisplayName();
}

public class Repository : IRepository
{
    private readonly Manager _manager;

    public Repository() {}

    public Repository(string connectionName)
    {
        _manager = new Manager(connectionName);
    }

    public string GetMemberDisplayName(string ID)
    {
        return "FooFoo";
    }
}

Now in another class that uses the repository class functionality has instantiated it as follows:

public class LogServiceHelper
{
    readonly IRepository _alrAttendance;
    readonly IRepository _alrUsers;

    public LogServiceHelper()
    {
        _alrAttendance = new Repository("value1");
        _alrUsers = new Repository("value2");
    }

    public string GetMemberName(string empId)
    {
        return _alrUsers.GetMemberDisplayName(empId);
    }
}

My question is if this is correct way to instantiate a class with parameterized constructor. And if yes, then second question is why do we need interface in this case. We could directly instantiate the class without creating the interface?

The interfaces are there so that you don't have new up concrete instances and can inject them instead. Right now your constructor is not parameterized. — JSteward
– JSteward, Commented Feb 6, 2019 at 21:58
This is of course fine but not the best practice. You should look at DI principle. — Johnny
– Johnny, Commented Feb 6, 2019 at 22:00

Johnny · Accepted Answer · 2019-02-06 22:15:34Z

2

Yes, that's how to invoke a parameterized constructor, but no, that's not what you should be doing.

As you have it, LogServiceHelper has a hard dependency on the Repository class, and so you are right, the interfaces don't buy you anything. However, if they were injected:

public LogServiceHelper(IRepository attendanceRepo, IRepository userRepo)
{
    _alrAttendance = attendanceRepo;
    _alrUsers = userRepo;
}

You suddenly gain the benefits of abstraction. Notably that a unit test could pass in fake repositories, and that you can switch to another implementation of IRepository without changing LogServiceHelper.

The next question is then "Who creates the Repository concrete class?". For that, I refer you to the variety of DI/IoC containers out there, such as Autofac, Unity, and NInject.

edited Feb 6, 2019 at 22:15

Johnny

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answered Feb 6, 2019 at 22:06

BradleyDotNET

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Comments

Johnny · Accepted Answer · 2019-02-06 22:16:57Z

We could directly instantiate the class without creating the interface?

This is indeed true and it might work without any problem. But then we are coming to the crucial questions:

Is my code testable, how would I test this piece of code?
Could I easily change the behavior without changing LogServiceHelper

If you don't rely on the abstraction, the answer to the above questions is unfortunately, no. Luckily there is something called SOLID and the D stands for Dependency Injection Principle.

public LogServiceHelper(IRepository alrAttendance, IRepository alrUsers)
{
    _alrAttendance = alrAttendance;
    _alrUsers = alrUsers;
}

So, with this simple change, you made decoupling of the moduls and all of a sudden you are relying on the abstraction which gains a lot of benefits to design.

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