c# what is the use of using Interface and class in controller Constructor

Question

In one of my interviews I face this problem as why you use your interface as below mention

Interface

Interface Irepo{
    void add(int a, int b);
}

Repo.cs

Public Class Repo:Irepo{
    public  void add(int a, int b){
}

HomeContrller

Public Clas HomeController:ApiController{

    Private readonly Irepo _Iobjrepo;
    public HomeController(){
        _Iobjrepo =new Repo();
    }
}

Please help me why to use Irepo in constructor of HomeController. What is its use?

I think this is because it's unclear what you asking for. We can't say why do **you** use **your** interfaces like this — vasily.sib, Oct 03 '19 at 05:06
Please, Take a look at the answer to the question [Creating objects from an interface in C#](https://stackoverflow.com/a/2186943/4977870) — Anas Alweish, Oct 03 '19 at 05:07
also, there is a lack of formatting and a lot of typos in your code, so it is almost unreadable — vasily.sib, Oct 03 '19 at 05:09
Because you can `_Iobjrepo = new AnotherRepo()` and not break existing code that relies on it, for one reason. — Richard Barker, Oct 03 '19 at 05:10
`public HomeController(Irepo repo)` you can use interface for "dependency injection" — TimChang, Oct 03 '19 at 05:38

Stefan · Answer 1 · 2019-10-03T06:50:14.303

Simple example of using interfaces which hopefully explains it a bit:

Interface

interface IAnimal
{
    bool HasFur;
}

Interface implementations

class Cat : IAnimal
{
    public bool HasFur => true;

    public Cat()
    {
    }
}

class Fish : IAnimal
{
    public bool HasFur => false;

    public FIsh()
    {
    }
}

Usage

IAnimal anyTypeOfAnimal = new Cat();
IAnimal anotherAnimal = new Fish();

public void TestInterface(IAnimal obj)
{
     Console.Log(obj.HasFur);
}

TestInterface(anyTypeOfAnimal.HasFur); //True
TestInterface(anotherAnimal.HasFur); //False

If you declare a variable of type IAnimal you can store any type that implements the interface. This can come in handy when you don't now which type of IAnimal you are getting and it doens't because in this example you just want to know if the animals have fur.

score 1 · Answer 2 · answered Oct 03 '19 at 06:26

In this context, one reason would be improve testability. Later when you write unit tests for your Home Controller, you do not want to use the actual Repo object inside Home controller. Because you don't want to read/write to the actual database when executing unit tests. The Interface IRepo will allow you to inject a "fake" or mock of the actual repo.

c# what is the use of using Interface and class in controller Constructor

2 Answers2