Interview FAQ on Interface in C#

What is Interface?

 An interface is collection of methods, properties, indexers and events with only signatures. Means In interface Methods, properties, indexers and events do not have their definition. So an Interface allows you to define behavioral characteristics and apply those behaviours to Class that implements this Interface. Example

public interface IHoliDay

    {

        List<DateTime> GetListOfHoliday();

    }

Or

interface IHoliDay

    {

          List<DateTime> GetListOfHoliday();

    }

If you use public List<DateTime> GetListOfHoliday() an error occurred.

Error  1      The modifier 'public' is not valid for this item       .

So there is no need of modifiers for Interface members. Interface members are automatically public and cannot be static.

Why Interface is came in existence?

 Interface is mainly used for two reasons

·         C# does not support multiple inheritances, so by using Interface, A class can implement multiple interfaces to achieve multiple inheritances.

public class HoliDay : Class1, Interface1,Interface2,Interface3

    {

    }

·         Data Hiding

You can call class members using Interface.

class Program

    {

        static void Main(string[] args)

        {

            //Hiding the Holiday class object

            IHoliday ii = new Holiday();

            ii.GetHoliDayList();

            Console.WriteLine(ii.val);

        }

    }

    public interface IHoliday

    {

        string val{get; set;}

        List<DateTime> GetHoliDayList();

       

    }

    public class Holiday : IHoliday

    {

        public String val{ get; set;}

        List<DateTime> hlist = new List<DateTime>();

        public  List<DateTime> GetHoliDayList()

        {

            //code

            val = "Class Members";

            Console.WriteLine("Calling by interface");

            return hlist;

        }

    }

Interface also used to make your code more reusable, maintainable, and scalable and provides the concept of component based programming.

Suppose you have two interfaces with method with same name. How to implement both in same class? 

User Intafacename.MethodName()

{

}

Example:

class Program

    {

        static void Main(string[] args)

        {

            //Hiding the Holiday class object

            IOfficeHoliday iioffice = new Holiday();

            iioffice.GetHoliDayList();

            IGuzzatedHoliday iiGuzzated = new Holiday();

            iiGuzzated.GetHoliDayList();

            //Console.WriteLine(ii.val);

        }

    }

    //first interface

    public interface IOfficeHoliday

    {

     

        List<DateTime> GetHoliDayList();

       

    }

    //second interface

    public interface IGuzzatedHoliday

    {

      

        List<DateTime> GetHoliDayList();

    }

    public class Holiday : IOfficeHoliday, IGuzzatedHoliday

    {

      

        List<DateTime> hlist = new List<DateTime>();

        //Method definintion for first interface

         List<DateTime> IGuzzatedHoliday.GetHoliDayList()

        {

            //code

            Console.WriteLine("IGuzzatedHoliday Method");

            return hlist;

        }

        //Method definintion for second interface

         List<DateTime> IOfficeHoliday.GetHoliDayList()

        {

            //code

            Console.WriteLine("IOfficeHoliday Method");

            return hlist;

        }

    }

//here if you use public with method definition in class that implements both of interfaces, an error occurred.

What is the use of is and as operator with respect to Interface concept?

 The is and as operators can be used to determine whether an interface is implemented by an object.

Holiday hh = new Holiday();

            IHoliday iholiday = hh as IHoliday;

            if (hh is IHoliday)

            {

                Console.WriteLine("Holiday class implemented IHoilday");

            }

Comments if you have other possible interview FAQ for Interface in C#.