Interface in C#: Part 1

Interface interface_name
{
}

interface IVehicle
{
     double Speed(int distance, int hours)
     {
         double speed = 0.0d;
         speed = distance / hours;
         return speed;
     }
 }

interface IVehicle
{
    public double Speed(int distance, int hours);       
}

private interface IVehicle
{
    double Speed(int distance, int hours);
}

Example: An interface IVehicle that has two fields, one speed (not initialized) and another hours (initialized).

interface IVehicle
{
    double speed;
    decimal hours = 0.0m;
}

We execute the above code and get two errors, the same error for each field; see:

Error : Interfaces cannot contain fields

So we can't declare a field in the interface.

5. When an interface is inherited by a class or a struct then need to implement all members of the interface in that class. An interface is inherited by a colon (:) sign on the class or struct.

Example

An interface "IVehicle" that has methods "Speed()" and "IVehicle" inherited by the Vehicle class.

interface IVehicle
{
    double Speed(int distance, int hours);       
}
class Vehicle:IVehicle
{       
}

We execute the above code and get an error such as:

Error : 'Vehicle' does not implement interface member 'IVehicle.Speed(int, int)'

So we should implement all members (method, properties etc) of the interface in classes that inherit an interface.

6. Should use a public access specifier for inherited members from an interface to a class. By default all members are public in interfaces but when we implement a method in a class we need to define it as public.

Example

An interface "IVehicle" that has a method "Speed()" signature and a class "Vehicle" that inherits the "IVehicle" interface.

interface IVehicle
{
    double Speed(int distance, int hours);
} 

class Vehicle:IVehicle
{
    double Speed(int distance, int hours)
    {
        double speed = 0.0d;
        speed = distance / hours;
        return speed;
     }
}

We execute the above code and get an error such as:

Error : 'Vehicle' does not implement interface member 'IVehicle.Speed(int, int)'. 'Vehicle.Speed(int, int)' cannot implement an interface member because it is not public.

So we need to use a public access specifier when we implement an inherited interface member in a class because all members are public in an interface.

7. We can't create an instance of an interface but we can declare a variable of a particular type interface.

Example: An interface "IVehicle" and a class "Vehicle" that inherits the "IVehicle" interface. See:

interface IVehicle
{       
} 

class Vehicle:IVehicle
{   
    IVehicle veh = new IVehicle();          
}

We execute the above code and get an error such as:

Error: Cannot create an instance of the abstract class or interface 'IVehicle'

So we can't create an instance of the interface as well as abstract class because an interface is always inherited but can create a variable of an interface type.

How can implement an Interface on a class

In the following code the "IVehicle" interface has a "Speed()" method signature and its inherited by the "Vehicle" class so the speed() method implementation is in the derived class. After that in the "Program" class a "Vehicle" instance calls the "Speed()" method:

using System;

namespace InterfaceExamples
 {
     interface IVehicle
    {
        double Speed(int distance, int hours);
    }
     class Vehicle : IVehicle
    {
        public double Speed(int distance, int hours)
        {
            double speed = 0.0d;
            speed = distance / hours;
            return speed;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            int distance =500,hours=2;
            double speed = 0.0;
            Vehicle objVehicle = new Vehicle();
            speed = objVehicle.Speed(distance, hours);
            Console.WriteLine("speed is {0}", speed);
            Console.Read();
        }       
    }
}