Interfaces in C#

//003: Extended Interface

public interface INokiaNew: INokiaOld

{

void SendMMS_AVI();

}

When a Class implements this new Interface INokiaNew, it should give basic functionality expected by INokiaOld plus Send the MMS in AVI format expected by INokiaNew.

Below is the class.

using System;

class Program
{
    static void Main(string[] args)
    {
        // Usage 003:
        MyNokiaNewPhone phone2 = new MyNokiaNewPhone();

        INokiaOld iface_basic = phone2;
        INokiaNew iface_delux = phone2;

        iface_basic.SendSMS(); // You can't send MMS as it is a basic interface
        iface_delux.SendSMS(); // You can access basic functionality
        iface_delux.SendMMS_AVI(); // You can access extended functionality also
    }
}

class MyNokiaNewPhone : INokiaNew
{
    public void SomeFunction1()
    {
        Console.WriteLine("Some Function 1");
    }

    public void SendSMS()
    {
        Console.WriteLine("SMS Sent");
    }

    public void HoldCall()
    {
        Console.WriteLine("Call Temporarily on Hold");
    }

    public void SendMMS_AVI()
    {
        Console.WriteLine("MMS Sent in AVI Format");
    }
}

public interface INokiaOld
{
    void SendSMS();
    void HoldCall();
}

public interface INokiaNew : INokiaOld
{
    void SendMMS_AVI();
}

Note that iface_basic can only access the functionality of the basic interface, which is nothing but Sending an SMS and Holding the call. But, through iface_delux, you access all three functionality; that is, two basic (SMS, Call Hold) and one extended (MMS).

Implementing More than One Interface [INokiaNew, ISamSung ]

Let Us Assume that below is the ISamSung interface contract.

using System;

public interface ISamsung
{
    void SendMMS_Mpeg();
    // Commented the Below Function as per Balaji's reply.
    // void PlayMusic();
}

class HybridPhone : INokiaOld, ISamsung
{
    public void SendSMS()
    {
        Console.WriteLine("SMS Sent");
    }

    public void HoldCall()
    {
        Console.WriteLine("Call Temporarily on Hold");
    }

    public void SendMMS_Mpeg()
    {
        Console.WriteLine("MMS Sent in MPeg format");
    }
}

class Program
{
    static void Main(string[] args)
    {
        HybridPhone phone = new HybridPhone();
        INokiaOld nokiaInterface = phone;
        ISamsung samsungInterface = phone;

        nokiaInterface.SendSMS();
        nokiaInterface.HoldCall();
        samsungInterface.SendMMS_Mpeg();
    }
}

public interface INokiaOld
{
    void SendSMS();
    void HoldCall();
}

The client code now can access the Samsung functionality as well as basic Nokia functionality from the Class object HybridPhone. Below is the code that will demonstrate this.

HybridPhone phone3 = new HybridPhone();

// 004_1: Use Nokia Interface
INokiaOld iface_nokia = phone3;
iface_nokia.SendSMS();

// 004_2: Use Samsung interface
ISamsung iface_samsung = phone3;

Note how the interface functionality is extracted from the same phone object using different interface types. You can not get the Samsung functionality through the INokiaOld interface and the Basic Nokia functionality through the SamSung interface.

Casting the Interface - Is versus As

Look at the below code.

HybridPhone phone3 = new HybridPhone();

INokiaOld iface_nokia = phone3;

// 005_1: Check the Type using is keyword
if (iface_nokia is ISamsung)
{
    // 005_2: Cast it to the other interface type
    ISamsung iface_samsung1 = (ISamsung)iface_nokia;
    iface_samsung1.SendMMS_Mpeg();
}

In the above code, first, we are checking iface_nokia is Samsung type. The first look will get you to give the answer NO. But, note that, here if you read it like "Does the object pointed by the interface type INokiaOld is ISamSung as well". You do say Yes now. Look at how (005_2) casting the object pointed by iface_nokia is done to ISamSung. Once the Casting is done, you can access the functionality of Samsung.

HybridPhone phone3 = new HybridPhone();

INokiaOld iface_nokia = phone3;

// 005_3: Cast and Assign
ISamsung iface_samsung2 = iface_nokia as ISamsung;
iface_samsung2?.SendMMS_Mpeg(); // Using the null conditional operator for safety

If you look at the above code, the 'As' keyword also does the same job. When Object is not ISamSung we will get NULL assigned to iface_samsung2.