In this part of my C# tutorial we’ll look at Interfaces using both a simple example and another that is rather complex. We’ll also look at Abstract Methods, Inheriting an Interface, How to Create Flexible Designs, and the Command Design Pattern.

For best results take notes on the cheat sheet provided below as you watch and leave any questions you have.

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Code & Transcript

// ---------- IDrivable.cs ----------

namespace CSharpTutA.cs
{
    // An interface is a class with nothing but
    // abstract methods. Interfaces are used
    // to represent a contract an object may
    // decide to support.

    // Interfaces commonly have names that
    // are adjectives because adjectives
    // modify nouns (Objects). The also
    // describe abstract things

    // It is common to prefix your interfaces with 
    // an I

    interface IDrivable
    {
        // Interfaces can have properties
        int Wheels { get; set; }
        double Speed { get; set; }

        // Classes that inherit an interface
        // must fulfill the contract and 
        // implement every abstract method
        void Move();
        void Stop();
    }
}

// ---------- Vehicle.cs ----------

using System;

namespace CSharpTutA.cs
{
    class Vehicle : IDrivable
    {
        // Vehicle properties
        public string Brand { get; set; }

        public Vehicle(string brand = "No Brand",
            int wheels = 0,
            double speed = 0)
        {
            Brand = brand;
            Wheels = wheels;
            Speed = speed;
        }

        // Properties and methods from
        // the interface
        public double Speed {get; set;}

        public int Wheels {get; set;}

        public void Move()
        {
            Console.WriteLine($"The {Brand} Moves Forward at {Speed} MPH");
        }

        public void Stop()
        {
            Console.WriteLine($"The {Brand} Stops");
            Speed = 0;
        }
    }
}

// ---------- Program.cs ----------

using System;

// A class can support multiple interfaces. 

// Create an interface Project -> Add New Item
// and select Interface

namespace CSharpTutA.cs
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create a Vehicle object
            Vehicle buick = new Vehicle("Buick",
                4, 160);

            // Check if Vehicle implements 
            // IDrivable
            if(buick is IDrivable)
            {
                buick.Move();
                buick.Stop();
            } else
            {
                Console.WriteLine("The {0} can't be driven", buick.Brand);
            }

            // We are now modeling the act of
            // picking up a remote, aiming it
            // at the TV, clicking the power
            // button and then watching as
            // the TV turns on and off

            // Pick up the TV remote
            IElectronicDevice TV = TVRemote.GetDevice();

            // Create the power button
            PowerButton powBut = new PowerButton(TV);

            // Turn the TV on and off with each 
            // press
            powBut.Execute();
            powBut.Undo();

            Console.ReadLine();

        }
        
    }

}

// ---------- IElectronicDevice.cs ----------

namespace CSharpTutA.cs
{
    // With interfaces you can create very
    // flexible systems. Here I'll model
    // generic electronic devices like 
    // TVs and Radios and remotes that 
    // control them and the buttons on the
    // remotes. 
    interface IElectronicDevice
    {
        // We want each device to have 
        // these capabilities
        void On();
        void Off();
        void VolumeUp();
        void VolumeDown();

    }
}

// ---------- Television.cs ----------

using System;

namespace CSharpTutA.cs
{
    // Because we implemented the 
    // ElectronicDevice interface any
    // other device we create will know
    // exactly how to interface with it.
    class Television : IElectronicDevice
    {
        public int Volume { get; set; }

        public void Off()
        {
            Console.WriteLine("The TV is Off");
        }

        public void On()
        {
            Console.WriteLine("The TV is On");
        }

        public void VolumeDown()
        {
            if (Volume != 0) Volume--;
            Console.WriteLine($"The TV Volume is at {Volume}");
        }

        public void VolumeUp()
        {
            if (Volume != 100) Volume++;
            Console.WriteLine($"The TV Volume is at {Volume}");
        }
    }
}

// ---------- ICommand.cs ----------

namespace CSharpTutA.cs
{
    interface ICommand
    {
        // We can model what happens when
        // a button is pressed for example
        // a power button. By breaking
        // everything down we can add
        // an infinite amount of flexibility
        void Execute();
        void Undo();
    }
}

// ---------- PowerButton.cs ----------

namespace CSharpTutA.cs
{
    class PowerButton : ICommand
    {
        // You can refer to instances using
        // the interface
        IElectronicDevice device;

        // Now we get into the specifics of
        // what happens when the power button
        // is pressed.
        public PowerButton(IElectronicDevice device)
        {
            this.device = device;
        }

        public void Execute()
        {
            device.On();
        }

        // You can provide a way to undo
        // an action just like the power 
        // button does on a remote
        public void Undo()
        {
            device.Off();
        }
    }
}

// ---------- TVRemote.cs ----------

namespace CSharpTutA.cs
{
    class TVRemote
    {
        // Now we are modeling the action of
        // picking up the remote with our hand
        public static IElectronicDevice GetDevice()
        {
            return new Television();
        }
    }
}