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SOLID (4/5) - Interface segregation principle

Interface segregation principle


In the field of software engineering, the interface-segregation principle (ISP) states that no client should be forced to depend on methods it does not use. ISP splits interfaces that are very large into smaller and more specific ones so that clients will only have to know about the methods that are of interest to them. Such shrunken interfaces are also called role interfaces. ISP is intended to keep a system decoupled and thus easier to refactor, change, and redeploy.

using System;

namespace interfacesegregation
{
    public class Document 
    {

    }

    public interface IMachine 
    {
        void Print(Document d);
        void Scan(Document d);
        void Fax(Document d);
    }

    public class MultiFunctionPrinter : IMachine 
    {
        public void Print(Document d) 
        {
            //
        }

        public void Scan(Document d)
        {
            //
        }

        public void Fax(Document d)
        {
            //
        }
    }

    // The interface has too much functionaly that won't be used
    public class Printer : IMachine 
    {
        public void Print(Document d) 
        {
            //
        }

        public void Scan(Document d)
        {
            throw new NotImplementedException();
        }

        public void Fax(Document d)
        {
            throw new NotImplementedException();
        }
    }    

    // We should separate the interface into smaller ones 
    public interface IPrinter 
    {
        void Print(Document d);
    }

    public interface IScanner 
    {
        void Scan(Document d);
    }

    public class Photocopier : IPrinter, IScanner
    {
        public void Print(Document d) 
        {
            //
        }

        public void Scan(Document d)
        {
            //
        }
    }

    public interface IMultiFunctionDevice : IPrinter, IScanner
    {        
    }

    public class MultiFunctionMachine : IMultiFunctionDevice
    {
        public void Print(Document d) 
        {
            Console.WriteLine("Printing...");
        }

        public void Scan(Document d)
        {
            Console.WriteLine("Scanning...");
        }
    }


    public class Program
    {
        static void Main(string[] args)
        {
            var machine = new MultiFunctionMachine();
            machine.Scan(new Document());
            machine.Print(new Document());
        }
    }
}

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