Interface Segregation Principle

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In this session we will discuss  

• Interface Segregation Principle
• Will look at a Case Study of Interface Segregation Principle
• And will implement Interface Segregation Principle with a simple example

In the first session of SOLID Introduction we have understood that "I" in SOLID is an acronym for Interface Segregation Principle

• The interface-segregation principle (ISP) states that "no client should be forced to depend on methods it does not use".
• This means, instead of one fat interface many small interfaces are preferred based on groups of methods with each one serving one sub-module.
• The ISP was first used and formulated by Robert C. Martin while consulting for Xerox. 

Let us now understand how the ISP was evolved with a case study.

Case Study

Problem

• As we all know Xerox Corporation manufactures printer systems. In their development process of new systems Xerox had created a new printer system that could perform a variety of tasks such as stapling and faxing along with the regular printing task. 
• The software for this system was created from the ground up. 
• As the software grew for Xerox, making modifications became more and more difficult so that even the smallest change would take a redeployment cycle of an hour, which made development nearly impossible.
• The design problem was that a single Job class was used by almost all of the tasks. Whenever a print job or a stapling job needed to be performed, a call was made to the Job class. 
• This resulted in a 'fat' class with multitudes of methods specific to a variety of different clients. 

Because of this design, a staple job would know about all the methods of the print job, even though there was no use for them.

Solution

• To overcome this problem Robert C Martin suggested a solution which is called the Interface Segregation Principle. 
• Which means, Instead of one fat interface many small interfaces are preferred based on groups of methods with each one serving one sub-module.

Below example demonstrates how we can achieve Single Responsibility Principle

Code before Interface Segregation Principle.

namespace ISPDemoConsole

{

    public interface IPrintTasks

    {

        bool PrintContent(string content);

        bool ScanContent(string content);

        bool FaxContent(string content);

        bool PhotoCopyContent(string content);

        bool PrintDuplexContent(string content);

    }

}

namespace ISPDemoConsole.Client

{

    class HPLaserJet : IPrintTasks

    {

        public bool FaxContent(string content)

        {

            Console.WriteLine("Fax Done"); return true;

        }

        public bool PhotoCopyContent(string content)

        {

            Console.WriteLine("PhotoCopy Done"); return true;

        }

        public bool PrintContent(string content)

        {

            Console.WriteLine("Print Done"); return true;

        }

        public bool PrintDuplexContent(string content)

        {

            Console.WriteLine("Print Duplex Done"); return true;

        }

        public bool ScanContent(string content)

        {

            Console.WriteLine("Scan Done"); return true;

        }

    }

}

namespace ISPDemoConsole.Client

{

    class CannonMG2470 : IPrintTasks

    {

        public bool PhotoCopyContent(string content)

        {

            Console.WriteLine("PhotoCopy Done"); return true;

        }

        public bool PrintContent(string content)

        {

            Console.WriteLine("Print Done"); return true;

        }

        public bool ScanContent(string content)

        {

            Console.WriteLine("Scan Done"); return true;

        }

        public bool PrintDuplexContent(string content)

        {

            return false;

        }

        public bool FaxContent(string content)

        {

            return false;

        }

     }

}

Code after Interface Segregation Principle

namespace ISPDemoConsole

{

    interface IPrintScanContent

    {

        bool PrintContent(string content);

        bool ScanContent(string content);

        bool PhotoCopyContent(string content);

    }

    interface IFaxContent

    {

        bool FaxContent(string content);

    }

    interface IPrintDuplex

    {

        bool PrintDuplexContent(string content);

    }

} 

namespace ISPDemoConsole.Client

{

    class HPLaserJet : IPrintScanContent, IFaxContent, IPrintDuplex

    {

        public bool FaxContent(string content)

        {

            Console.WriteLine("Fax Done"); return true;

        }

        public bool PhotoCopyContent(string content)

        {

            Console.WriteLine("PhotoCopy Done"); return true;

        }

        public bool PrintContent(string content)

        {

            Console.WriteLine("Print Done"); return true;

        }

        public bool PrintDuplexContent(string content)

        {

            Console.WriteLine("Print Duplex Done"); return true;

        }

        public bool ScanContent(string content)

        {

            Console.WriteLine("Scan Done"); return true;

        }

    }

}

namespace ISPDemoConsole.Client

{

    class CannonMG2470 : IPrintScanContent

    {

        public bool PhotoCopyContent(string content)

        {

            Console.WriteLine("PhotoCopy Done");

            return true;

        }

        public bool PrintContent(string content)

        {

            Console.WriteLine("Print Done");

            return true;

        }

        public bool ScanContent(string content)

        {

            Console.WriteLine("Scan Done");

            return true;

        }

    }

}

Also, we strongly recommend you to refer to our design pattern tutorial for more details on creational design patterns 

I believe this session has given you a good idea on how we can implement Interface segregation principle. 

In our next video we will focus on Open closed principle.