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SOLID-Open-closed principle (OCP) Real-Time Example in C#

The SOLID Principles are the design principles that enable us to manage several software design problems. These principles provide us with ways to move from tightly coupled code to loosely coupled and encapsulated real business needs properly. Also readable, adaptable, and scalable code.

The SOLID Principles guide developers as they write readable, adaptable, and scalable code or design an application.

The SOLID Principles can be applied to any OOP program.

The SOLID Principles were developed by computer science instructor and author Robert C. Martin. Now, SOLID principles have also been adopted in both agile development and adaptive software development.

The 5 principles of SOLID are:

1.      Single Responsibility Principle (SRP)

2.      Open-Closed Principle (OCP)

3.      Liskov Substitution Principle (LSP)

4.      Interface Segregation Principle (ISP)

5.      DependencyInversion Principle (DIP)

 Open-closed principle (OCP):

The Open/closed Principle says that a software module/class/function is open for extension and closed for modification. That means a class should be easily extended but there is no need to change its core implementations.

“Software entities … should be open for extension, but closed for modification.” Robert C. Martin.


Implementation:

Let’s take an example of bank accounts like regular savings, salary savings, corporate, etc. for different customers. As for each customer type, there are different rules and different interest rates.

 

The below Account class code violates the OCP principle, if the bank introduces a new Account type in that case, we need to modify CalcInterest method for adding a new account type.

 

The code looks like,

// Does not follow a Open-closed principle (OCP)

public class Account

{

               public decimal InterestRate { get; set; }

               public decimal BalanceAmount { get; set; }

               //members and function declaration

               public decimal CalcInterest(string accountType)

               {

                              // saving Account type

                              if (accountType == "Regular")

                              {

                                     InterestRate = (BalanceAmount * 4) / 100; 

                                    if (BalanceAmount < 1000) InterestRate -= (BalanceAmount * 2) / 100;

                                    if (BalanceAmount < 5000) InterestRate += (BalanceAmount * 4) / 100;

                              } 

                              // salary Account type

                              if (accountType == "Salary")

                              {

                                             InterestRate = (BalanceAmount * 5) / 100;

                              } 

                              // Corporate Account type

                              if (accountType == "Corporate")

                              {

                                             InterestRate = (BalanceAmount * 3) / 100;

                              } 

                              return InterestRate;

               }

}

We can apply the Open-closed principle (OCP) by using interface, abstract class, abstract methods, and virtual methods when we want to extend functionality.

Here in the below, we have used interface (IAccount) to follow an Open-closed principle (OCP) or remove the code that violates OCP principle,

interface IAccount

{

               // members and function declaration, properties

               decimal BalanceAmount { get; set; }

               decimal CalcInterest();

}

 

//regular savings account

public class RegularSavingAccount : IAccount

{

               public decimal BalanceAmount { get; set; } = 0;

               public decimal CalcInterest()

               {

                              decimal InterestRate = (BalanceAmount * 4) / 100;

                              if (BalanceAmount < 1000) InterestRate -= (BalanceAmount * 2) / 100;

                              if (BalanceAmount < 5000) InterestRate += (BalanceAmount * 4) / 100;

                              return InterestRate;

               }

}

//Salary savings account

public class SalarySavingAccount : IAccount

{

               public decimal BalanceAmount { get; set; } = 0;

               public decimal CalcInterest()

               {

                              decimal InterestRate = (BalanceAmount * 5) / 100;

                              return InterestRate;

               }

}

//Corporate Account

public class CorporateAccount : IAccount

{

               public decimal BalanceAmount { get; set; } = 0;

               public decimal CalcInterest()

               {

                              decimal InterestRate = (BalanceAmount * 3) / 100;

                              return InterestRate;

               }

}

In the above code, three new classes are created by extending them from IAccount.

1.      RegularSavingAccount

2.      SalarySavingAccount

3.      CorporateAccount

This is solving the problem of modification of class and by extending the interface, we can extend functionality.

The above code implements both the Open-closed principle (OCP) as well as the Single-responsibility principle (SRP) principle, as each class does a single task and we are not modifying the class and only doing an extension.

 

Use Cases of Open-Closed Principle in C#:

The Open-Closed Principle (OCP) is one of the SOLID principles of object-oriented programming, and it states that a class should be open for extension but closed for modification. In C#, there are several ways to apply the Open-Closed Principle to improve the flexibility and maintainability of your code.

Here are some use cases:

1.      Adding new functionality through inheritance

2.      Using interfaces for extension

3.      Using strategy pattern

4.      Using Abstract Factories

5.      Using Dependency Injection

By applying the Open-Closed Principle in these ways, you can enhance the flexibility of your code, make it easier to extend, and reduce the risk of introducing bugs when adding new features.

 

Advantages of Open-Closed Principle:

1.      Code Reusability

2.      Maintainability

3.      Reduced Risk of Regression Bugs

4.      Encourages Design Patterns

5.      Scalability

Disadvantages of Open-Closed Principle:

1.      Performance Impact: added extra layers might decrease performance due to added indirection

2.      Abstraction Overhead: added unnecessary abstraction overhead

 

By Anil Singh | Rating of this article (*****)

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