Open close principle in C#

https://www.c-sharpcorner.com/UploadFile/pranayamr/open-close-principle/

Example of open close principle-

namespace SolidPrinciples.Open.Closed.Principle.After{
   public abstract class Shape{
      public abstract double Area();
   }
   public class Rectangle: Shape{
      public int Width { get; set; }
      public int Height { get; set; }
      public override double Area(){
         return Width * Height;
      }
   }
   public class Circle : Shape{
      public double Radius { get; set; }
      public override double Area(){
         return Radius * Radius * Math.PI;
      }
   }
   public class CombinedAreaCalculator{
      public double Area (Shape[] shapes){
         double area = 0;
         foreach (var shape in shapes){
            area += shape.Area();
         }
         return area;
      }
   }
}

Hi Naresh Beniwal,

Open Closed Principle (OCP) states that a class must be open to be extended but closed to changes. Example in C# is as following:

• BankAccount
• BankAccountBase
• SavingsAccount
• LongTermSavingsAccount

BankAccount

// Not following the Open Closed Principle  
  
public enum AccountType  
{  
    SavingsAccount = 1,  
    LongTermSavingsAccount = 2  
}  
  
public class BankAcount  
{  
    public string AccountNumber { get; set; }  
  
    public string AccountTitle { get; set; }  
  
    public AccountType BankAccountType { get; set; }  
  
    public float CalculateInterest(float accountBalance)  
    {  
        // Add an IF statement for each type of new bank account -- violates OCP  
        if (BankAccountType == AccountType.SavingsAccount)  
            return accountBalance * 0.02F;  
        if (BankAccountType == AccountType.LongTermSavingsAccount)  
            return accountBalance * 0.04F;  
        else  
            return accountBalance;  
    }  
}

BankAccountBase

// Following the Open Closed Principle  
public interface IBankAccount  
{  
    float CalculateInterest(float accountBalance);  
}  
  
public abstract class BankAccountBase  
{  
    public string AccountNumber { get; set; }  
    public string AccountTitle { get; set; }  
    public AccountType BankAccountType { get; set; }  
}  

SavingsAccount

public class SavingsAccount : BankAccountBase, IBankAccount  
{  
    public float CalculateInterest(float accountBalance)  
    {  
        return accountBalance * 0.02F;  
    }  
}  

LongTermSavingsAccount

public class LongTermSavingsAccount : BankAccountBase, IBankAccount  
{  
    public float CalculateInterest(float accountBalance)  
    {  
        return accountBalance * 0.04F;  
    }  
} 

How to use

var bankAccount = new BankAcount()  
{  
    AccountNumber = "100AAA",  
    AccountTitle = "Mr. John Doe",  
    BankAccountType = AccountType.SavingsAccount  
};  
  
Console.WriteLine($"The Interest amount for {bankAccount.AccountTitle} is ${bankAccount.CalculateInterest(1000)}");  
  
//Calling class following Open Closed Principle  
  
var savingsBankAccount = new SavingsAccount()  
{  
    AccountNumber = "100AAA",  
    AccountTitle = "Mr. John Doe",  
    BankAccountType = AccountType.SavingsAccount  
};  
Console.WriteLine($"The Interest amount for {savingsBankAccount.AccountTitle} is ${savingsBankAccount.CalculateInterest(1000)}");  

Hope, this will help you!

The open-closed principle states that a software component like method, class or module should be open for extension, but closed for modification.

Please check below links for your reference 

https://code-maze.com/open-closed-principle/

https://dotnettutorials.net/lesson/open-closed-principle/

https://levelup.gitconnected.com/4-ways-to-achieve-open-closed-principle-in-c-d2d0ebea4b86

The Open Close principle is one that is used to open extend and close for any changes.

For eg., 

RBI has a set of norms and would like to extend the logic in SBI  business cases. So we are extending the RBI library and changing in our SBI Library, In this approach we are not modifying anything on Main placeholder however we are molding it in on required place on demand basis. 

https://www.c-sharpcorner.com/article/solid-principles-in-c-sharp-open-closed-principle/

https://www.c-sharpcorner.com/blogs/open-close-principle-with-example1