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Open Closed Principal

Open Closed Principle (DIP) is one of the five software design principles. What programming languages you know, you should know these five principles written below :
  • Open Close Principle (OCP)
  • Dependency Inversion Principle (DIP)
  • Interface Segregation Principle (ISP)
  • Single Responsibility Principle (SRP)
  • Liskov’s Substitution Principle (LSP)
In this column i will explain Open Closed Principle.
Purpose/Reason
The reason to use open closed principle is to add new features without changing existing software entities. Especially in enterprise application, due to marketing conditions, softwares need to be changed. In order to reduce time, cost and risk for implementation new features, you should consider how to expand software easily. Open closed principle is one of the five answers written above.
Key Principle
Open Close Principle states software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.
Implementation
Suppose that, you have two different database products which are MySQL and Oracle. In order to connect right database, you built a DatabaseManager to decide that which database product is used to connect. Here is the solution which is not recommended.
Bad Example :
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public class DatabaseManager
{
    public void Connect(Database d)
    {
         if (d.DBType == 1)
            ConnectMySQL((MySQLDB)d);
         else if (d.DBType == 2)
            ConnectOracle((OracleDB)d);
    }
 
    public void ConnectMySQL(MySQLDB db)
    {
        // connect
    }
 
    public void ConnectOracle(OracleDB db)
    {
        // connect
    }
     
}
 
public class Database
{
    public int DBType;
}
 
public class MySQLDB : Database
{
    public MySQLDB()
    {
        base.DBType = 1;
    }
}
 
public class OracleDB : Database
{
    public OracleDB()
    {
        base.DBType = 2;
    }
}
For instance, one day ago, your manager said that we need to support Microsoft SQL database. You need to modify almost all classes. But if you cared open closed principle before, you wouldn’t need to change all classes. Let’s see good example about how it should have built.
Good Example :
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public abstract class Database
{
    public abstract void Connect();
}
 
public class DatabaseManager
{
    private Database db;
    public DatabaseManager(Database db)
    {
        this.db = db;
    }
 
    public void Connect()
    {
        this.db.Connect();
    }
}
 
public class MySQLDatabase : Database
{
    public void Connect()
    {
        //connect MySQL
    }
}
 
public class OracleDatabase : Database
{
    public void Connect()
    {
        //connect Oracle
    }
}
If you had built your database manager like above, you wouldn’t have needed to modify all classes. The only thing you should do is add new MicrosoftSQL class into current situation.
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public class MicrosoftSQL : Database
{
    public void Connect()
    {
        // connect Microsoft SQL
    }
}
Summary : 
As you see, this principle should be applied software application which is most likely to be modified or added new features into current situation. Otherwise, your software application has more abstractions between classes and this occurs more complication in your code.
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