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Understanding “I” - ISP (Interface Segregation principle)

Now assume that our customer class has become a SUPER HIT component and it’s consumed across 1000 clients and they are very happy using the customer class.
solid in c#
Now let’s say some new clients come up with a demand saying that we also want a method which will help us to “Read” customer data. So developers who are highly enthusiastic would like to change the “IDatabase” interfaceas shown below.
But by doing so we have done something terrible, can you guess ?
HINT: - Think about the effect of this change on the above image. 

interface IDatabase
{
        void Add(); // old client are happy with these.
voidRead(); // Added for new clients.
}

If you visualize the new requirement which has come up, you have two kinds of client’s: -
  • Who want’s just use “Add” method.
  • The other who wants to use “Add” + “Read”.

Now by changing the current interface you are doing an awful thing, disturbing the 1000 satisfied current client’s , even when they are not interested in the “Read” method. You are forcing them to use the “Read” method.
So a better approach would be to keep existing clients in their own sweet world and the serve the new client’s separately.
So the better solution would be to create a new interface rather than updating the current interface. So we can keep the current interface “IDatabase” as it is and add a new interface “IDatabaseV1” with the “Read” method the “V1” stands for version 1. 

interface IDatabaseV1 : IDatabase // Gets the Add method
{
Void Read();
}

You can now create fresh classes which implement “Read” method and satisfy demands of your new clients and your old clients stay untouched and happy with the old interface which does not have “Read” method. 
class CustomerwithRead : IDatabase, IDatabaseV1
    {

public void Add()
{
	Customer obj = new Customer();
Obj.Add();
}
	Public void Read()
	{
	// Implements  logic for read
}
    }

So the old clients will continue using the “IDatabase” interface while new client can use “IDatabaseV1” interface. 
IDatabase i = new Customer(); // 1000 happy old clients not touched
i.Add();

IDatabaseV1 iv1 = new CustomerWithread(); // new clients
Iv1.Read();

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