“A Class should have only one reason to change”
Let's go through the problem first. Have a look at the code
given below:
public class BankAccount
{
public BankAccount() { }
public string AccountNumber { get;
set; }
public decimal AccountBalance { get;
set; }
public decimal CalculateInterest()
{
// Code
to calculate Interest
}
} |
Its Model class for object BankAccount, and it's also saving the
Calculating Interest for the bank account. Now look at the few change Request we received from
business:
1.
Please add a new Property AccountHolderName .
2.
Some new rule has been introduced to calculate
interest.
These 2 are totally different type
of change request. One is changing on features; where as other one is impacting
the functionality. So that's SRP Principle says, we should have 2 different
type of reason to change one class. Most of the times we keep functions in our domain
classes, which contain business rules. This violates Single Responsibility Principle.
Now let's go through how we can
achieve this thing by implementing Single
Responsibility Principle. Look at the code below:
public interface IBankAccount
{
string AccountNumber { get; set; }
decimal AccountBalance { get; set; }
}
public class BankAccount
: IBankAccount
{
public string AccountNumber { get;
set; }
public decimal AccountBalance { get;
set; }
} |
Now our class BankAccount is just
responsible for properties of the BankAccount. If we have add any new Business
Rule in Calculation of Interest, we don't need to change BankAccount class.
Now
about saving the data, we will create another class. Let's say InterstCalculator, as below: public interface IInterstCalculator
{
decimal CalculateInterest();
}
public class InterstCalculator : IInterstCalculator
{
public decimal CalculateInterest(IBankAccount account)
{
// Write your logic here
return 1000;
}
} |
InterestCalculator class requires no
changes, in case we need to add a new Property AccountHolderName. So that's correct implementation of Single
Responsibility Principle. We have also used Interfaces to communicate
between InterestCalculator and BankAccount classes. That will help us to manage
dependencies between classes. I will talk about this more, in my next blogs.
Cheers!!