SOLID Principles

SOLID is an a mnemonic acronym for five design principles intended to make software designs more understandable.
These principles aim to make the code more readable, easy to maintain, extensible, reusable and without repetition.
The principles are a subset of many principles Robert C. Martin (Uncle Bob) in his 2000 paper Design Principles and Design Patterns, although the SOLID acronym was introduced later by Michael Feathers.

S - Single-responsiblity principle
O - Open-closed principle
L - Liskov substitution principle
I - Interface segregation principle
D - Dependency Inversion Principle

Single Responsibility Principle

A class must have only one reason to change and only one responsibility.

Bad

public sealed class Invoice
{
    public int Id { get; set; }

    public double Price { get; set; }

      public float TaxPercentage { get; set; }

      public double GetTaxAmount()
      {
        return Price * TaxPercentage / 100;
      }

    public void Insert()
    {
      /// Implementation for ADDING to DATABASE
    }

    public void Delete()
    {
      /// Implementation for DELETING to DATABASE
    }
}

The code is incorrect because it has two responsibilities, business rules and database persistence.

Good

public sealed class Invoice
{
    public int Id { get; set; }

    public double Price { get; set; }

      public float TaxPercentage { get; set; }

      public double GetTaxAmount()
      {
          return Price * TaxPercentage / 100;
      }
}

public sealed class InvoiceRepository
{
    public void Insert(Customer customer)
    {
        /// Implementation for ADDING to DATABASE
    }

    public void Delete(Customer customer)
    {
        /// Implementation for DELETING to DATABASE
    }
}

The code is correct because the responsibilities have been split, and each class has only one reason to change.

xshaheen/solid

SOLID Principles

Bad

Good