The SOLID principles are a set of five design guidelines that help produce maintainable, scalable software.

Principle	Focus
SRP	One reason to change
OCP	Open for extension, closed for modification
LSP	Subtypes must be substitutable
ISP	Small, focused interfaces
DIP	Depend on abstractions, not concretions

Single Responsibility

A class should have only one reason to change.

class Invoice {
    void calculateTotal() { /* ... */ }
    void print() { /* violates SRP */ }
}

Split print() into a dedicated InvoicePrinter class and each concern evolves independently.

Open/Closed Principle

A module should be open for extension but closed for modification — new behavior arrives by adding code, not by rewriting existing code.

public interface IDiscount
{
    decimal Apply(Order order);
}

public class SeasonalDiscount : IDiscount
{
    public decimal Apply(Order order) => order.Total * 0.10m;
}

public class LoyaltyDiscount : IDiscount
{
    public decimal Apply(Order order) => order.Total * 0.15m;
}

Adding a new discount type means adding a class; the existing ones stay untouched.

Liskov Substitution

Objects of a subtype should be usable anywhere the parent type is expected, without surprising behavior.

public class Rectangle
{
    public virtual int Width { get; set; }
    public virtual int Height { get; set; }
}

public class Square : Rectangle
{
    public override int Width { set { base.Width = value; base.Height = value; } }
    public override int Height { set { base.Width = value; base.Height = value; } }
}

A Square is not a faithful substitute for Rectangle — resizing one side silently changes the other. Model them separately or behind a common abstraction.

Interface Segregation

A client should not be forced to depend on methods it never uses. Prefer several narrow interfaces over one broad one.

// one fat interface
public interface IWorker
{
    void Work();
    void Eat();
}

// focused interfaces
public interface IWorkable
{
    void Work();
}

public interface IEatable
{
    void Eat();
}

Now each class implements only what it actually needs.

Dependency Inversion

High-level modules should not depend on low-level modules. Both should depend on abstractions.

public interface IClock
{
    DateTime UtcNow { get; }
}

public class SystemClock : IClock
{
    public DateTime UtcNow => DateTime.UtcNow;
}

This keeps business logic testable and decoupled from the clock.

More in-depth articles will follow — this one just sets the stage.