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How would you implement the 'Liskov Substitution Principle' correctly when designing an abstract class hierarchy, and what's a classic violation example?

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Answer

LSP requires that any subclass must be fully usable anywhere its base class is expected, WITHOUT breaking the caller's expectations — a classic violation is a 'Square extends Rectangle' hierarchy where setting Square's Width also changes its Height (to stay square), silently breaking code that assumes setting Rectangle.Width leaves Height unaffected.

public class Rectangle {
  public virtual double Width { get; set; }
  public virtual double Height { get; set; }
}
public class Square : Rectangle {
  public override double Width { get => base.Width; set { base.Width = value; base.Height = value; } } // LSP violation!
}
// Code assuming 'rect.Width = 5' doesn't affect Height breaks silently for a Square

Real-world example Recognizing and avoiding a classic LSP violation when a seemingly natural 'is-a' relationship (Square is-a Rectangle) breaks behavioral expectations.

Common follow-ups: What's a better design that avoids this specific Square/Rectangle LSP trap entirely?

Design Patterns in C#