What does the letter L in the SOLID principles signify and how does it influence software system design?

The letter L in SOLID principles stands for Liskov Substitution Principle.

The essence of the principle: subclass objects should be interchangeable with superclass objects without violating the correctness of the program. That is, if class B inherits from class A, then anywhere A is used, B can be substituted without changing the expected behavior.

Impact on design:

• Ensures proper inheritance and polymorphism.
• Helps avoid unexpected errors when extending functionality.
• Promotes the creation of more reliable and maintainable systems.

Example of LSP violation:

class Bird {
  public virtual void Fly() { /*...*/ }
}

class Ostrich : Bird {
  public override void Fly() {
    throw new NotSupportedException(); // ostrich cannot fly
  }
}

Here, Ostrich violates LSP because it cannot replace Bird without changing behavior.

A proper approach is to reconsider the hierarchy or use interfaces to avoid such situations.