Inheritance: reuse with `base`

One class built on another

Inheritance declares that one class is a kind of another:

class Account
{
    public decimal Balance { get; protected set; }

    public void Deposit(decimal amount)
    {
        if (amount <= 0) throw new ArgumentException("non-positive deposit");
        Balance += amount;
    }
}

class SavingsAccount : Account
{
    public void AddMonthlyInterest(decimal rate)
    {
        Balance += Balance * rate;   // reuses the inherited Balance
    }
}

SavingsAccount carries everything Account has — Deposit with its validation included — and adds its own behavior. Callers holding a SavingsAccount can Deposit without knowing the derived type exists.

Extension, replacement, and base

A derived class can add members, or replace a non-virtual member with new (hiding) — but replacing is a smell for beginners: the hidden base method still runs when called through a base reference.

The honest tool for "same idea, better implementation" is the constructor chain:

class SavingsAccount : Account
{
    public SavingsAccount(string owner, decimal opening) : base(owner, opening) { }
}

base(...) forwards construction to the base constructor — the base's invariants run first, always. base.Member also reaches the base's implementation from inside an override.