Custom Exceptions

class InsufficientFundsException : Exception {
    public InsufficientFundsException(string message) : base(message) {}
}

class BankAccount {
    private double _balance;

    public void Withdraw(double amount) {
        if (amount > _balance) {
            throw new InsufficientFundsException(
                $"Cannot withdraw {amount}; balance is {_balance}"
            );
        }
        _balance -= amount;
    }
}

Exactly the same reasoning as Java's version of this lesson: a named, specific exception type lets calling code react to that particular failure differently from any other.