throw and custom exceptions

Throwing deliberately

Your own code throws when its contract is broken:

static decimal Withdraw(decimal balance, decimal amount)
{
    if (amount <= 0)
        throw new ArgumentException("amount must be positive");
    if (amount > balance)
        throw new InvalidOperationException("insufficient funds");
    return balance - amount;
}

Two rules make throw messages useful:

• ArgumentException (and friends) for bad arguments right now — with the argument's name if possible.
• InvalidOperationException for a bad operation given the current state — withdrawing from an overdrawn account.

Custom exceptions

When callers must tell your failures apart from the framework's, define a type:

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

static void Pay(decimal amount)
{
    if (amount > 1000m)
        throw new PaymentDeclinedException("over the single-payment limit");
}

Now a caller can catch (PaymentDeclinedException) and know exactly whose rule fired. The convention: name ends in Exception, derive from Exception, pass the message to base.