Functions, parameters and guard clauses — Clean Code, Testing & Best Practices

2. Functions, parameters and guard clauses

Basic★ Must knowLesson 2 of 12

Small; do one thing; one level of abstraction.
Few parameters (≤ 3); group many into a parameter object/record.
No hidden side effects; avoid boolean "flag" parameters that switch behaviour.
Return early with guard clauses instead of deep nesting.

csharp

// ❌ Arrow code
public decimal Ship(Order o)
{
    if (o != null) { if (o.IsPaid) { if (o.Lines.Count > 0) { return Calculate(o); } else throw ...; } else throw ...; }
    else throw ...;
}
// ✅ Guard clauses
public decimal Ship(Order? o)
{
    ArgumentNullException.ThrowIfNull(o);
    if (!o.IsPaid) throw new InvalidOperationException("Order not paid");
    if (o.Lines.Count == 0) throw new InvalidOperationException("Order is empty");
    return Calculate(o);
}