Edge cases and boundary thinking

The edge taxonomy

For any function ask: what about —

• Empty — no elements at all?
• Null — the absence of a collection, not the same as empty?
• One — a single element?
• Many — does it still work at scale?
• Boundary — exactly at the limit (<= vs < bugs live here)?
• Weird — duplicates, negative numbers, unicode, whitespace?

// the classic boundary bug
static bool IsAdult(int age) => age > 18;   // an 18-year-old disagrees!

Off-by-one errors concentrate at boundaries. When the spec says "18 or older", write the test for exactly 18 first.

Test both directions

An incorrect solution must fail — that's what makes a test a test. For every claim, the two-sided habit: a case where correct code passes, and a near-miss implementation that must not. A test suite where anything passes proves nothing.