Assertions as executable specs

The pattern

// Arrange: build the world
var items = new List<int> { 3, 1, 2 };
// Act: one behavior
items.Sort();
// Assert: the claim
// items should be 1,2,3

A test has one job: prove one claim about one behavior. When it fails, the message should name the claim, the actual value, and the expected value — that trio is what makes failures diagnosable at a glance.

Good tests share three traits

• Fast — milliseconds, so nobody skips running them.
• Deterministic — same input, same result, every run. No clocks, no randomness, no "sometimes fails".
• Isolated — each test builds its own world; no test depends on another having run first.

The habit this course drills: when a bug appears, don't just fix it — write the test that would have caught it, then fix. The test is the regression guard; the fix is temporary without it.