Check behaviour: catch a boundary bug before it reaches a learner

The code builds, but exactly 25 minutes fails a 25-minute goal. What check would catch that mistake next time? Turn the boundary rule into repeatable checks.

Predict a bug

The requirement is: 25 minutes or more meets a 25-minute goal. Before running, predict this program. Then decide whether the observed result satisfies the requirement.

using System;

Console.WriteLine(IsGoalMet(25, 25));

static bool IsGoalMet(int minutes, int goal)
{
    return minutes > goal;
}

Choose checks from the requirement

Requirement: minutes >= goal

24 minutes ──> below ──> False
25 minutes ──> equal ──> True
26 minutes ──> above ──> True

A check needs an input, an expected result and a comparison with the actual result. “The program ran” is not the same as “the result meets the requirement.” The three neighbouring values make the boundary visible.

Run repeatable checks

The small Check method below prints a pass or throws on failure. It uses no extra packages. This is a first step toward automated tests; a formal test framework gives you richer discovery and reporting later.

using System;

Check("below the goal", IsGoalMet(24, 25) == false);
Check("exactly the goal", IsGoalMet(25, 25) == true);
Check("above the goal", IsGoalMet(26, 25) == true);
Check("zero goal", IsGoalMet(0, 0) == true);

static bool IsGoalMet(int minutes, int goal)
{
    return minutes >= goal;
}

static void Check(string name, bool passed)
{
    if (!passed)
    {
        throw new InvalidOperationException("FAIL: " + name);
    }
    Console.WriteLine("PASS: " + name);
}

Change only >= to > inside IsGoalMet and run again. The “exactly the goal” check should fail. Restore >= afterward. A check that can catch the original mistake gives you useful evidence about this requirement.

Your turn: test a different rule

Write CanStartSession(int minutes) for the rule “a session must be at least 5 minutes and at most 90 minutes.” Before coding, write the expected results for 4, 5, 90 and 91. Implement the method and add four Check calls in your own project.

Stop here and run your attempt. Each check should name the input and compare the returned bool with the rule.

Hint

Both comparisons must hold: at least 5, and at most 90.

Compare your result

The answer sequence is False, True, True, False. An implementation can return minutes >= 5 && minutes <= 90.

Explain it back

Explain why a passing compile is weaker evidence than these four checks. What does CanStartSession(-1) return, and which comparison rejects it? The predicate already returns false for negative values. Separately decide whether the input screen should explain that a duration cannot be negative.

Reference: Microsoft — testing in .NET.