Debugging failing tests
Make failures reproducible, remove time/randomness, shrink to a minimal case, and improve assertion messages for fast diagnosis.
Debugging failing tests is a free C# Academy lesson on CoddyKit — lesson 3 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Game plan
Plan to fix failing tests:
- Reproduce reliably
- Remove flakiness (time/random I/O)
- Shrink to a smallest failing case
- Add clear assertion messages
Flaky by design
Using UtcNow and default Random makes behavior vary between runs → flaky tests.
using System;
// Demo: occasionally fails because it depends on current second and default Random()
public static class Promo
{
// Gives bonus on even seconds; random adds small jitter
public static int BonusNow()
{
int baseBonus = (DateTime.UtcNow.Second % 2 == 0) ? 10 : 9;
Random r = new Random(); // time-based seed → unpredictable
return baseBonus + (r.Next(0, 2)); // 0 or 1 more
}
}
public class Program
{
public static void Main(string[] args)
{
// Simulated test: sometimes expects exactly 10, but result varies
int got = Promo.BonusNow();
if (got != 10) Console.WriteLine("FAIL (flaky): expected 10 but got " + got);
else Console.WriteLine("PASS (maybe): got " + got);
}
}
Control dependencies
Inject time and Random. With a fixed seed and fixed timestamp, the failure becomes reproducible.
using System;
public interface IClock { DateTime NowUtc(); }
public sealed class FakeClock : IClock
{
private readonly DateTime _fixedUtc;
public FakeClock(DateTime fixedUtc) { _fixedUtc = fixedUtc; }
public DateTime NowUtc() { return _fixedUtc; }
}
public sealed class PromoDeterministic
{
private readonly IClock _clock;
private readonly Random _rng;
public PromoDeterministic(IClock clock, Random rng) { _clock = clock; _rng = rng; }
public int BonusNow()
{
int baseBonus = (_clock.NowUtc().Second % 2 == 0) ? 10 : 9;
return baseBonus + _rng.Next(0, 2);
}
}
public static class Assert
{
public static void AreEqual(int expected, int actual, string msg)
{
if (expected != actual) throw new Exception("Expected " + expected + " but got " + actual + " :: " + msg);
}
}
public class Program
{
public static void Main(string[] args)
{
try
{
IClock clock = new FakeClock(new DateTime(2016, 1, 1, 0, 0, 20, DateTimeKind.Utc)); // even second
Random rng = new Random(123); // fixed seed
PromoDeterministic p = new PromoDeterministic(clock, rng);
int got = p.BonusNow(); // predictable now
Assert.AreEqual(10, got, "even second, first draw should be 0");
Console.WriteLine("PASS: deterministic promo");
}
catch (Exception ex)
{
Console.WriteLine("FAIL: " + ex.Message);
}
}
}
Minimal repro
Shrink inputs until a tiny example fails (here: empty array). Small cases are easier to reason about.
using System;
// Buggy function: average of ints, but divides by count without guarding empty input
public static class Stats
{
public static double Average(int[] xs)
{
int sum = 0;
for (int i = 0; i < xs.Length; i++) sum += xs[i];
return sum / xs.Length; // BUG: divide by zero if empty; also int division truncates
}
}
public static class Assert
{
public static void Throws<T>(Action a, string msg) where T : Exception
{
try { a(); }
catch (Exception ex)
{
if (ex is T) return;
throw new Exception("Expected " + typeof(T).Name + " but got " + ex.GetType().Name + " :: " + msg);
}
throw new Exception("Expected " + typeof(T).Name + " but no exception thrown :: " + msg);
}
}
public class Program
{
public static void Main(string[] args)
{
// Minimal failing case: empty array
Assert.Throws<DivideByZeroException>(delegate { Stats.Average(new int[0]); }, "empty input");
Console.WriteLine("Reproduced: empty input triggers the bug.");
}
}
Fix and message quality
Guard against invalid input and use descriptive assertion messages (include inputs/expected/epsilon).
using System;
public static class Stats
{
public static double Average(int[] xs)
{
if (xs == null) throw new ArgumentNullException("xs");
if (xs.Length == 0) throw new ArgumentException("xs must not be empty", "xs");
double sum = 0.0; // use double to avoid integer truncation
for (int i = 0; i < xs.Length; i++) sum += xs[i];
return sum / xs.Length;
}
}
public static class Assert
{
public static void AreAlmostEqual(double expected, double actual, double eps, string msg)
{
if (Math.Abs(expected - actual) > eps)
throw new Exception("Expected " + expected + " but got " + actual + " (eps " + eps + ") :: " + msg);
}
}
public class Program
{
public static void Main(string[] args)
{
try
{
double a = Stats.Average(new int[] { 2, 4, 6 });
Assert.AreAlmostEqual(4.0, a, 1e-9, "average of 2,4,6");
Console.WriteLine("PASS: fixed average with clear message");
}
catch (Exception ex)
{
Console.WriteLine("FAIL: " + ex.Message);
}
}
}
Debugging checklist
Debugging checklist:
- Stabilize dependencies (time, random, I/O)
- Reproduce locally and repeatedly
- Shrink to a minimal failing input
- Add precise assertion messages
- Prefer pure functions and guard checks
First step for flaky tests
Recap
Recap: Reproduce reliably by controlling time/randomness, shrink the input, and write helpful assertion messages to pinpoint the bug fast.
Frequently asked questions
Is the “Debugging failing tests” lesson free?
Yes — the full text of “Debugging failing tests” is free to read here on the web, and the C# Academy course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO.
What will I learn in “Debugging failing tests”?
Make failures reproducible, remove time/randomness, shrink to a minimal case, and improve assertion messages for fast diagnosis. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start C# Academy?
No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “Debugging failing tests” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this C# Academy lesson?
Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- xUnit/MSTest setup, assertions
- Fakes via interfaces; test data builders
- Debugging failing tests