It's a great start

public static bool IsItTeaTimeYet(
	int hour)
{
    return hour > 5 && hour < 22;
}

[Test]
public void Perfect_tea_time()
{
    Assert.That(
        Tea.IsItTeaTimeYet(3),
        Is.False);

    Assert.That(
        Tea.IsItTeaTimeYet(12),
        Is.True);
}

All code covered 😬

Profit? 💰

TDD requires discipline

Coverage = code was run

Mutation testing 👾

public static bool IsItTeaTimeYet(
	int hour)
{
    return hour >= 5 && hour < 22; // '> 5' mutated to '>= 5'
}

[Test]
public void Perfect_tea_time()
{
    Assert.That(
        Tea.IsItTeaTimeYet(3),
        Is.False);

    Assert.That(
        Tea.IsItTeaTimeYet(12),
        Is.True);
}

Mutant survived 👾

MutationTestingExample\TeaTime.cs:8
  original: return hour > 5 && hour < 22;
  mutated: return hour >= 5 && hour < 22;

Testing the tests

Java

JavaScript

Ruby

PHP

...and C# ?

How hard could it be?

Fettle

From google dictionary

What does it need to do?

Mutate the code

Run the tests

.NET Framework

NUnit 3

Mutate the code

CIL

public static int Sum(int a, int b)
{
   return a + b;
}

IL_0000: nop
IL_0001: ldarg.0
IL_0002: ldarg.1
IL_0003: add			# the interesting bit
IL_0004: stloc.0
IL_0005: br.s IL_0007
IL_0007: ldloc.0
IL_0008: ret
					

First Pass

Finds gaps in our tests 👍

False positives 😬

Takes hours 😬

string result = "";
switch (input)
{
 	case "a": result = "F"; break;
 	case "b": result = "G"; break;
 	case "c": result = "H"; break;
 	...
	case "j": result = "O"; break;
}

string result = "";
uint num = .ComputeStringHash(input);
if (num <= 3826002220u)
{
  if (num <= 3775669363u)
  {
    if (num != 3758891744u)
    {
      ...
					

🤔

Roslyn

public static int Sum(int a, int b)
{
   return a + b;
}

MethodDeclaration
	PublicKeyword
	StaticKeyword
	...
	Block
		OpenBraceToken
		ReturnStatment
			ReturnKeyword
			AddExpression
				IdentifierName		// 'a'
				PlusToken		// '+'
				IdentifierName		// 'b'
			SemicolonToken
		CloseBraceToken

Second Pass

Finds gaps in our tests 👍

False positives 👍

Takes hours 😬

Optimisations

for every class
	for every method
		for every instruction
			run all the tests

😱

Interrupted flow 🙈

Only run relevant tests

How to tell?

public static bool IsItTeaTimeYet(
	int hour)
{
	return hour > 5 && hour < 22;
}

public static bool IsItTeaTimeYet(
	int hour)
{
	Console.WriteLine(
		"Fettle: d3044986-2942-4d5b-b157-99c0d9488f54");

	return hour > 5 && hour < 22;
}

TestA

MethodA
MethodC

TestB

MethodC
MethodD

Third Pass

Finds gaps in our tests 👍

False positives 👍

Takes hours minutes 👍

Part Of The Pipeline

15K lines of code

96% coverage

12 minutes

Overnight and on-demand

3K lines of code

95% coverage

2 minutes

As part of CI

Issues

Cognitive Overload 🤯

Infinite loops 💥

Flaky tests 👻

Coverage still important 👀

...But

2 - 3 mutants a month on average

Plug gaps in our tests 👍

Deleted redundant code 👍

Encourages TDD 👍

...Greater confidence 😁

The Future

Faster feedback?

The Future?

Have a go

Get in touch @owennell

Slides: oliwennell.github.io

Fettle: github.com/ComparetheMarket/fettle

Stryker.NET: github.com/stryker-mutator/stryker-net