Write your first unit tests in C# with MSTest

# Unit Testing 101

## "Write your first unit tests in C#"

### Cesar Aguirre

_Software engineer, lifelong learner_

---

## Agenda

### What and Why

* What is a Unit Test
* Why you should write Unit Tests?
* What code should I test?
* What is a good test?

### How

* How to Organize my tests
* How to Name my tests
* Arrange/Act/Assert
* Assertion Frameworks
* MsTests
* Example: Stringie Remove method
	
---

## What and Why

---

## What is a Unit Test?

> "A unit test is an automated piece of code that invokes a unit of work in the system and then checks a single assumption about the behavior of that unit of work."
  
.footnote[Source: [Unit Test - Definition](https://www.artofunittesting.com/definition-of-a-unit-test/)]
  
---

## But, what is a Unit of Work?

A Unit of Work is any logic exposed through public methods that

* returns value,
* changes the state of the system
* or makes an external invocation

We shouldn't be testing private methods directly

---

## Why you should write Unit Tests?

* Allow change
* Prevent bugs from happening again
* Have living documentation
  
---

## Analogy

![](https://images.unsplash.com/photo-1561900478-5001f6b4d8ed?crop=entropy&cs=tinysrgb&fit=crop&fm=jpg&h=400&ixid=MXwxfDB8MXxhbGx8fHx8fHx8fA&ixlib=rb-1.2.1&q=80&utm_campaign=api-credit&utm_medium=referral&utm_source=unsplash_source&w=600)

.footnote[Photo by [Loic Leray](https://unsplash.com/@loicleray?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText) on [Unsplash](https://unsplash.com/s/photos/circus?utm_source=unsplash&utm_medium=referral&utm_content=creditCopyText)]

---

## What code should I test?

Test any code that contains logic in the form of

* Conditionals
* Loops
* Calculations

---

## What is a good test? Write F.I.R.S.T

* Fast
* Independent
* Repeatable
* Self-validating
* Timely

---

## When should I write my unit tests?

Before or after the production code?

1. Test first: Test-Driven Development
2. Test later

Make sure you always see your tests failing

---

## How

---

## How to Organize my tests

### Project

```bash
MyLibrary
MyLibrary.UnitTests
```

### Class

```bash
MyClass.cs
MyClassTests.cs
MyClassTests.AnAwesomeFeature.cs
```

---

## How to Name my tests

### A sentence

```csharp
ItShouldThrowAnExceptionWhenDividingByZero
```

### UnitOfWork_Scenario_ExpectedResult

```csharp
Divide_Zero_ThrowsException
```

---

## Arrange/Act/Assert

```csharp
public void Divide_Zero_ThrowsException()
{
	// Arrange
	
	// Act
  
	// Assert
}
```

---

## Assertion Frameworks

* [NUnit](https://github.com/nunit/nunit)
* [MSTest](https://github.com/Microsoft/testfx-docs)
* [XUnit](https://github.com/xunit/xunit)

---

## MSTest

### Create a test method

```csharp
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace MyLibrary.Tests
{
	[TestClass]
	public class MyClassTests
	{
		[TestMethod]
		public void MyTestMethod()
		{
		}
		
		[TestMethod]
		public async Task MyAsyncTestMethod()
		{
		}
	}
}
```

---

## MSTest

### Assertion methods

```csharp
Assert.AreEqual(expected, actual)
Assert.AreNotEqual(notExpected, actual)

Assert.IsFalse(condition)
Assert.IsTrue(condition)

Assert.IsNotNull(value)
Assert.IsNull(value)

Assert.ThrowsException<Exception>(() => SomeAction());
Assert.ThrowsExceptionAsync<Exception>(async () => await SomeActionAsync());
```

---

## Example: Stringie

Let's write some tests for Stringie, a library to transform strings with a fluent interface.

```csharp
string transformed = "Hello, world!".Remove();
// ""

string transformed = "Hello, world!".Remove("Hello");
// ", world!"
```

---

## Example: Stringie Remove method

```csharp
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Stringie;

namespace Stringie.Tests
{
	[TestClass]
	public class RemoveTests
	{
		[TestMethod]
		public void Remove_SingleWordToRemove_ReturnsEmpty()
		{
			var str = "123456";
			
			var removed = str.Remove("123456");
			
			Assert.AreEqual(0, remove.Length);
		}
	}
}
```

---

## It's your turn

Write at least 3 unit tests for Stringie. Clone this repo and follow the instructions on the README file

.center[[![canro91/Testing101 - GitHub](https://gh-card.dev/repos/canro91/Testing101.svg)](https://github.com/canro91/Testing101)]

---

> _"Legacy code is code without tests"_
>
> Working Effectively with Legacy Code, Michael Feathers

---

## To learn more about this subject

Check one of my posts

* [The C# Definitive Guide](https://canro91.github.io/2018/11/17/TheC-DefinitiveGuide/)
* [How to write good unit tests? Avoid these two issues](https://canro91.github.io/2020/11/02/UnitTestingTips/)
* [How to write good unit tests? Have a failing test](https://canro91.github.io/2021/02/05/FailingTest/)
* [How to create fakes with Moq](https://canro91.github.io/2020/08/11/HowToCreateFakesWithMoq/)
* [The Art of Unit Testing: Four Takeaways](https://canro91.github.io/2020/03/06/TheArtOfUnitTestingReview/)

---

# That's all folks!

## Cesar Aguirre

https://canro91.github.io/