meet it

A checks one small piece, every time you run it.

A unit test runs one small piece of code with known inputs and checks the answer. Tests catch a mistake the moment you make it, and let you change code without fear. Testing tools exist, but the core is tiny: an , a check that complains loudly when something is wrong.

Write AssertEqual(expected, actual, name) once and call it for every case. Make it generic, and compare with Equals, which works for any T. Test every too: the odd inputs at the limits, like zero, an empty list or a 100 percent discount.

using System;

int passed = 0;
int failed = 0;
AssertEqual(90m, AfterDiscount(100m, 10), "ten percent off");
AssertEqual(0m, AfterDiscount(0m, 50), "free stays free");
AssertEqual(100m, AfterDiscount(100m, 0), "no discount");
AssertEqual(0m, AfterDiscount(100m, 100), "everything off");
Console.WriteLine($"{passed} passed, {failed} failed");

decimal AfterDiscount(decimal price, int percent) => price - price * percent / 100;

void AssertEqual<T>(T expected, T actual, string name)
{
    if (Equals(expected, actual))
    {
        passed++;
        Console.WriteLine($"PASS {name}");
        return;
    }
    failed++;
    Console.WriteLine($"FAIL {name}: expected {expected}, got {actual}");
}

Equals(a, b) is a built-in method every type understands, which is why the generic version can use it.

guess first

Two tests for one method. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

using System;

AssertEqual("Ana", Capitalise("ana"), "lower case");
AssertEqual("Ana", Capitalise("ANA"), "upper case");

string Capitalise(string name) => char.ToUpper(name[0]) + name.Substring(1);

void AssertEqual<T>(T expected, T actual, string name)
{
    if (Equals(expected, actual))
    {
        Console.WriteLine($"PASS {name}");
        return;
    }
    Console.WriteLine($"FAIL {name}: expected {expected}, got {actual}");
}

your turn

Your turn. Test that bad prices are refused.

On line 4, choose a price that should be refused, and name the test. Then, on line 20, write the exception type a refused price throws.

Your code

output

press run to see what happens

fix it

The test helper will not compile. Can you fix it?

Broken code to fix

output

press run to see what happens

C# says == cannot be used on two values of type T, on line 11.
T could be any type, and not every type has ==. Every type does have Equals.
Change the test on line 11 to Equals(expected, actual), then run.

nailed it

you test your code like a professional. :D

A unit test checks one small piece with known inputs.
Compare generic values with Equals, not ==.
Test the edge cases, and test that bad input throws.

new words in your collection

unit test
assertion
edge case

This lesson isn’t saved. A free account remembers every lesson you finish.

A unit test checks one small piece, every time you run it.

Two tests for one method. What will this show?

Your turn. Test that bad prices are refused.

The test helper will not compile. Can you fix it?

you test your code like a professional. :D

A checks one small piece, every time you run it.