Don't duplicate logic in Asserts: The most common mistake on unit testing

We have covered some common mistakes when writing unit tests. Some of them may seem obvious. But, we all have made this mistake when we started to write unit tests. This is the most common mistake when writing unit tests and how to fix it.

Don’t repeat the logic under test when verifying the expected result of tests. Instead, use known, hard-coded, pre-calculated values.

Let’s write some tests for Stringie, a (fictional) library to manipulate strings with a fluent interface. Stringie has a Remove() method to remove substrings from the end of a string.

We can use Stringie Remove() method like this,

var hello = "Hello, world!";

string removed = hello.Remove("world!").From(The.End);
// "Hello,"

Don’t Copy and Paste the tested logic

When writing unit tests, don’t copy the tested logic and paste it into private methods to use them inside assertions.

If we bring the tested logic to private methods in our tests, we will have code and bugs in two places. Duplication is the root of all evil. Even, inside our tests.

Please, don’t write assertions like the one in this test.

[TestMethod]
public void Remove_ASubstring_RemovesThatSubstringFromTheEnd()
{
    string str = "Hello, world!";

    string transformed = str.Remove("world!").From(The.End);

    Assert.AreEqual(RemoveFromEnd(str, "world!"), transformed);
    //              ^^^^^
    // We duplicate the Remove logic in another method
}

private string RemoveFromEnd(string str, string substring)
{
    var index = str.IndexOf(substring);
    return index >= 0 ? str.Remove(index, substring.Length) : str;
}

Don’t make internals public

Also, by mistake, we expose the internals of the tested logic to use them in assertions. We make private methods public and static. Even to test those private methods directly.

From our Unit Testing 101, we learned to write unit tests through public methods. We should test the observable behavior of our tested code. A returned value, a thrown exception, or an external invocation.

Again, don’t write assertions like the one in this test.

[TestMethod]
public void Remove_ASubstring_RemovesThatSubstringFromTheEnd()
{
    string str = "Hello, world!";

    string transformed = str.Remove("world!").From(The.End);

    Assert.AreEqual(Stringie.PrivateMethodMadePublicAndStatic(str), transformed);
    //              ^^^^^
    // An "internal" method exposed to our tests 
}

Use known values to Assert

Instead of duplicating the tested logic, by exposing internals or copy-pasting code into assertions, use a known expected value.

For our sample test, let’s simply use the expected substring "Hello,". Like this,

[TestMethod]
public void Remove_ASubstring_RemovesThatSubstringFromTheEnd()
{
    string str = "Hello, world!";

    string transformed = str.Remove("world!").From(The.End);

    Assert.AreEqual("Hello,", transformed);
    //              ^^^^^^^^
    // Let's use a known value in our assertions
}

If we end up using the same expected values, we can create constants for them. Like,

const string Hello = "Hello";
// or
const string HelloAndComma = "Hello,";

Voilà! That’s the most common mistake when writing unit tests. It seems silly! But often, we duplicate Math operations and string concatenations and it passes unnoticed. Remember, don’t put too much logic in your tests. Tests should be only assignments and method calls.

If you’re new to unit testing, read my post on how to write your first unit tests in C# with MSTest and check the 4 common mistakes when writing your first tests. Also, don’t miss my Unit Testing 101 series where I cover more subjects like the ones from this post.

Happy testing!