Home ยป Is it OK to have multiple asserts in a single unit test?

Is it OK to have multiple asserts in a single unit test?

Solutons:


I don’t think it’s necessarily a bad thing, but I do think we should strive towards only having single asserts in our tests. This means you write a lot more tests and our tests would end up testing only one thing at a time.

Having said that, I would say maybe half of my tests actually only have one assert. I think it only becomes a code (test?) smell when you have about five or more asserts in your test.

How do you solve multiple asserts?

Tests should fail for one reason only, but that doesn’t always mean that there should be only one Assert statement. IMHO it is more important to hold to the “Arrange, Act, Assert” pattern.

The key is that you have only one action, and then you inspect the results of that action using asserts. But it is “Arrange, Act, Assert, End of test“. If you are tempted to continue testing by performing another action and more asserts afterwards, make that a separate test instead.

I am happy to see multiple assert statements that form parts of testing the same action. e.g.

[Test]
public void ValueIsInRange()
{
  int value = GetValueToTest();

  Assert.That(value, Is.GreaterThan(10), "value is too small");
  Assert.That(value, Is.LessThan(100), "value is too large");
} 

or

[Test]
public void ListContainsOneValue()
{
  var list = GetListOf(1);

  Assert.That(list, Is.Not.Null, "List is null");
  Assert.That(list.Count, Is.EqualTo(1), "Should have one item in list");
  Assert.That(list[0], Is.Not.Null, "Item is null");
} 

You could combine these into one assert, but that’s a different thing from insisting that you should or must. There is no improvement from combining them.

e.g. The first one could be

Assert.IsTrue((10 < value) && (value < 100), "Value out of range"); 

But this is not better – the error message out of it is less specific, and it has no other advantages. I’m sure you can think of other examples where combining two or three (or more) asserts into one big boolean condition makes it harder to read, harder to alter and harder to work out why it failed. Why do this just for the sake of a rule?

NB: The code that I am writing here is C# with NUnit, but the principles will hold with other languages and frameworks. The syntax may be very similar too.

I have never thought that more than one assert was a bad thing.

I do it all the time:

public void ToPredicateTest()
{
    ResultField rf = new ResultField(ResultFieldType.Measurement, "name", 100);
    Predicate<ResultField> p = (new ConditionBuilder()).LessThanConst(400)
                                                       .Or()
                                                       .OpenParenthesis()
                                                       .GreaterThanConst(500)
                                                       .And()
                                                       .LessThanConst(1000)
                                                       .And().Not()
                                                       .EqualsConst(666)
                                                       .CloseParenthesis()
                                                       .ToPredicate();
    Assert.IsTrue(p(ResultField.FillResult(rf, 399)));
    Assert.IsTrue(p(ResultField.FillResult(rf, 567)));
    Assert.IsFalse(p(ResultField.FillResult(rf, 400)));
    Assert.IsFalse(p(ResultField.FillResult(rf, 666)));
    Assert.IsFalse(p(ResultField.FillResult(rf, 1001)));

    Predicate<ResultField> p2 = (new ConditionBuilder()).EqualsConst(true).ToPredicate();

    Assert.IsTrue(p2(new ResultField(ResultFieldType.Confirmation, "Is True", true)));
    Assert.IsFalse(p2(new ResultField(ResultFieldType.Confirmation, "Is False", false)));
}

Here I use multiple asserts to make sure complex conditions can be turned into the expected predicate.

I am only testing one unit (the ToPredicate method), but I am covering everything I can think of in the test.

Related Solutions

When should I not kill -9 a process?

Generally, you should use kill (short for kill -s TERM, or on most systems kill -15) before kill -9 (kill -s KILL) to give the target process a chance to clean up after itself. (Processes can't catch or ignore SIGKILL, but they can and often do catch SIGTERM.)...

Default value for UUID column in Postgres

tl;dr Call DEFAULT when defining a column to invoke one of the OSSP uuid functions. The Postgres server will automatically invoke the function every time a row is inserted. CREATE TABLE tbl ( pkey UUID NOT NULL DEFAULT uuid_generate_v1() , CONSTRAINT pkey_tbl...

comparing five integers with if , else if statement

try this : int main () { int n1, n2, n3, n4, n5, biggest,smallest; cout << "Enter the five numbers: "; cin >> n1 >> n2 >> n3 >> n4 >> n5 ; smallest=biggest=n1; if(n2>biggest){ biggest=n2; } if(n2<smallest){ smallest=n2;...

How to play YouTube audio in background/minimised?

Here's a solution using entirely free and open source software. The basic idea is that although YouTube can't play clips in the background, VLC for Android can play clips in the background, so all we need to do is pipe the clip to VLC where we can listen to it...

Why not use “which”? What to use then?

Here is all you never thought you would ever not want to know about it: Summary To get the pathname of an executable in a Bourne-like shell script (there are a few caveats; see below): ls=$(command -v ls) To find out if a given command exists: if command -v...

Split string into Array of Arrays [closed]

If I got correct what you want to receive as a result, then this code would make what you want: extension Array { func chunked(into size: Int) -> [[Element]] { return stride(from: 0, to: self.count, by: size).map { Array(self[$0 ..< Swift.min($0 + size,...

Retrieving n rows per group

Let's start with the basic scenario. If I want to get some number of rows out of a table, I have two main options: ranking functions; or TOP. First, let's consider the whole set from Production.TransactionHistory for a particular ProductID: SELECT...

Don’t understand how my mum’s Gmail account was hacked

IMPORTANT: this is based on data I got from your link, but the server might implement some protection. For example, once it has sent its "silver bullet" against a victim, it might answer with a faked "silver bullet" to the same request, so that anyone...

What is /storage/emulated/0/?

/storage/emulated/0/Download is the actual path to the files. /sdcard/Download is a symlink to the actual path of /storage/emulated/0/Download However, the actual files are located in the filesystem in /data/media, which is then mounted to /storage/emulated/0...

How can I pass a command line argument into a shell script?

The shell command and any arguments to that command appear as numbered shell variables: $0 has the string value of the command itself, something like script, ./script, /home/user/bin/script or whatever. Any arguments appear as "$1", "$2", "$3" and so on. The...

What is pointer to string in C?

argv is an array of pointers pointing to zero terminated c-strings. I painted the following pretty picture to help you visualize something about the pointers. And here is a code example that shows you how an operating system would pass arguments to your...

How do mobile carriers know video resolution over HTTPS connections?

This is an active area of research. I happen to have done some work in this area, so I'll share what I can about the basic idea (this work was with industry partners and I can't share the secret details ๐Ÿ™‚ ). The tl;dr is that it's often possible to identify an...

How do I change the name of my Android device?

To change the hostname (device name) you have to use the terminal (as root): For Eclair (2.1): echo MYNAME > /proc/sys/kernel/hostname For Froyo (2.2): (works also on most 2.3) setprop net.hostname MYNAME Then restart your wi-fi. To see the change, type...

How does reverse SSH tunneling work?

I love explaining this kind of thing through visualization. ๐Ÿ™‚ Think of your SSH connections as tubes. Big tubes. Normally, you'll reach through these tubes to run a shell on a remote computer. The shell runs in a virtual terminal (tty). But you know this part...

Difference between database vs user vs schema

In Oracle, users and schemas are essentially the same thing. You can consider that a user is the account you use to connect to a database, and a schema is the set of objects (tables, views, etc.) that belong to that account. See this post on Stack Overflow:...