What's the idiomatic way to verify collection size in xUnit?

Question

I have in my test suite a test that goes something like this:

[Fact]
public void VerifySomeStuff()
{
    var stuffCollection = GetSomeStuff();

    Assert.Equal(1, stuffCollection.Count());
}

This test works as I expect, but when I run it xUnit prints a warning:

warning xUnit2013: Do not use Assert.Equal() to check for collection size.

However, no alternative is suggested in the warning, and a google search takes me to the source code in xUnit for the test that verifies this warning is printed.

If Assert.Equal() isn't the correct way to verify the length of a collection, what is?

To clarify: I realize that I could "trick" xUnit into not emitting this warning by e.g. extracting a variable or using Assert.True(stuff.Count() == 1) instead. The latter is just hacky, and the former feels like if xUnit is e.g. trying to avoid multiple iterations of an IEnumerable<T>, then this is the wrong way to go (because I'll get compiler hints about that separately if it's an issue), and xUnit itself should never have to evaluate the input more than once (in fact it probably will get the same input regardless of variable extraction, because of how C# function calling works).

So, I'm not just interested in removing that warning from my output. An answer to my question also explains why that warning is included in the library in the first place and why whatever approach I should use instead is better.

if you store `stuffCollection.Count()` in a separate variable and pass it to the assert does it give you the same error? — M Y, Oct 09 '17 at 19:21
Maybe [this one](https://stackoverflow.com/a/30815958/107625)? — Uwe Keim, Oct 09 '17 at 19:22

score 185 · Accepted Answer · answered Oct 09 '17 at 20:15

185

Xunit offers quick fixes for most of its warnings, so you should be able to see what it thinks is "right".

In your case, it wants you to use Assert.Single since you are expecting exactly one item. If you were asserting an arbitrary number, like 412, then it would not give you a warning about using Count. It will only suggest using Single if you are expecting one item, or Empty if you are expecting no items.

answered Oct 09 '17 at 20:15

vcsjones

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Thanks, that makes sense. FWIW, I was seeing this when building in VS Code, where the quick action did not show up, so actually including the fix suggestion in the warning message would have been much more helpful. – Tomas Aschan Oct 09 '17 at 21:03
2

@TomasLycken - ah. Yes there is an issue for that here: https://github.com/xunit/xunit/issues/1423 – vcsjones Oct 09 '17 at 21:05
15

I'm not a fan of that behavior; sometimes the 1 count is just incidental, and it seems less expressive to enforce the call to .Single(). The test may change to expect a different count, and it seems annoying to have to make the change to call a completely different method rather than just changing a number. – vargonian Feb 15 '19 at 00:23
2

Single is cool for single Item, I have 3 items, and I don't want to write full Assert.Collection, does xUnit have Assert.Triple? haha – Pawel Cioch Nov 13 '19 at 17:19
1

@PawelCioch according to https://xunit.net/xunit.analyzers/rules/xUnit2013.html they have `Empty`, `Single` and `NotEmpty` - if you expect a dynamic value xUnit2013 shouldn't trigger. – mbx Apr 17 '20 at 05:16
@mbx thus it should not show warning. I can do Assert.True(3 == list.Count) to avoid warning, but this is worse than Assert.Equal(3, list.Count) in testing the error for first will be "expected true found false" not very informative, while equal "expected 3, found 1" so Equal for collection count is definitely better than "hacking" to avoid warning. I get the intention of xUnit creator but sometimes it's really better to check count than to write complex Assert.Collection – Pawel Cioch Apr 22 '20 at 17:06
@PawelCioch xUnit only warns for 0 and 1, I don't get a warning for `var list = new List { 1, 2, 3 };Assert.Equal(3, list.Count);` – mbx Apr 23 '20 at 06:25
1

@mbx I'll try some day, maybe new version does it better now... Thanks! – Pawel Cioch Jul 01 '20 at 14:45
But why would it give an error about checking for size? The error suggests you should never do it, not that there's a better way in this specific instance. – Captain Prinny Jul 07 '21 at 15:00

Tikall · Answer 2 · 2021-11-19T13:34:02.120

20

The rule only applies when testing for 0 or 1 items in collection.

Assert.Equal(0, result.Length) // rule warning, use .Empty
Assert.Equal(1, result.Length) // rule warning, use .Single
Assert.Equal(2, result.Length) // ok

To satisy rule:

Assert.Empty(result); // for 0 items
Assert.Single(result); // for 1 item
Assert.NotEmpty(result); // for 1 or more items

When using Assert.NotEmpty we may as well be precise with a count

Assert.Equal(2, result.Length) // Does not violate rule xUnit2013

https://xunit.net/xunit.analyzers/rules/xUnit2013

edited Nov 19 '21 at 13:34

answered Jul 08 '21 at 10:34

Tikall

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13

`Assert.NotEmpty(result)` is not a good equivalent for `Assert.Equal(2, result.Length)` nor for *2 or more items* it tests a different thing. – jakubiszon Nov 17 '21 at 16:46
1

Agree, it was an "Off by 1"-error in comment. Edited comment for Assert.NotEmpty(result) from 2 to 1. – Tikall Nov 19 '21 at 13:36

score 5 · Answer 3 · edited Apr 21 '23 at 04:45

If you have more than one item, you can't use Assert.Single.

The expectation seems to be that you should use Assert.Collection:

var stuffCollection = GetSomeStuff();

Assert.Collection(stuffCollection, 
    item => Assert.True(true), // this lambda verifies the first item
    item => Assert.True(true), // second item
);

The assertion above verifies that there are exactly two items in the collection. You can provide stricter lambdas (such as item => Assert.Equals(7, item.property1) for each item if you want.

Personally, I'm not a fan; this seems like a very verbose way of saying how long you want the collection to be.

score 3 · Answer 4 · answered Nov 28 '18 at 16:33

3

I found this give me the same error:

Assert.Equal(2, vm.Errors.Count());

And casting it stopped the error from appearing.

Assert.Equal(2, (int)vm.Errors.Count());

answered Nov 28 '18 at 16:33

devjc

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I'm quite sure, this is *not* the *ideomatic* way. – mbx Apr 16 '20 at 12:16
You _could_ also do `Assert.Equal(2, vm.Errors.Count() + 1)` =) – Per Erik Stendahl Sep 19 '21 at 10:31

score 2 · Answer 5 · answered Apr 12 '18 at 02:59

2

I had same issue when I used Count property as below in xUnit.

After, I use Count() function on collection, it fixed my issue.

answered Apr 12 '18 at 02:59

Bhuwan Maharjan

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Fixed the issue, but you still don't use XUnit like you should! – Daniel Eisenreich Sep 19 '18 at 06:41
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@DanielEisenreich what is the correct way to assert count for a specific number if it's greater than 1? – SomeGuyOnAComputer Sep 28 '18 at 19:42
1

@SomeGuyOnAComputer and the other 4 upvotes. Forget what I said, I was too cheeky. If it's greater one you have no other choice. – Daniel Eisenreich Nov 09 '18 at 06:45

score 2 · Answer 6 · answered Aug 10 '20 at 16:24

2

For single element in a list, it's best to use this instead: Assert.Single(resultList);

answered Aug 10 '20 at 16:24

Bosco Han

103
5

score 1 · Answer 7 · answered Nov 01 '22 at 23:20

Look I won't tell anyone if you just ignore the warning. Personally I think its more hassle than its worth. If you have an .editorconfig:

[*.cs]
dotnet_diagnostic.xUnit2013.severity = none # warning xUnit2013: Do not use Assert.Equal() to check for collection size.

score -1 · Answer 8 · answered Nov 18 '22 at 05:01

-1

To check the length of a collection with one element, you can use:

Assert.Single(yourCollection)

answered Nov 18 '22 at 05:01

Alex

1

What's the idiomatic way to verify collection size in xUnit?

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