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Clarify CollectionAssert.Contains parameters and failure message #1272

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Clarify CollectionAssert.Contains parameters and failure message
SeanKilleen Oct 22, 2026
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Merge remote-tracking branch 'origin/master' into 672_collectionasser…
SeanKilleen Oct 26, 2026
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31 changes: 27 additions & 4 deletions ...articles/nunit/writing-tests/assertions/classic-assertions/Collection-Assert.md
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Original file line number Diff line number Diff line change
Expand Up @@ -46,13 +46,13 @@ CollectionAssert.AreNotEquivalent(IEnumerable expected, IEnumerable actual);
CollectionAssert.AreNotEquivalent(
IEnumerable expected, IEnumerable actual, string message, params object[] args);

CollectionAssert.Contains(IEnumerable expected, object actual);
CollectionAssert.Contains(IEnumerable collection, object actual);
CollectionAssert.Contains(
IEnumerable expected, object actual, string message, params object[] args);
IEnumerable collection, object actual, string message, params object[] args);

CollectionAssert.DoesNotContain(IEnumerable expected, object actual);
CollectionAssert.DoesNotContain(IEnumerable collection, object actual);
CollectionAssert.DoesNotContain(
IEnumerable expected, object actual, string message, params object[] args);
IEnumerable collection, object actual, string message, params object[] args);

CollectionAssert.IsSubsetOf(IEnumerable subset, IEnumerable superset);
CollectionAssert.IsSubsetOf(
Expand All @@ -79,6 +79,29 @@ CollectionAssert.IsOrdered(IEnumerable collection,
IComparer comparer, string message, params object[] args);
```

## Contains and DoesNotContain

@stevenaw stevenaw Oct 25, 2026

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Thanks for putting a PR up for this one @SeanKilleen

I'll have to reacquaint myself with the issue before reviewing. Hopefully soon. I had thought we supported subset-style contains through this API too, but perhaps that's just at constraint level


The parameters of `Contains` and `DoesNotContain` do not follow the usual *expected, actual* pattern of the other
classic assertions. The first argument is the collection to search, and the second, named `actual`, is the single item
being looked for. Despite its name, that item is what the assertion *expects* to find, so the failure message reports it
on the **Expected** line and the collection on the **But was** line:

```csharp
CollectionAssert.Contains(new[] { 1, 2 }, 3);

// Expected: some item equal to 3
// But was: < 1, 2 >
```

Passing a collection as the second argument does not check that one collection contains another. It checks whether the
collection contains that collection as a single element. The parameter names are kept for compatibility, since renaming
them would break callers that use named arguments. The constraint-based equivalents read more naturally:

```csharp
Assert.That(new[] { 1, 2 }, Does.Contain(3));
Assert.That(new[] { 1, 2 }, Has.Member(3));
```

## Examples

### Comparing collections
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