xUnit2027 v2 v3 Warning

Comparison of sets to linear containers have undefined results

Cause

A violation of this rule occurs when calling an equality assertion with a set and a linear container.

Reason for rule

Sets do not have a predefined order when enumerating their contents, so comparing them against a linear container (like an array or list) can cause false negatives/positives, depending on how the set data is enumerated. The most common tipping point causing unpredictably tends to be when containers grow larger in size.

For more information, see Equality with hash sets vs. linear containers.

How to fix violations

Create an order to the set data. A common way to do this is to use OrderBy from LINQ.

Examples

Violates

using System.Collections.Generic;
using Xunit;

public class xUnit2027
{
    [Fact]
    public void TestMethod()
    {
        var expected = new List<int> { 42, 2112 };
        var actual = new HashSet<int> { 42, 2112 };

        Assert.Equal(expected, actual);
    }
}

Does not violate

using System.Collections.Generic;
using System.Linq;
using Xunit;

public class xUnit2027
{
    [Fact]
    public void TestMethod()
    {
        var expected = new List<int> { 42, 2112 };
        var actual = new HashSet<int> { 42, 2112 };

        Assert.Equal(expected, actual.OrderBy(x => x));
    }
}