xUnit2028 v2 v3 Warning

Do not use Assert.Empty or Assert.NotEmpty with problematic types

Cause

Some problematic types (ArraySegment<T> and StringValues) should not used with Assert.Empty or Assert.NotEmpty.

Reason for rule

The reason ArraySegment<T> is problematic is because it has an implementation of GetEnumerator that can throw exceptions, and the assertion library uses enumerator to determine when a collection is empty (or not).

The reason StringValues is problematic is because it contains an implicit cast to string, which the compiler favors over its implementation of IEnumerable, and when the collection is empty, the string that it returns is null (which is not legal for Assert.Empty). Further, you can’t use Assert.NotNull because there is no string overload, so you’re not checking the implicit string for null-ness.

How to fix violations

To fix a violation of this rule, it is recommended that you examine the item count via the Count property with Assert.Equal or Assert.NotEqual. Doing these operations with these known problematic types will not trigger xUnit2013 (unlike other collections which do).

Examples

Violates

using System;
using Xunit;

public class xUnit2028
{
    [Fact]
    public void TestMethod()
    {
        var arraySegment = new ArraySegment<int>();

        Assert.Empty(arraySegment);
    }
}

using Microsoft.Extensions.Primitives;
using Xunit;

public class xUnit2028
{
    [Fact]
    public void TestMethod()
    {
        var stringValues = StringValues.Empty;

        Assert.Empty(stringValues);
    }
}

Does not violate

using System;
using Xunit;

public class xUnit2028
{
    [Fact]
    public void TestMethod()
    {
        var arraySegment = new ArraySegment<int>();

        Assert.Equal(0, arraySegment.Count);
    }
}

using Microsoft.Extensions.Primitives;
using Xunit;

public class xUnit2028
{
    [Fact]
    public void TestMethod()
    {
        var stringValues = StringValues.Empty;

        Assert.Equal(0, stringValues.Count);
    }
}