MSTEST0065: Avoid Assert.AreEqual on collection types

Property	Value
Rule ID	MSTEST0065
Title	Avoid Assert.AreEqual on collection types
Category	Usage
Fix is breaking or non-breaking	Non-breaking
Enabled by default	Yes
Default severity	Warning
Introduced in version	4.3.0
Is there a code fix	No

Note

This rule is available starting with MSTest 4.3.

Cause

A call to Assert.AreEqual or Assert.AreNotEqual is made on a value whose static type implements IEnumerable<T> (other than String).

Rule description

AreEqual and AreNotEqual use EqualityComparer<T>.Default. For most collection types — for example arrays, List<T>, or any user-defined type implementing IEnumerable<T> — this falls back to reference equality (or to whatever equality the type defines for itself), and not to element-wise comparison. As a result, the assertion almost never asserts what the test author intended.

[TestClass]
public class TestClass
{
    [TestMethod]
    public void Test()
    {
        var expected = new[] { 1, 2, 3 };
        var actual = new[] { 1, 2, 3 };

        // Violation: this compares references, not contents, and fails.
        Assert.AreEqual(expected, actual);
    }
}

How to fix violations

Choose the assertion that matches your intent:

• Use Assert.AreSequenceEqual for ordered element-wise comparison.
• Use Assert.AreSequenceEqual(expected, actual, SequenceOrder.InAnyOrder) for unordered element-wise comparison.
• Use Assert.AreEquivalent for deep structural comparison.

[TestClass]
public class TestClass
{
    [TestMethod]
    public void Test()
    {
        var expected = new[] { 1, 2, 3 };
        var actual = new[] { 1, 2, 3 };

        Assert.AreSequenceEqual(expected, actual);
    }
}

When to suppress warnings

Suppress this warning only if the type defines its own Equals/GetHashCode to compare contents and you intentionally rely on that.

Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable MSTEST0065
// The code that's violating the rule is on this line.
#pragma warning restore MSTEST0065

To disable the rule for a file, folder, or project, set its severity to none in the configuration file.

[*.{cs,vb}]
dotnet_diagnostic.MSTEST0065.severity = none

For more information, see How to suppress code analysis warnings.