Using Should().Equal() on two List<object> fails even though list contents are identical

Description

Using Should().Equal() to compare two List<object> always fails, even though the items in the list are equal. I have reproduced below with strings, and confirmed it is also true with ints.

Complete minimal example reproducing the issue

A reproducible example below. The exact same test but the type of the list goes from List<string> to List<object>

// Two identical strings - should succeed
var strings = new List<string>{"foo"};
var identicalStrings = new List<string>{"foo"};
strings.Should().BeEquivalentTo(identicalStrings); // Passes
strings.Should().Equal(identicalStrings); // Passes

// Same test, but here the specified type is List<object>
var objects = new List<object>{"bar"};
var identicalObjects = new List<object>{"bar"};
objects.Should().BeEquivalentTo(identicalObjects); // Passes
objects.Should().Equal(identicalObjects); // This fails with
// Expected objects to be equal to {{"bar"}}, but {"bar"} differs at index 0.

Full stack trace:

 Error Message:
   Expected objects to be equal to {{"bar"}}, but {"bar"} differs at index 0.
  Stack Trace:
     at FluentAssertions.Execution.XUnit2TestFramework.Throw(String message)
   at FluentAssertions.Execution.TestFrameworkProvider.Throw(String message)
   at FluentAssertions.Execution.DefaultAssertionStrategy.HandleFailure(String message)
   at FluentAssertions.Execution.AssertionScope.FailWith(Func`1 failReasonFunc)
   at FluentAssertions.Execution.AssertionScope.FailWith(Func`1 failReasonFunc)
   at FluentAssertions.Execution.AssertionScope.FailWith(String message, Object[] args)
   at FluentAssertions.Execution.GivenSelector`1.FailWith(String message, Object[] args)
   at FluentAssertions.Execution.GivenSelector`1.FailWith(String message, Func`2[] args)
   at FluentAssertions.Execution.GivenSelectorExtensions.AssertCollectionsHaveSameItems[TActual,TExpected](GivenSelector`1 givenSelector, ICollection`1 expected, Func`3 findIndex)
   at FluentAssertions.Collections.CollectionAssertions`2.AssertSubjectEquality[TActual,TExpected](IEnumerable expectation, Func`3 equalityComparison, String because, Object[] becauseArgs)
   at FluentAssertions.Collections.SelfReferencingCollectionAssertions`2.Equal(T[] elements)
   at AIP.FastlogFG.Test.FastlogReaderRegresssionTets.FOO() in /home/geewee/programming/OAI/FastlogFG/FastlogFG.Test/FastlogReaderRegressionTests.cs:line 42

I have confirmed the same happens with List<int> as List<object> where the error looks similar: Expected objects to be equal to {{2}}, but {2} differs at index 0.

Note that strangely enough this snippet passes succesfully:

var objects = new List<object> {"foo"};
objects.Should().Equal("foo"); 

So it seems to only happen when comparing two List<object>

Versions

Fluent Assertion 5.10.3 .NET Core 3.1

[dennisdoomen]

It’s an combination of an suboptimal API and compiler behavior. You’re using the Equal method that takes a params array. Because of that, you’re actually comparing a collection with a collection of collections of type object. The double braces on the expectation part of the message should be the hint.

[ricirt]

@jnyrup it’s done. Thank you very much! 😃