Global AssertionOptions are not thread-safe

Description

As discussed in #808 I’m opening a separate issue.

Global AssertionOptions appear to not be thread-safe.

Assume the following scenario.

Test 1 is using Should().BeEquivalentTo() and the current execution is at CloneDefaults Test 2 is executing AssertionOptions.AssertEquivalencyUsing() at the same time on a different thread. This modifies the collection the CloneDefaults is currently iterating through. Which is of course not allowed.

Complete minimal example reproducing the issue

Since this is a multi-threading issue it’s a bit difficult to reproduce. The more tests there are run in parallel, the higher the chance it will occur. But basically this is an outline of what happens in various tests that eventually lead to the exception. I’m trying to get a failure with only these 2 tests using Riders Run tests until failure feature

namespace FluentAssertionsRepro
{
    using FluentAssertions;
    using Xunit;

    public class Test1
    {
        [Fact]
        public void Test()
        {
            const string Text = "Some text";
            var container = new Container(Text);

            container.Should().BeEquivalentTo(new Container(Text));
        }
    }

    public class Test2
    {
        static Test2()
        {
            AssertionOptions.AssertEquivalencyUsing(
                options => options
                    .Using<Container>(
                        context =>
                        {
                            context.Subject.Should().BeEquivalentTo(context.Expectation);
                        })
                    .WhenTypeIs<Container>());
        }

        [Fact]
        public void Test()
        {
            const string Text = "Some text";
            var x = new Container(Text);

            x.Should().BeEquivalentTo(new Container(Text));
        }
    }

    public class Container
    {
        public Container(string text) => this.Text = text;

        public string Text { get; }
    }
}

Expected behavior:

Have Test 1 run normally Have Test 2 run with updated AssertionOptions

Actual behavior:

I get this exception.

System.InvalidOperationException : Collection was modified; enumeration operation may not execute.
  Stack Trace:
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Linq.Enumerable.ConcatIterator`1.MoveNext()
   at System.Collections.Generic.List`1.InsertRange(Int32 index, IEnumerable`1 collection)
   at System.Collections.Generic.List`1.AddRange(IEnumerable`1 collection)
   at FluentAssertions.Equivalency.SelfReferenceEquivalencyAssertionOptions`1..ctor(IEquivalencyAssertionOptions defaults)
   at FluentAssertions.Equivalency.EquivalencyAssertionOptions`1..ctor(IEquivalencyAssertionOptions defaults)
   at FluentAssertions.AssertionOptions.CloneDefaults[T]()
   at FluentAssertions.Primitives.ObjectAssertions.BeEquivalentTo[TExpectation](TExpectation expectation, Func`2 config, String because, Object[] becauseArgs)
   at FluentAssertions.Primitives.ObjectAssertions.BeEquivalentTo[TExpectation](TExpectation expectation, String because, Object[] becauseArgs)

Versions

• Which version of Fluent Assertions are you using? 5.10.3
• Which .NET runtime and version are you targeting? E.g. .NET framework 4.6.1 or .NET Core 2.1. .NET Core 3.1 (netcoreapp3.1)

Additional Information

Any additional information, configuration or data that might be necessary to reproduce the issue.

[eNeRGy164]

You could add an additional if (alreadyLoaded) { return; } before the lock, if it’s already true, nobody will set it to false and saves a lock

[jnyrup]

I just tried fiddling with creating a custom XunitTestFramework and registering it using [assembly: TestFramework()].

So far my experiments show that it:

• is invoked exactly once,
• before any tests are run and
• lets tests run in parallel.

XUnitTestProject3.zip