I'm not sure if this is a bug or not, which is why I am starting a discussion.

I have placed a CollectionDefinition in a shared assembly that we use across all of our test projects and I have set DisableParallelization=true. We then use this definition via the new type-based attribute in a test project that references this shared assembly. When running the tests I can see that if I inspect TestContext.Current.TestCollection.CollectionDefinition that Xunit correctly picks up my definition in the shared assembly but, TestContext.Current.TestCollection.DisableParallelization is false. However if I move the collection into the test project, TestContext.Current.TestCollection.DisableParallelization is true.

Per @bradwilson 's comment in #1541 (comment) it sounds like this should now be a supported with xunit3:

I'm closing this issue because I believe it has been resolved through the use of Type-based collection references in v3, which allow collection references across projects.

The official documentation says the following and I'm not sure if it's outdated or not:

[...] but collection definitions must be in the same assembly as the test that uses them.

Obviously it seems to be partially working in-so-much that a test collection is being identified and applied to the tests, but the fact it drops the DisableParallelization value suggests that its either still not intended to work that way or it's a bug. Any ideas on which?

Here's an example. With the CollectionDefinition in a shared assembly, the first test will pass but the second will fail. If moved into the test's assembly then both tests will pass.

Shared assembly:

[CollectionDefinition(DisableParallelization = true)]
public sealed class NoParallelCollection;

Test assembly

[Collection<NoParallelCollection>]
public class MyTests 
{
    [Fact]
    public void TestType()
    {
        var tc = TestContext.Current.TestCollection;

        Assert.NotNull(tc);
        var xtc = Assert.IsType<IXunitTestCollection>(tc, false);
        Assert.Equal(typeof(NoParallelCollection), xtc.CollectionDefinition);
    }
    [Fact]
    public void TestParallel()
    {
        var tc = TestContext.Current.TestCollection;

        Assert.NotNull(tc);
        var xtc = Assert.IsType<IXunitTestCollection>(tc, false);
        Assert.True(xtc.DisableParallelization);
    }
}