Skip to content

How Do I Share a Value Across a Test?

Use [Shared] when a composed parameter and a value used inside another composed parameter need to be the exact same instance, not two independently-composed look-alikes.

Basic case

[Theory]
[Compose]
public void ServiceUsesTheSharedRepository([Shared] Repository repository, OrderService service)
{
    service.Repository.Should().BeSameAs(repository);
}

[Shared] Repository repository composes Repository first; when OrderService's constructor needs a Repository, it reuses that exact instance instead of composing a new one.

Sharing a substitute so you can assert on it

The most common real use — compose a substitute, hand it to a dependent service, and assert against it directly:

[Theory]
[Compose<NSubstituteTestProfile>]
public async Task Saves_order([Shared] IOrderRepository repository, CreateOrderHandler handler, PlaceOrder command)
{
    await handler.Handle(command);
    await repository.Received(1).SaveAsync(Arg.Any<Order>(), Arg.Any<CancellationToken>());
}

Without [Shared], repository and the IOrderRepository composed inside handler would be two separate substitutes — asserting Received(1) on repository would fail even though handler genuinely called a repository, just not this one.

Common mistakes

  • Expecting [Shared] to match by parameter name — sharing is type-keyed; every parameter or nested dependency requesting exactly that type in the same row reuses the value, regardless of name.
  • Declaring two [Shared] parameters of the same type on one method — there's no way to tell which is "the" shared instance, so this is a signature error.
  • Reaching for [Shared] on a core (non-Compono.XunitV3) Composer — [Shared] is scoped to a Compono.XunitV3 [Compose] row; programmatic composition doesn't have this concept.

Next