gistfile1.cs

public class Test
    {
        private Fixture fixture;
        private IRepository sut;

        public Test()
        {
            fixture = new Fixture(); // with some configuration
            sut = fixture.Freeze<IRepository>();
        }

        [Theory, NSubData]
        public void nsubData_gets_the_same_fixture_instance([Frozen]IRepository repo)
        {
            repo.Should().BeSameAs(sut);
        }
    }