Customization with timed Do

Contact.cs

    public class Contact : IHubSpotEntity
    {
        [DefaultProperty("vid")]
        public long Id { get; set; }

        [DefaultProperty("portal-id")]
        public long PortalId { get; set; }

        [DefaultProperty("is-contact")]
        public bool IsContact { get; set; }

        [DefaultProperty("profile-token")]
        public string UserToken { get; set; }

        [DefaultProperty("profile-url")]
        public Uri ProfileUrl { get; set; }

        [CustomProperty("email")]
        public string Email { get; set; }

        [CustomProperty("createdate", IsReadOnly = true)]
        public DateTimeOffset Created { get; set; }

        [CustomProperty("firstname")]
        public string FirstName { get; set; }

        [CustomProperty("lastname")]
        public string LastName { get; set; }

        [CustomProperty("lastmodifieddate", IsReadOnly = true)]
        public DateTimeOffset LastModified { get; set; }

        [CustomProperty("associatedcompanyid")]
        public long AssociatedCompanyId { get; set; }

        IReadOnlyDictionary<string, object> IHubSpotEntity.Properties { get; set; } = new Dictionary<string, object>();
    }

ContactAutoDataAttribute.cs

    public class ContactAutoDataAttribute : AutoDataAttribute
    {
        public ContactAutoDataAttribute() : base(CreateFixture)
        {
            
        }

        public static IFixture CreateFixture()
        {
            IFixture fixture = new Fixture();

            fixture.Customize<Contact>(c => c.Do(contact =>
            {
                ((IHubSpotEntity)contact).Properties = new Dictionary<string, object>
                {
                    ["firstname"] = contact.FirstName,
                    ["lastname"] = contact.LastName,
                    ["email"] = contact.Email,
                    ["createdate"] = contact.Created,
                    ["associatedcompanyId"] = contact.AssociatedCompanyId
                };
            }));

            return fixture;
        }
    }

Test.cs

[Test, ContactAutoData]
public void Customization_works(Contact contact)
{
    IHubSpotEntity hubSpot = contact;

    Assert.That(contact.FirstName, Is.EqualTo(hubSpot.Properties["firstname"])); // BOOM!
}