asp.net-mvc – 实体框架数据库 – 第一个uniqueidentity结果没

Violation of PRIMARY KEY constraint ‘PK_User’. Cannot insert duplicate
key in object ‘dbo.User’. The duplicate key value is
(00000000-0000-0000-0000-000000000000). The statement has been
terminated.

码：

public partial class User
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public System.Guid Id { get; set; }
    public string UserName { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
}

我的UserController代码

[HttpGet]
public ActionResult Create() {
    return View();
}

[HttpPost]
public ActionResult Create(User user) {
        if (ModelState.IsValid) {
            db.Users.Add(user);
            db.SaveChanges();

            return RedirectToAction("Index");
        }

        return View(user);
}

如何自动生成新的guid？