我有 4 个 class 和 class 属性，但我应该如何关联这些类？ （一对一，一对多……）

Question

which table should be associated with which table?

How do I add foreign keys?

I am confused. Can you help me?

ex: user and department ---> one to many?

  public class User
    {
        public int UserId { get; set; } 
        public string Username { get; set; } 
        public string Email { get; set; } 
        public string Password { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int DepartmentId { get; set; }
        public int TitleId { get; set; }
        public int ManagerUserId { get; set; }
    }
    public class Department
    {
        public int DepartmentId { get; set; }
        public string DepartmentCode { get; set; }
        public string Name { get; set; }
        public int ManagerDepartmentId { get; set; }
        public int ManagerUserId { get; set; }
    }
    public class Position
    {
        public int PositionId { get; set; }
        public string PositionCode { get; set; }
        public string Name { get; set; }
        public int UserId { get; set; }
        public byte Status { get; set; }
    }
    public class Title
    {
        public int TitleId { get; set; }
        public string Name { get; set; }
        public byte IsIntegrationData { get; set; }
    }

Answer 1

User and department is one-to-many relationship. I would recommend you to look here: An example how configure One-to-Many relationship between two entities in Entity Framework 6 using the code-first approach.

Answer 2

Try something like this:

  public class User
    {
        public int UserId { get; set; } 
        public string Username { get; set; } 
        public string Email { get; set; } 
        public string Password { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }

        [ForeignKey("Department")]
        public int DepartmentId { get; set; }
        public virtual Department Department {get; set;}
        
        [ForeignKey("Title")]
        public int TitleId { get; set; }
        public virtual Title Title {get;set;}

        [ForeignKey("Manager")]
        public int ManagerUserId { get; set; }
        public virtual User Manager {get; set;}
        
        [InverseProperty("Manager")]
        public virtual List<Department> DepartmentsManaged {get; set;}
    }


    public class Department
    {
        public int DepartmentId { get; set; }
        public string DepartmentCode { get; set; }
        public string Name { get; set; }

        [ForeignKey("Manager")]
        public int ManagerUserId { get; set; }
        public virtual User Manager {get; set;}

        public virtual List<User> Users {get; set;}
    }


    public class Position
    {
        public int PositionId { get; set; }
        public string PositionCode { get; set; }
        public string Name { get; set; }

        [Required]
        [ForeignKey("User")]
        public int UserId { get; set; }
        public virtual User User {get; set;}

        public byte Status { get; set; }
    }
    public class Title
    {
        public int TitleId { get; set; }
        public string Name { get; set; }
        public byte IsIntegrationData { get; set; }

        public virtual List<User> Users {get; set;}
    }

I removed a few fields such as ManagerDepartmentId because this should be accessed through the navigation property Department.Manager.DepartmentId.