public class USER
    {
       [Key]
        public int USERID { get; set; }
        public string NAME { get; set; }
        public string DEPTID{ get; set; }
        public string TYPEID{ get; set; }
        public virtual DEPT depts { get; set; }
        public virtual USERTYPE usertypes{ get; set; }
     ｝
public class DEPT 
    {
       [Key]
        public string DEPTID{ get; set; }
        public string NAME{ get; set; }
        public virtual ICollection<USER> users { get; set; }
     ｝
public class USERTYPE
    {
       [Key]
        public string TYPEID{ get; set; }
        public string NAME{ get; set; }
        public virtual ICollection<USER> users { get; set; }
     ｝

public class USERDB:DbContext
    {
        public DbSet<USER> users { get; set; }
        public DbSet<DEPT> depts { get; set; }
        public DbSet<USERTYPE> usertypes{ get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            
            modelBuilder.Entity<USER>()
                .HasRequired(u => u.depts)
                .WithMany(d => d.users)
                .HasForeignKey(a => a.DEPTID)
                .WillCascadeOnDelete(false);
             modelBuilder.Entity<USER>()
                .HasRequired(u => u.usertypes)
                .WithMany(t => t.users)
                .HasForeignKey(a => a.TYPEID)
                .WillCascadeOnDelete(false);
               
        }
    }

public ActionResult TestList()
        {
            var db=new USERDB();

            var users = db.users.ToList();

            
            return View(users);
        }

@model IEnumerable<testlist.Models.USER>

@foreach (var item in Model) {
    <tr>       
        <td>
            @item.ID
        </td>
        <td>
            @item.NAME
        </td>
        <td>
            @item.depts.NAME   //<--------这里报错
        </td>
<td>
            @item.usertypes.NAME
        </td>
        <td>