我为“asp.net核心mvc 3.0”网站制作过滤器。我有以下过滤器：“语言”、“专业”、“成本”。我可以从网站上得到所有过滤器的ID。但我不能返回到符合这些过滤器的网站用户。下面是我的代码：
类用户

public class User : IdentityUser
 {
     public virtual ICollection<UserAndLanguage> UserAndLanguages { get; set; } = new List<UserAndLanguage>();
     public virtual ICollection<TutorAndProfession> andProfessions { get; set; } = new List<TutorAndProfession>();
     //public List<UserAndLanguage> andLanguages { get; set; }
     public virtual ICollection<TutorProfessionAndCost> TutorProfessionAndCosts { get; set; } = new List<TutorProfessionAndCost>();

     [Not Mapped]
     public List<Language> Languages { get; set; }

     [Not Mapped]
     public List<TutorProfessionAndCost> ProfessionAndCosts { get; set; }

     public User()
     {
         Languages = new List<Language>();
         ProfessionAndCosts = new List<TutorProfessionAndCost>();
     }
 }

class TutorProfessionAndCost

public class TutorProfessionAndCost
 {
     public int Id { get; set; }

     public string UserId { get; set; }
     [Not Mapped]
     public List<User> Users { get; set; }
     public TutorProfessionAndCost()
     {
         Users = new List<User>();
     }

     //profession
     public int IdTutorProfession { get; set; }
     public string EducationalSubject { get; set; }
    
     public int Cost_One { get; set; } //the cost of the lesson is individual
     public int Cost_Group { get; set; } //cost of class group class

     public virtual ICollection<TutorAndProfession> UserAndProfs { get; set; } = new List<TutorAndProfession>();
 }

UserAndLanguage类

public class UserAndLanguage
 {
     public int Id { get; set; }
    
     public string UserId { get; set; }
     public User User { get; set; }
    
     public int LangId { get; set; }
     public Language Language { get; set; }
 }

数据库：

public DbSet<UserAndLanguage> UserAndLanguages { get; set; }
     public DbSet<TutorAndProfession> TutorAndProfessions { get; set; }

    protected override void OnModelCreating(ModelBuilder builder)
     {
         base.OnModelCreating(builder);

         builder.Entity<User>().HasKey(u => u.Id);

         //many-to-many communication for the languages in which the user teaches
         builder.Entity<UserAndLanguage>()
             .HasKey(ul => new { ul.UserId, ul.LangId });

         builder.Entity<UserAndLanguage>()
             .HasOne(ul => ul.User)
             .WithMany(u => u.UserAndLanguages)
             .HasForeignKey(ul => ul.UserId);

         builder.Entity<UserAndLanguage>()
             .HasOne(ul => ul.Language)
             .WithMany(u => u.UserAndLanguages)
             .HasForeignKey(ul => ul.LangId);

         //connection for professions and user
         //builder.Entity<TutorProfessionAndCost>()
         //.HasOne(p => p.User)
         //.WithMany(u => u.ProfessionAndCosts)
         //.HasForeignKey(p => p.UserId);

         //many-to-many relationship for professions and user
         builder.Entity<TutorAndProfession>()
             .HasKey(up => new { up.UserId, up.ProfessionId });

         builder.Entity<TutorAndProfession>()
             .HasOne(up => up.User)
             .WithMany(u => u.andProfessions)
             .HasForeignKey(up => up.UserId);

         builder.Entity<TutorAndProfession>()
             .HasOne(up => up.Profession)
             .WithMany(u => u.UserAndProfs)
             .HasForeignKey(up => up.ProfessionId);

     }

控制器

public IActionResult TutorsInfo(int ProfesionTutor, int LanguageTutor, string OneOrGroupTutor, int PriseTutor)
    {
        //List<User> Tutors = new List<User>();

        List<UserAndLanguage> tLang = new List<UserAndLanguage>();
        //List<string> tLang = new List<string>();

        if (LanguageTutor == 0)
            tLang = _context.UserAndLanguages.ToList();
        else
            tLang = _context.UserAndLanguages.Where(t => t.LangId == LanguageTutor).ToList();

        List<TutorProfesionAndCost> tProf = new List<TutorProfesionAndCost>();
        
        if(PriseTutor != 0)
        {
            if (OneOrGroupTutor == "all")
                tProf = _context.TutorProfesionAndCosts.Where(t => t.IdTutorProfesion == ProfesionTutor).ToList();
            else if (OneOrGroupTutor == "One")
                tProf = _context.TutorProfesionAndCosts.Where(t => t.IdTutorProfesion == ProfesionTutor).Where(t => t.Cost_One != 0 && t.Cost_One <= PriseTutor).ToList();
            else if (OneOrGroupTutor == "Group")
                tProf = _context.TutorProfesionAndCosts.Where(t => t.IdTutorProfesion == ProfesionTutor).Where(t => t.Cost_Group != 0 && t.Cost_Group <= PriseTutor).ToList();
            else if (OneOrGroupTutor == "Work")
                tProf = _context.TutorProfesionAndCosts.Where(t => t.IdTutorProfesion == ProfesionTutor).Where(t => t.ControlWork == true).ToList();
        }
        else
        {
            if (OneOrGroupTutor == "all")
                tProf = _context.TutorProfesionAndCosts.Where(t => t.IdTutorProfesion == ProfesionTutor).ToList();
            else if (OneOrGroupTutor == "One")
                tProf = _context.TutorProfesionAndCosts.Where(t => t.IdTutorProfesion == ProfesionTutor).Where(t => t.Cost_One != 0).ToList();
            else if (OneOrGroupTutor == "Group")
                tProf = _context.TutorProfesionAndCosts.Where(t => t.IdTutorProfesion == ProfesionTutor).Where(t => t.Cost_Group != 0).ToList();
            else if (OneOrGroupTutor == "Work")
                tProf = _context.TutorProfesionAndCosts.Where(t => t.IdTutorProfesion == ProfesionTutor).Where(t => t.ControlWork == true).ToList();
        }

        if (tProf == null && ProfesionTutor == 0)
            tProf = _context.TutorProfesionAndCosts.ToList();

        
        //tProf not null
        //tLang not null
        List<User> tut = _context.Users
             .Where(u => u.TutorProfesionAndCosts == tProf)
             .Where(u => u.UserAndLanguages == tLang)
             .ToList();

         //error. tut = null

        return View(tut);
    }