public class ProjectModel
{
    [Key]
    public int? Id { get; set; }
    [Required]
    public string Title { get; set; }
    public UserModel createByUser { get; set; }
    public virtual ICollection<TaskTableModel> TaskList { get; set; }

    public virtual ICollection<UserModel> User { get; set; }
    public ProjectModel()
    {
        TaskList = new List<TaskTableModel>();
        User = new List<UserModel>();
    }
}

public class UserModel : IdentityUser
{
    public UserModel()
    {
        Project = new List<ProjectModel>();
        Tasks = new List<TaskModel>();
    }

    public virtual ICollection<ProjectModel> Project { get; set; }
    public virtual ICollection<TaskModel> Tasks { get; set; }

    public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<UserModel> manager)
    {
        // Обратите внимание, что authenticationType должен совпадать с типом, определенным в CookieAuthenticationOptions.AuthenticationType
        var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
        // Здесь добавьте утверждения пользователя
        return userIdentity;
    }
}

public class TaskModel
{
    [Key]
    public int? Id { get; set; }
    [Required]
    public string Title { get; set; }
    public Nullable<DateTime> finishDate { get; set; }
    [Required]
    public string Content { get; set; }
    public CheckBoxModel CheckBox { get; set; }
    public virtual ICollection<CommentModel> Comment { get; set; }
    public virtual ICollection<UserModel> User { get; set; }
    public PriorityTaskModel Priority { get; set; }

    public TaskModel()
    {
        Comment = new List<CommentModel>();
        User = new List<UserModel>();
    }

}