在导航属性中添加新项将导致“集合导航属性必须>实现目标类型的ICollection<>错误”

我有以下代码,需要根据条件在导航属性中添加新项。 NotificationToUser 财产 Notification 班级是 IEnumerable 键入。

  Notification notification = new Notification
  {
      DateCreated = DateTime.Now,          
      ToUsers = _context.PeerGroupMemberships
         .Where(pg => pg.PeerGroup.SubmissionId == assessmentItem.SubmissionId && pg.UserId != currentUser.Id)                                       
         .Select(pg => new NotificationToUser { IsRead = false, UserId = pg.UserId })
  };


  if(submissionOwnerId != currentUser.Id)
  {
      notification.ToUsers = notification.ToUsers.Append(new NotificationToUser { IsRead = false, UserId = submissionOwnerId });
  }

  _context.Notifications.Add(notification);
  _context.SaveChanges();

但是,将新项添加到导航属性会导致此错误:

System.invalidooperationexception:'导航属性的类型 实体类型“Notification”上的“ToUsers”是 “AppendPrepend1Iterator”未实现 收集。集合导航属性必须 实现目标类型的ICollection<gt;'

通知类是:

public class Notification
{
    [Key]
    public int Id { get; set; }

    public string Text { get; set; }
    public string Url { get; set; }
    public DateTime DateCreated { get; set; }

    public IEnumerable<NotificationToUser> ToUsers { get; set; }

}

我想知道怎样才能减轻这个问题。