Adding a new item into navigation property causes “Collection navigation properties must > implement ICollection<> of the target type” error

Question

I have the following code, where I need to add a new item into the navigation property depending on the condition. NotificationToUser property of Notification class is IEnumerable type.

  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();

However, adding a new item to the navigation property causes this error:

System.InvalidOperationException: 'The type of navigation property 'ToUsers' on the entity type 'Notification' is 'AppendPrepend1Iterator' which does not implement ICollection. Collection navigation properties must implement ICollection<> of the target type.'

The Notification class is:

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; }

}

I wonder how I can mitigate this issue.

Answer 1

Since your ToUsers is IEnumerable<NotificationToUser> type, you need to use ToList() before you save the data.In your situation,it is IQueryable<NotificationToUser> after Select .

Modify your code as follow:

if(submissionOwnerId != currentUser.Id)
{
  notification.ToUsers = notification.ToUsers
                         .Append(new NotificationToUser { IsRead = false, UserId = submissionOwnerId })
                         .ToList()
}else//for the situation you do not need to append new NotificationToUser 
{
 notification.ToUsers = notification.ToUsers.ToList()
}
_context.Notifications.Add(notification);
_context.SaveChanges();