Question

Using C# and LINQ to entities I have a problem with child and parent entity searching. In conceptual terms I am trying to get a Collection of IEnumerable children where these children have certain properties and also the parents of these children have certain properties.

In concrete terms I have Routes and Streets which have a many to many relationship. I am trying to find Streets on a specific Route where the Street has a positive property of LeftNote or RightNote (LeftNote and RightNote are strings and I am searching for strings that are not empty space).

I have the following entities (cut down for clarity)

public class Route
{
    public int RouteID { get; set; }

    public virtual ICollection<Street> Streets { get; set; }
}

public class Street
{
    public string LeftNote { get; set; }

    public string RightNote { get; set; }

    public virtual ICollection<Route> Routes { get; set; }
}

I have the following LINQ expression:

var streets = this.repository.Routes
                        .Where(r => r.RouteID == routeId).FirstOrDefault()
                        .Streets
                        .Where(s => s.LeftNote.Length > 0 || s.RightNote.Length > 0);

This works perfectly until I run this against entity data where the Route exists but there are no streets that have LeftNotes or RightNotes. In these cases I get a NullReference exception. I am trying to find a way to properly express this query which handles the absence of Streets with LeftNotes (there should always be a Route which matches routeId. If not that is a valid exception case and should throw and error).

EDIT: The issue seems to be around null strings rather than anything related to the LINQ construct.

Answer 1

Do you actually mean

var streets = repository.Streets.Where
       (s => s.Routes.Any(r => r.RouteID == routeId )
             && (s.LeftNote.Length > 0 || s.RightNote.Length > 0));