Linq query including null child entities

Question

Using entity framework, i am trying to select records and their related child records. The child records could be null, so i would just like to return an empty string if they are. I am getting a

null reference exception

when attempting to run the following code.

var query = ctx.UserAlerts.Include(x=>x.DocumentType).Where(x => x.UserId == userId).ToList();
var alerts = query.Select(u => new AlertConfigVM
            {
                UserId = u.UserId,
                Destination = u.AlertDestination,
                TypeofAlert = u.TypeofAlert,
                HourInterval = u.IntervalAsHours,
                DocumentTypeName= u.DocumentType.Name??string.Empty
            }).ToList();

here are my Entities

public class UserAlert
{
    public int Id { get; set; }
    public string UserId { get; set; }
    public User User { get; set; }
    public int TypeofAlert { get; set; }
    public string AlertDestination { get; set; }
    public int? DocumentTypeId { get; set; }
    public virtual DocumentType DocumentType { get; set; }
    public int? IntervalAsHours { get; set; }
}

public class DocumentType
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Key { get; set; }
}

and here is my return type.

public class AlertConfigVM
{
    public int Id { get; set; }
    public string UserId { get; set; }
    public User User { get; set; }
    public int TypeofAlert { get; set; }
    public string Destination { get; set; }
    public int? DocumentTypeId { get; set; }
    public string DocumentTypeName { get; set; }
    public int? HourInterval { get; set; }
}

Make sure documentType is not null before checking if documenttype.name is not null. — Forklift, Feb 22 '17 at 16:46
Possible duplicate of [What is a NullReferenceException, and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Rahul Singh, Feb 22 '17 at 16:56

tjcertified · Accepted Answer · 2017-02-22T16:55:38.810

3

Change this:

DocumentTypeName= u.DocumentType.Name??string.Empty

to this:

DocumentTypeName= u.DocumentType?.Name??string.Empty

This will allow the DocumentType to default to string.Empty if null.

edited Feb 22 '17 at 16:55

answered Feb 22 '17 at 16:48

tjcertified

694
7
18

Awesome, that worked perfectly. I didnt realize that you could do that. Where did you pick up that handly little trick? – Rick james Feb 22 '17 at 16:53
Honestly, I "accidentally" discovered it using ReSharper, if I'm honest. – tjcertified Feb 22 '17 at 16:54
2

Note, that this solution **ONLY** works in Visual Studio 2015 and later with MSBuild using Roslyn. – Cameron Feb 22 '17 at 17:19

score 1 · Answer 2 · edited May 23 '17 at 12:25

Since tjcertified's answer only works in Visual Studio 2015 and later, this solution should work regardless of environment:

var query = ctx.UserAlerts.Include(x=>x.DocumentType).Where(x => x.UserId == userId).ToList();
var alerts = query.Select(u => new AlertConfigVM
            {
                UserId = u.UserId,
                Destination = u.AlertDestination,
                TypeofAlert = u.TypeofAlert,
                HourInterval = u.IntervalAsHours,
                DocumentTypeName= u.DocumentType != null 
                                  ? u.DocumentType.Name != null 
                                      ? u.DocumentType.Name 
                                      : string.Empty 
                                  : string.Empty
            }).ToList();

The ?: operation is the ternary operator and is the same as a regular if-else statement.

You could make it shorter: (`u.DocumentType == null ? null : u.DocumentType.Name) ?? string.Empty` — Anderson Pimentel, Feb 23 '17 at 17:10

Linq query including null child entities

2 Answers2