Linq left join null reference error

Question

When i am making left join i have this error. How to prevent error if id's doenot intersect?

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.

class Program
{
    static void Main(string[] args)
    {
        List<Component> firstlist = new List<Component>();
        List<Component> secondlist = new List<Component>();
        firstlist.Add(new Component { id = 1, Name = "Jhon" });
        secondlist.Add(new Component { id = 2, Name = "Jhon" });

        var test = from d in firstlist
                                 join i in secondlist
                                 on d.id equals i.id
                                 into a
                                 from b in a.DefaultIfEmpty()
                                 select new Component { id = b.id, Name = b.Name };
        List<Component> Result = test.ToList();
    }

    public class Component
    {
        public int id { get; set; }

        public string Name { get; set; }
    }
}

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) — rory.ap, Mar 17 '16 at 16:37
Is it caused by `b`? You may want to check if `b` is not `null` — Ian, Mar 17 '16 at 16:41
Either add a `where b != null` or really don't do a left join since you only pull values from the right. Really it depends on what you expect, an empty list or a list that contains `null` items or `Component`s with default values. — juharr, Mar 17 '16 at 16:49

score 3 · Accepted Answer · answered Mar 17 '16 at 16:50

Try something like the following:

class Program
{
    static void Main(string[] args)
    {
        List<Component> firstlist = new List<Component>();
        List<Component> secondlist = new List<Component>();
        firstlist.Add(new Component { id = 1, Name = "Jhon" });
        secondlist.Add(new Component { id = 2, Name = "Jhon" });

        var test = from d in firstlist
                                 join i in secondlist
                                 on d.id equals i.id
                                 into a
                                 from b in a.DefaultIfEmpty()
                                 select new Component { id = (b != null) ?b.id : default(int), Name = (b != null) ? b.Name : default(string) };
        List<Component> Result = test.ToList();
    }

    public class Component
    {
        public int id { get; set; }

        public string Name { get; set; }
    }
}

As you can see, I added a null check on b before setting it.

select new Component { id = (b != null) ? b.id : default(int), Name = (b != null) ? b.Name : default(string) };

You might also be able to do something like

select (b == null) ? default(Component) : new Component { id = b.id, Name = b.Name };

I'm not 100% sure about the syntax for that second option, but something like that might work.

In C# 6 you could do `new Component { id = b?.id ?? 0, Name = b?.Name }` — juharr, Mar 17 '16 at 16:52

Ian · Answer 2 · 2016-03-17T17:01:25.343

1

To fix it, you need to ensure that b is not null before you call/use its method/property/field. Change this:

select new Component { id = b.id, Name = b.Name };

into:

select new Component { id = b == null ? 0 : b.id, Name = b == null ? null : b.Name };

In C#6, you could use null conditional operator ?.:

select new Component { id = b?.id ?? 0, Name = b?.Name };

edited Mar 17 '16 at 17:01

answered Mar 17 '16 at 16:54

Ian

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the `id = b?.id` will not work because `id` is an `int` and `b?.id` would return an `int?`. So you really need something like `id = b?.id ?? 0`. – juharr Mar 17 '16 at 17:00
@juharr you are right, thanks for the correction. `id` is `int` – Ian Mar 17 '16 at 17:01

score 0 · Answer 3 · answered Mar 17 '16 at 17:01

you problem lies in

select new Component { id = b.id, Name = b.Name };

using a.DefaultIfEmpty() can return NULL, and so b.id will throw exception. Correct code should be:

select new Component { id = b==null?return_safe_value:b.id, Name =b==null?rturn_safe_value:b.Name };

Linq left join null reference error

3 Answers3