Linq select with new inside and list property throw null exceprion

Question

Good day. I'm trying perform select query with linq from collection of establishments. Establishment contains list of products it selling, but list of products can be empty. Well, I guess cause of that I get null exception. How I fix it and avoid null exception? This is my linq query:

var result = Establishments.Select(e => new
                {
                    ID = e.ID,
                    Name = e.Name,
                    Category = e.Category.Name,
                    XAdress = e.XAdress,
                    YAdress = e.YAdress,
                    CompanyName = e.Company.Name,
                    ProductsSelling = e.ProductsSelling.Select(p => new // error here
                    {
                        ID = p.ID,
                        Name = p.Name,
                        Category = p.Category.Name,
                        Price = p.Price,
                        Additives = p.PossibleAdditives.Select(a => new
                        {
                            ID = a.ID,
                            Name = a.Name,
                            Price = a.Price
                        })
                    })                    
                });

Looks like `e.ProductsSelling` is null for some value of `e`. Your computer must be broken. — 15ee8f99-57ff-4f92-890c-b56153, Oct 07 '16 at 17:11
Most likely, `ProductsSelling` is null for some `Establishment`. There's not enough info to know why. — Gasper, Oct 07 '16 at 17:14
what is data type of Establishments? if it is of type System.Collections.Generic.List where T is Establishment datatype, then you must check whether e.ProductsSelling is null or not null before expanding it. — sam, Oct 07 '16 at 17:18

YuvShap · Accepted Answer · 2016-10-07T17:28:54.967

3

You can check that ProductsSelling is not null before querying it using Null-Conditional Operator:

var result = Establishments.Select(e => new
            {
                ID = e.ID,
                Name = e.Name,
                Category = e.Category.Name,
                XAdress = e.XAdress,
                YAdress = e.YAdress,
                CompanyName = e.Company.Name,
                ProductsSelling = e.ProductsSelling?.Select(p => new /* note the '?' operator*/
                {
                    ID = p.ID,
                    Name = p.Name,
                    Category = p.Category.Name,
                    Price = p.Price,
                    Additives = p.PossibleAdditives.Select(a => new
                    {
                        ID = a.ID,
                        Name = a.Name,
                        Price = a.Price
                    })
                })                    
            });

If e.ProductsSelling is not null it will perform the linq query otherwise it will return null, this will prevent the NullReferenceException you are getting.

edited Oct 07 '16 at 17:28

answered Oct 07 '16 at 17:17

YuvShap

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thanks. query now doesn't thow error, but result list is null... but before: Establishments are not empty, all fine with them. – Amelina Oct 07 '16 at 17:28
How do you know result list is null, what are you trying to do with it? do you get any error? – YuvShap Oct 07 '16 at 17:31
I'm trying to return it from server. and in return Json(result); I get null exception again. – Amelina Oct 07 '16 at 17:33
Maybe try to use the ToList() extension in the end of the query? LINQ use a deferred execution mechanism and I think this causes you the problem. – YuvShap Oct 07 '16 at 17:39
1

Can you put a break point after the query to assure it is null? I think your problem is not in the query itself but in the Json parsing part. – YuvShap Oct 07 '16 at 17:43
I commented strings with json - no errors, the I wrote foreach (var r in result) Log.Logger.Debug(r.ProductsSelling.ToList().Count.ToString()); and null exception've come back! – Amelina Oct 07 '16 at 19:07
Look at the exception message, find out which of the class propeties is null and then you can use '?' oprator before it too. I would guess it is the PossibleValues in the inner query but it might be Category or Company properties also. – YuvShap Oct 07 '16 at 19:14

Linq select with new inside and list property throw null exceprion

1 Answers1