Linq foreach loops once then throws error

Question

I'm trying to iterate over all the results from a linq statement. I understand that the linq statement is not executed until the foreach loop starts. The code steps through the foreach loop once with a result but when it tries to loop through a second time it throws the error "Object references not set to an instance of an object" on the object "SelectectedEntity".

List<FeedEntity> EntityList = feed.entity.ToList();

IEnumerable<FeedEntity> SelectectedEntity = 
     from entity in EntityList  
     where entity.trip_update.trip.trip_id == "9571620-BCC 17_18-BCC_FUL-M-Tu-W-Th-01" 
     select entity;

foreach (FeedEntity e in SelectectedEntity)
{
    string s = e.trip_update.trip.route_id.ToString();
}

I'm expecting a few dozen elements in the SelectedEntity collection. What am I missing or have I misunderstood how linq operates.

The question is not what is a null reference, as I know what that is ..but why was I getting a null reference at this point in the code.

It had to do with the data and how linq interprets it. I'll post the correct code below as the answer.

It would certainly help if you learned the basics of c# naming: public properties are named with PascalCase and without `_` (underscores) and variables are named with camelCase. Your problem is that `e.trip_update.trip.route_id` is `null`, so `ToString` fails — Camilo Terevinto, Feb 01 '18 at 00:58
You are getting an error because in the loop trip_update or trip or route_id is null and trying to dereference null generates an exception. You need to be able to handle missing values. — Ray Fischer, Feb 01 '18 at 01:11
Yes , you are right the return list did contain objects that did not contain a trip_route or trip object. However the failure occurs during the iteration foreach loop not the string cast. In regards to the naming convention if you are refering to .trip_update.trip.route_id These are the object names as defined in the protobuf-net Nuget assembly use to deserialize real time transport data into C# objects. You'll need to talk to Google as it's their approved cross language standard which is why it doesn't follow c# naming standards. — ErickTreetops, Feb 01 '18 at 02:02
I'd like to provide the answer on how the linq should be written but as it was marked duplicate that option is missing. Knowing what a null reference is won't stop you making this linq mistake.. — ErickTreetops, Feb 01 '18 at 02:10

score -1 · Answer 1 · answered Feb 01 '18 at 13:11

"Object references not set to an instance of an object" means that you try to access an object, while this object equals null:

MyClass myObject = null;
string name = myObject.Name;

So you have an entity list, and some horrible string, and you want only those elements form your entity list where entity.trip_update.trip.trip_id equals horrible string.

Are your sure that entity not is null, and entity.trip_update not is null and ... not is null?

You should check this in your query

const string horribleString = "9571620-BCC 17_18-BCC_FUL-M-Tu-W-Th-01";
var result = entityList
   .Where(entity => entity != null
       && entity.trip_update != null
       && entity.trip_update.trip != null
       && entity.trip_update.trip.trip_id == horribleString);

If you use a modern version of C# you can use the null conditional operator. which makes your query better readable:

 var result = entityList
    .Where(entity => entity?.trip_update?.trip?.trip_id == horribleString);

I wonder what is wrong with my answer that I get downvoted. At least give some feedback, so I can improve my answer. This is not encouraging to answer people's questions event if I think RTFM — Harald Coppoolse, Feb 02 '18 at 07:28

Linq foreach loops once then throws error

1 Answers1