C# Recieving NullReference error when using Where Contains

Question

When filtering a list using lambda where and contains, I get a NullReference error

var servers = _context.Servers.ToList();
servers = servers.Where(t => t.Technology.Contains(technology)).ToList();

But I get a this error:

Exception thrown: 'System.NullReferenceException' in ServerBuildApp.dll ServerBuildApp.Models.Servers.Technology.get returned null.

The list of servers contains a 'Technology' property and it contains the string I am passing it, for example "BIZ"

Any ideas? Or am I doing this completely wrong?

The title of question and the question itself has nothing to do with each other. Apparently you have at least one element in the list whose `Technology` property is null. — Ofir Winegarten, Dec 19 '18 at 11:32
You are not exactly "filtering in place without creating a new list", more like "filtering and creating a new list, but assigning it to the same variable" - which is not a problem, just to keep things accurate. As for the error - it's not related, you just have some `Technology` properties with a value of `null` - check for it before you call a function on the value (e.g. `t.Technology?.Contains(technology)`) — Nimrod Dolev, Dec 19 '18 at 11:32
I have updated the question and the content to represent the real problem, thanks for everyone's quick responses. Love this community. — Gareth Doherty, Dec 19 '18 at 12:00

score 4 · Accepted Answer · answered Dec 19 '18 at 11:31

4

You need to check the t.Technology for null.

var servers = _context.Servers.ToList();
servers = servers.Where(t => t.Technology?.Contains(technology) is true).ToList();

You should check t.Technology for null and then check it contains.

answered Dec 19 '18 at 11:31

lindexi

1

Why not avoid making an extra list? `var servers = _context.Servers.Where(t => t.Technology?.Contains(technology)).ToList();` – Matthew Watson Dec 19 '18 at 11:39
t.Technology?.Contains(technology) return bool? but the where is Func that you should check the value – lindexi Dec 19 '18 at 11:40
@MatthewWatson And you do need the `== true` check due to the nullable bool. – DavidG Dec 19 '18 at 11:48
This is what I should have typed: `var servers = _context.Servers.Where(t => t.Technology?.Contains(technology) is true).ToList();` - this avoids making an extra unneccessary list. The main point here is to avoid making an extra list. – Matthew Watson Dec 19 '18 at 11:50
Lindexi's answer worked a treat. Thank you. – Gareth Doherty Dec 19 '18 at 11:58
1

This helped me as well, thanks lindexi! – user2768479 May 24 '20 at 22:45

1 Answers1