This LINQ statement crashes if one of the Properties is NULL. How can I fix this?

Question

I have the following linq statement which works great, when each gameServer in the list has a collection of connectedClients.

but when the connectedClient is null, the query crashes.

How can I prevent this from crashing?

var connectedClients = (from x in gameServers
                        from y in x.ConnectedClients
                        select new
                        {
                            x.Name,
                            x.GameType,
                            ConnectedClients = new
                            {
                                y.ClientName,
                                y.ConnectedOn,
                                y.ClientIpAddressAndPort
                            }
                        }).ToList();

and..

public class GameServer
{
    public int Id;
    public ICollection<Client> ConnectedClients;
    ...
}

`where y != null`? It seems too easy, I must have wrongly understood your question. — jv42, Nov 17 '11 at 10:22
I don't see a variable called `connectedClient`. Am I missing something? — Polynomial, Nov 17 '11 at 10:22
Personally I would change your code such that `ConnectedClients` is **empty** rather than `null` when there are no connected clients (see eg [this](http://stackoverflow.com/questions/1969993/is-it-better-to-return-null-or-empty-collection) ). — AakashM, Nov 17 '11 at 10:43

score 8 · Accepted Answer · answered Nov 17 '11 at 10:23

8

If it is null, use a value that isn't null instead:

var connectedClients = (
    from x in gameServers
    from y in x.ConnectedClients ?? Enumerable.Empty<Client>()
    // ...

The ?? is called the null-coalescing operator.

answered Nov 17 '11 at 10:23

Merlyn Morgan-Graham

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now that's tricky :) :) :) I'm a fan of `??` .. use it all the time .. didn't think of using it, that way! – Pure.Krome Nov 17 '11 at 10:26

score 7 · Answer 2 · answered Nov 17 '11 at 10:22

add a where to check for null before second from

var connectedClients = (from x in gameServers
                        where x.ConnectedClients != null
                        from y in x.ConnectedClients
                        select new
                        {
                            x.Name,
                            x.GameType,
                            ConnectedClients = new
                            {
                                y.ClientName,
                                y.ConnectedOn,
                                y.ClientIpAddressAndPort
                            }
                        }).ToList();

+1 because this could be just as important to people with similar questions. — Merlyn Morgan-Graham, Nov 17 '11 at 11:02

score 0 · Answer 3 · answered Nov 17 '11 at 10:37

IEnumerable<GameServer> gameServesWIthConnectedClients = from x in gameServers 
                       where x.ConnectedClients != null  
                       select x;

var connectedClients = from y in gameServesWIthConnectedClients
                       select
                           new
                               {
                                   y.Name,
                                   y.GameType,
                                   ConnectedClients =
                           new
                               {
                                   y.ConnectedClients.ClientName,
                                   y.ConnectedClients.ConnectedOn,
                                   y.ConnectedClients.ClientIpAddressAndPort
                               }
                               };
connectedClients = connectedClients.ToList();

This LINQ statement crashes if one of the Properties is NULL. How can I fix this?

3 Answers3