Question

当列表中的每个gameServer都有connectedClients的集合时，我有以下linq语句，该语句效果很好。

但是当connectedClient为null时，查询会崩溃。

如何防止崩溃？

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();

和..

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

Answer 1

如果为null，则使用非空值：

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

??被称为the null-coalescing operator。

Answer 2

在

之后添加一个在第二个之前检查null的位置

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();

Answer 3

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();

如果其中一个属性为NULL，则此LINQ语句崩溃。我怎样才能解决这个问题？

3 个答案: