“对象引用未设置为对象的实例”即使我检查了 null

Question

I have Room and Bed EF classes which each of Room have some Bed s.when I use this LINQ statement:

IEnumerable<Room> room=...
if (room == null || !room.Any())
    return null;
return room.SelectMany(r=>r.Beds); 

give me this error:

Object reference not set to an instance of an object.

in return line.

Answer 1

One of your rooms in the enumerable is null. Do this:

return room.Where(r => r != null)
           .SelectMany(r => r.Beds);

Answer 2

I found out that my Collection of Room s were not null and also none of Room 's Beds were null . problem was that at least one item of my Room Collection was null . so according to YK1 's answer I should use:

return room.Where(r => r != null).SelectMany(r => r.Beds);

Answer 3

The error can only happen when a Room has Beds == null .

You state: "I only have one Room with two Beds in it" but the question also mentions EF.

So the problem is in the ... in IEnumerable<Room> room=... .

When your EF query uses lazy loading the Beds property will be null even if there are records.

For a complete solution you'll have to post all details about the EF part: Code|DB first, the query, which type Context class, EF version etc.

With the latest versions of EF this kind of problem is rare, my guess is that you have a ToList() in the query where you shouldn't.

Answer 4

You can try to use count too, like this:

IEnumerable<Room> room=...
if (room == null)    // Check for nulls
       return null;
else if (room.count() == 0)     // Check if empty
       return null;
else if(!room.Any(x => x.Beds == null)      // Check if there is no null Beds
       return room.SelectMany(r=>r.Beds);

Answer 5

Please try this..

return room.Where(r => r != null & r.Beds != null).SelectMany(r => r.Beds);