Linq Group加入DefaultIfEmpty

Question

I have this GroupJoin:

var groupjoin = cData.GroupJoin(
            aData,
            c => c.Id,
            a => a.Id,
            (c, joined) => new { c, a = joined.DefaultIfEmpty() })
            .ToList();

In my test data, there are NO matches. So, I have this code:

            var difference = groupjoin.FirstOrDefault(g => 
            g.a == null);

I was expecting difference to be an anonymous object with a "c" property that was an object from cData , and an "a" property that was null.

However, ga == null is never true, so FirstOrDefault gives me a null for difference . ga is, in fact, a DefaultIfEmptyIterator and gaToList() gives me a count of 1, and gaToList[0] == null is true.

What have I done wrong here?

Answer 1

That's how DefaultIfEmpty works. This method returns a collection with one element (type parameter's default) if the collection is empty, not null.

So in your case, if there are no matches, joined.DefaultIfEmpty() will return a collection with just one element, that is null for reference types.

If you want null when joined is empty try something like this:

joined.Any() ? joined : null

You can read more about DefaultIfEmpty here .