Linq To Objects Outer Join?

This will compile to the exact same code as the example above, except that the parameter named "groupJoinItem" in the SelectMany will be named something like "temp0" because that parameter doesn't explicitly exist in the comprehension version of this code. I think you can appreciate how much simpler this version of the code is. I always use this syntax when doing a left outer join.

I never use GroupJoin with SelectMany. However, at first glance it makes very little sense. A join followed by an into creates a GroupJoin.

You first have to know this, and why you'd want this. Then the second from indicates a SelectMany, which is non-obvious. When you have two from keywords you are effectively creating a cross join (cartesian product), which is what SelectMany is doing.

It's a basic cross join. So, back to our original left join LINQ query: The first from of the query is the group join, and the second from expresses a cross join between each groupJoinItem and its own collection of matching right letters. In fact, we could actually write it as such!

That selectMany expresses the following: "for each item in groupJoin, cross join it with its own MatchingRightLetters property, and concatenate all of the results together." This gives the exact same result as any of our left join code above. That's probably way too much of an explanation for this simple question, but I dislike cargo cult programming (google it).

You should know exactly what your code is doing, and why, or else you won't be able to tackle more difficult problems.