[RESOLVED] LINQ is Missing Something, Or I Am

There is this example from MS documentation:

Code:

---

Dim queryResults = From cust In customers
                  Group Join ord In orders On
                    cust.CustomerID Equals ord.CustomerID
                    Into CustomerOrders = Group,
                          OrderTotal = Sum(ord.Amount)
                  Select cust.CompanyName, cust.CustomerID,
                          CustomerOrders, OrderTotal

---

A simple little example, but for me, the Into keyword is always an error. I have tried a very simple example such as this:

Code:

---

                Dim store1 As IEnumerable(Of Guid)
                Dim store2 As IEnumerable(Of Guid)

                Dim res=From gd In store1 Join gd2 In store2 On gd Equals gd2 into

---

which is as simple as possible, but 'into' is underlined with the error End of Statement Expected. Normally, this means that I overlooked something, but I sure can't see what it would be.

What am I missing?

The example is using Group Join while you're just using Join. I've never used Group Join but I assume that Into is specific to that.

Well, that's interesting, as it appears to be the case. I had tried both join and group join before I stripped it down to that simplified example, and into caused the same error both times, but now that I think about it, I never did try it with JUST a Group Join.

When I change the example to Group Join, the error goes away. Into becomes valid and accepted. What I was doing before I stripped down the example was to join two datatables followed by a Group Join of a further datatable with the intent of doing what amounted to a Left Outer Join on that third table. Into is not allowed if there is a Join, but is allowed if there is not.

My mental model was that Into was essentially creating a temporary table, in which case it shouldn't matter whether there was a join or group join, but that is not the case, so my mental model must be wrong. I guess I don't understand what "into" is doing.

As per documentation, the Into Clause applies to aggregate functions or groupings to apply to a collection which is used in Aggregate, Group By and Group Join clauses (the example that you provided).

Edit: The C# docs has a better explanation of how into works.