Object reference not set to an instance of a diggity...

I came across something at work the other day that I found interesting, and since I am never afraid to admit that I don’t know something, I thought I would post it.  Other than the fact that I used anonymous types to create the sample objects as opposed to writing classes, the code explains itself:

Public Sub TestLinqOrderBy()
        Dim f1 = New With {Key .Commodity = "Commod1", .Qty = 1000}
        Dim f2 = New With {Key .Commodity = "Commod2", .Qty = 2000}
        Dim f3 = New With {Key .Commodity = "Commod3", .Qty = 3000}

        Dim list1 = GetAnonymousList(f1)
        list1.Add(f1)
        list1.Add(f2)
        list1.Add(f3)

        'functionally these 2 calls are identical, since they are the same the rest of the example will use var
        Dim var = list1.OrderByDescending(Function(p) p.Qty) ' sort the list descending by quantity
        Dim var2 = From p In list1 Order By p.Qty Descending

        ' the list var now looks like this:
        'Commodity = "Commod3", Qty = 3000}
        'Commodity = "Commod2", Qty = 2000}
        'Commodity = "Commod1", Qty = 1000}

        'however, these will return you an IOrderedEnumerable (not an IEnumerable like most other simple linq queries), essentially a sorted list that will keep it sorted on the key
        'therefore, as statement like this may give you a list back you didnt expect because after the set has occurred, the IOrderedEnumerable will resort and keep itself sorted
        For i As Integer = 0 To 2
            var(i).Qty = New Random(i).Next(3000)
        Next

        ' since the above code changes the key of the list (Qty), it will set and reorder
        ' since you are looping through the list by index, you may not be changing the value in the list you thought you were when the loop began, 
        ' and the runtime will not tell you that you have modified the collection while looping

        ' the thing to note here is that if you specifiy an order by in your linq statement, that you are getting back a list that will always be sorted by the key, unless…
        ' if you want to sort a list with linq and then get it back without the live sorting, you can ToList your IOrderedEnumerable to return an IEnumerable
        Dim var3 = (list1.OrderByDescending(Function(p) p.Qty)).ToList  ' sort the list descending by quantity without live sorting
        'or
        Dim var4 = (From p In list1 Order By p.Qty Descending).ToList

        'YOU CAN PASTE THIS CODE INTO A NUNIT TEST CLASS AND STEP THROUGH IT IF YOU WANT TO SEE WHAT IS HAPPENING
    End Sub

    Private Function GetAnonymousList(Of T)(ByVal itemOfType As T) As List(Of T)
        Return New List(Of T)
    End Function

That may be old news to some, but the it seems like every time I pull LINQ out of my toolbox I am more and more impressed with it.  I have a list of favorite interview questions I like to ask a candidate, and they are mostly centered around conversation as opposed to test questions.  One of my favorite interview questions to ask someone is “What is your favorite thing about .NET?".  There is no right or wrong answer there, but I am looking for an answer that shows some kind of passion.  If I were to answer my own question, for me it used to be reflection, but now my answer would be LINQ.  It is awesome to have a set based tool in your kit.

Since LINQ seems to teach me something every time I use it, this post was prompted by what I thought was a pretty cool query.  Say you were given an collection object called Dealerships, and there is a shared method on a Dealership a list that contains all of the valid makes of car that the Dealership is allowed to sell.  Then you have a user in UserRequestedObject that has requested to see the Dealerships that are possible matches for the makes he/she wants to buy.

The data would look something like this:

So, one way to find the dealerships the user is asking for is you could write a query like this :

    Dim commonDealerships = (From t In dealerships Where _
            (From p In UserRequestedObject Select p.MakeOfCar).Any _
            (Function(u) GetValidMakesForDealership(t).Contains(u)))

Which will return you Dealerships 1, 2 and 4, and is seriously cool stuff if you ask me.

In addition, if you have an object that is a collection that contains another object that is a collection, like this:

If a user requested to see all dealerships that have a 1997 BMW M3, you could write a query like this:

        Dim m3Dealers = (From t In dealerships Where _
           (From p In t.Cars Select p.Make, p.Model, p.Year).All _
           (Function(u) u.Make = requestedMake AndAlso u.Model = requestedModel AndAlso u.Year = requestedYear))

Nested Loops no more!  This effectively gives you all parent rows that have children that are an exact match to the key you passed in.  I understand that under the hood this is doing the same thing as a nested loop, but if you can think in set based syntax, sometimes that is the best way to solve the problem.   Very useful!