Hi. In this video, we're going to think about grouping. Grouping collections of objects within LINQ queries.

So, I've added some code to the program that we were looking at a while ago, and it's allowing us to group a collection of objects into subsets of data.

To begin with, it's fairly straight forward, same stuff as before. We are-, we've got a collection of pets and we're going to order the pets into breed sequence. But then we're going to group them into a, a set of collections, a subset of collections.

We are grouping by animal type. So, this is the key field that we're using. And, as we look at the underlying data, we'll notice that there are different types of animal in our collection. We have dogs, we have cats and we have fish.

So, we're creating a collection within the collection. A collection of fish, a collection of dogs, a collection of cats. And we're ordering that set of collections into key sequence, animalTypes.Key, which in this case will be the pet type.

At this point, we therefore have a collection of objects that we have no type name for. That collection of objects is animal types, and it will contain only two properties. The first property being the key field of that collection, and the second property being the collection itself.

So, I can instantiate a new object when I have no type name for it, and the first property I'm going to call animal type, which will be the key of that collection, and the second property will be a collection of just cats, or just dogs, or just fish.

Therefore, I'm bringing a collection that I have no type name for, I have to use the word var, I have no other data type I can use.

When it comes to looking at the underlying data, we now have a collection inside a collection. The outer collection contains three groups, dogs, cats and fish, but in key sequence, so it will actually be cats, dogs and fish. And so I can write out to the console by looking at the properties of this object.

Notice, I can only again use the var type here because I have no class name for this, but I do know there's an animal type property and I do know there's an animal types property, that I can use a count method for, to find out how many objects there are of that particular type. How many dogs are there, how many fish and so on.

I can then iterate around that collection using a for-each, and then I, kind of, know again that I've got objects with particular properties, but I have no type name for them, so, again, var is the only choice I have, but I can print out the name of those pets and the breed of those pets.

So, I should be able to run the application to its conclusion and we should see that data being printed out to the console. And you'll notice that we do indeed get a first group out, which is the cats groups. There are two cats called Marmy and Fifi. Second group coming out, which is dogs, there's only one dog, it's Fido. Third group, fish, there are three of those, Bob, Bobby, and Bobina.

So, in this video we looked at how to group collections of objects. When we group objects, they can only ever be anonymous, we have no type name for them, but that's fine because the var data type gets us around those problems and allows us to use those anonymous types.

Practical examples of using LINQ to perform grouping

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