- All About LINQ 3:20
- All About LINQ 2 questions
- Writing Your First LINQ Query 8:12
- Using Query Syntax 2 objectives
- Object Initialization 4:14
- Object Initialization 1 objective
- Selecting, Projecting and Anonymous Types 8:24
- Selecting, Projecting and Anonymous Types 2 objectives
- Ordering and Grouping 6:04
- Ordering and Grouping 5 questions
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Writing Your First LINQ Query
8:12Dive into LINQ and write your first query.
Here we are in workspaces.
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You can follow along by opening up
workspaces by clicking the button below.
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We're going to use the C# REPL and
the console of our workspace here.
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To launch, we will enter csharp and
press Enter.
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The first thing we need to do is make
sure we've got the system.link namespace.
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In the C# REPL we can just enter,
using system.link.
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It says we've already got a directive for
it.
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That's because it includes the using
directives for system, system.link,
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system.collections and
system.collections.generic by default.
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Let's start out with a list of integers.
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List<int> numbers =
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new List<int> { 2,
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4, 8, 16,
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32, 64 };.
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If you haven't seen a list before, a list
is a type of generic collection in C#.
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When a type is generic, it means that
it's designed to handle any type.
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So we can declare a list and
put whatever we want into it.
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But we have to tell it what
to expect when we declare it.
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We've passed int as the type
parameter to the list.
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Then we're instantiating a new list and
initializing it with some integers.
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If you haven't seen this before,
it's similar to initializing an array.
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We don't have to do it that way but it's
a nice short cut to save us some time.
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We'll get into some more object
initialization later on.
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It's important to note that the list
collection implements I innumerable of T,
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which means we can use Linc with it.
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Say we need to only get the integers out
of our numbers list that are greater
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than 10.
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How would we go about that?
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Well, we would write a for
or a for each loop.
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First we need to create a new
list to store our result.
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Then we'd write the for each loop that
would iterate through our numbers list.
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Inside the loop,
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we'd write an expression that would check
if the current number fits our criteria.
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And if it does we'd add
it to the new list.
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Once loop is done, we've only got numbers
that are greater than 10 in our new list.
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Let's see how we would do that.
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Declare a new list.
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numbersGreaterThanTen =
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new List<int>();.
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Then our for
each loop foreach(int number in
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numbers).</int> our expression.
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If the number is indeed
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greater than 10,
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then numbers greater than
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ten.Add(number).
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And then our number's greater than ten.
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Should only have 16, 32 and 64.
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Now let's do the same thing
using a link query expression.
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from number in numbers where
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the number is greater than
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10, select number;.
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And that also returned 16, 32 and 64.
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We just wrote a link query that
did the same thing as our for
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each loop but was way faster to write and
easier to read.
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And our original numbers
list wasn't modified.
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Our query just returned a new collection.
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Writing that query may feel a little
different than what you might
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be used to in C#.
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If you've worked with SQL,
the clauses FROM, WHERE and
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SELECT might be familiar to you, but
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the words have moved around a little
bit from how you'd write them in SQL.
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Let's talk about how we wrote
that statement a little more.
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A link query is composed of a range
variable, a source, a query, and a result.
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Looking at our query expression we just
wrote, the range variable is this part,
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from number.
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Then we have our source, and
numbers, and then our query,
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where number > 10, and
finally our result, select number.
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The range variable part
might be a little confusing.
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Why did we have to do that?
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Its job is to be a reference
inside the query.
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Take a look at our previous
example with the for each loop.
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We have to declare an int number, so
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we could refer to it inside the for
each loop.
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That's exactly what a range
variable is doing from number.
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In fact, I could call it anything I want,
from xyz.
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I named it number though
to be more descriptive.
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Some developers prefer to use a single
letter as the range variable name.
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And it's generally okay to do that
when you deal with variables that
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have such a small scope
as with link queries.
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You'll probably see both out in the wild.
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Let's see how readable this is.
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from N in numbers
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where N is greater
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than 10 select n.
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A lot of developers like to increase
readability even more by separating
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out each component of the link
query onto its own line like this.
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from n in numbers where
n is greater than 10.
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Select n.
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We can see that it's returning
a sequence of integers, but
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is it returning a list of
integers like we want?
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Let's assign the result of
our query to a variable.
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List of int numbersGreaterThanTen
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Equals from N in numbers
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where n is greater than
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10, select N.
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Ha, we got an error cannot explicitly
convert type System.Collections.Generic.IE
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numerable to
system.collections.generic.list.
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Let's change it to IEnumerable then and
see if that works.
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IEnumerable numbersGreaterThanTen = and
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one more time a feeling from numbers.
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Whoops, did that wrong.
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Let's cancel out of there, and
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IEnumerable<int> numbersGreaterThanTen
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= from n in numbers</int>
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Where N is greater than 10.
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Select n.
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Link queries always return
an enumerable of the type that was in
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the original collection.
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This is important because it means that
the link query isn't evaluated until
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we enumerate over it.
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This is called deferred execution.
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The statement isn't evaluated until we
iterate through it with a for each or
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use a method on it like count
that forces it to be evaluated.
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So let's do that.
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NumbersGreaterThanTen.Count().
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But when we're here in the REPL,
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our statements are automatically
being evaluated.
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We'll see some more examples of deferred
execution later on in this course.
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