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Yeild can also be used in methods that return either IEnumerator or IEnumerable.
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There's a bit more you want to
know about the yield keyword.
0:00
We can use the yield keyword in any method
that returns either I enumerator or
0:03
I enumerable.
0:09
We just saw an example of using yield
in a method that returns I enumerator.
0:10
When implementing the git
numerator method,
0:14
a more common use of it is in
methods the return I enumerable.
0:17
To demonstrate how this works,
0:21
let's rewrite a method that
is already provided by link.
0:23
It's a great example of how to use yield.
0:27
We will write this method in a new class.
0:30
I'll just call it utils.
0:33
I'll start by getting rid of these
using directives we don't need up here.
0:45
And we'll make this a public static class.
0:50
We'll write a method called Take.
0:55
Take returns the first n items
in the collection passed in.
0:58
And it returns them as a collection
of type IEnumerable of T.
1:02
So I'll say public
static IEnumerable of T,
1:06
Of course I need to add
the using directive for that.
1:10
And the method's called Take.
1:18
And here's where we specify
the generic parameter.
1:22
I'll say IEnumerable<t>,
1:25
and call this parameter source.
1:32
And the second parameter
we'll just call N.
1:35
So take we'll take the first N items
from the source collection and
1:41
return them as an I numerable of T.
1:46
Notice that take is a generic method,
1:49
you can have generic methods
outside of a generic class.
1:51
In fact, in my experience generic methods
are more common than generic classes.
1:55
The type parameter is specified right
here after the name of the method.
2:01
Now T can be used anywhere
in the method and
2:05
this method we'll find the first n items
by looping through the source collection.
2:09
We'll need a counter to keep track
of how many items we've returned.
2:14
So I'll say int i = 0 and we'll have a for
2:18
each loop that loops through
each of the items and source.
2:23
So we'll say, var item in source and
2:26
we'll yield return each item.
2:31
Once we've yielded an item's,
2:39
we can signal the end of the collection
with the yield break statement.
2:41
So I'll say if I don't need to increment
i here in order to keep track of how many
2:45
items we've seen so far and
I'll do it with a pre-increment.
2:50
So I'll say plus plus i.
2:54
And then I'll check to see if that's N.
2:58
And if it is, we'll say yield break.
3:02
The take method is returning
an IEnumerable of T,
3:07
instead of an IEnumerator of T.
3:11
What's happening,
is by using yield return,
3:13
We are telling C sharp to construct
an empty I Enumerable collection.
3:17
The body of the take method is called,
when get a numerator is called,
3:21
by a foreach loop that's looping
through the I numerable of T.
3:25
that's returned by the take method.
3:30
Yield break here tells the move next
method of the numerator to return false.
3:33
Thus signaling the end of the enumeration.
3:39
Over in main,
et's see how to use this method.
3:42
So here I'll make a variable
of type I Enumerable of
3:49
T and I'll call it first three.
3:52
And a set equal to Utils.Take.
3:59
And it will just take list1 from above and
get the first 3 items.
4:05
Instead of T here.
4:16
This should be int.
4:17
Now, we can loop through
the first three collection.
4:21
So we'll say foreach
var item in firstThree.
4:24
The neat thing about
numerators is that even after
4:28
calling the take method on this line,
firstThree still doesn't contain anything.
4:34
not until the foreach loop is run but
the body of the take method is executed.
4:40
This is true of all
methods that use yield.
4:45
It's called lazy evaluation.
4:49
The items in the collection aren't
retrieved until they're needed.
4:51
Yield is a really handy
feature of C-sharp.
4:55
It provides data just in time,
right when it's needed.
4:59
It's actually quite versatile.
5:03
I've used it to write methods that
read from files and network buffers.
5:05
I've included some links in the teacher's
notes if you'd like to learn
5:09
more about yield.
5:12
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