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So far we've been using operators
that return an Enumerable.

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The next set of operators deal
with returning a single element

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from a collection.

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They're used quite often in LINQ queries.

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The Single method will return
a single element from our collection.

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We can use it without a parameter,
like in combination with the Where method.

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birds.Where, and don't forget,
if you don't have birds,

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load up your birds using
the instructions in Workspaces.

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Where b goes to b.Name == Crow.

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And then we'll call .Single.

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And there's our crow.

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Or we can use the predicate
as a parameter.

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birds.Single b goes to b.Name

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== Crow, and there it is.

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What happens if we use the Single method
but there isn't a match in the collection?

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birds.Single where b goes to

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b.Name == Chickadee.

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I don't think there's
a chickadee in there.

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Ouch.

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Invalid operation exception.

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Sequence contains no matching element.

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There's another version of this method,
and all of the element operators in LINQ

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have this version of their method,
SingleOrDefault.

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So let's try that one again,
but instead of just Single,

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I'll tack on OrDefault.

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And then it returns null.

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So what this method does is if it doesn't
find a match, it returns a default value.

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So in this case,
a bird has a default value of null.

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If we were dealing with integers, say,

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a list like we were using before,

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var numbers = new List of int 2, 4, 8.

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All right, that's enough.

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And if we use SingleOrDefault to try and
get a number that doesn't exist,

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Numbers.SingleOrDefault where

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n goes to n == 99,

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then it returns 0 since the default
value for an integer is 0.

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So what happens if we use
the Single method but

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our predicate matches more than one bird?

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birds.Single, burned again,
InvalidOperationException.

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Sequence contains more than one element.

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So the Single method
will return an exception

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if the condition returns more than
one element from the sequence.

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When we use Single,

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we want to make sure that there's only
one of its kind in the collection.

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You might think, well,
I should use SingleOrDefault so

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that I don't get an exception.

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It depends on what the situation is.

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You might want an exception if you
are truly looking for an unique object in

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the collection and there's more than one,
or it doesn't exist.

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The next two methods are pretty similar
to each other, the First method and

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the Last method.

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They can be used the same way as Single,
but

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it's not expected that
the element is unique.

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Let's get the first bird in our list.

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birds.First.

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And what's the last bird in our list?

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birds.Last.

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We can use a predicate just
like with the single method.

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birds.First where b

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goes to b.Color = Red.

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And like the single method,
if there are no elements that match,

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we get an exception.

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So let's try that again with Chickadee.

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These methods also have the or
default versions.

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So if we're not sure an element exists,
we'll use FirstOrDefault.

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Default.

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Whoops, forgot the Or.

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FirstOrDefault.

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Null, and we don't get an exception.

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The last method we'll talk
about is the ElementAt method.

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We can use it to get an element at
a certain position in the sequence.

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So like with using an indexer on an array,

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int of numbers = 0, 1, 2, 3.

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And I can access with an indexer like.

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But we can also use LINQ
to use ElementAt(2) and

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it does the same thing.

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I personally haven't seen
the ElementAt method used much, but

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I'm sure there's some edge
case where it might be handy.

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If your Enumerable is also a list,
you can still use the indexer, so

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like birds[0].

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But if you've got the result of a LINQ
query, which is a pure enumerable, so

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var redBirds = birds.Where b

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goes to b.Color == Red.

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We wouldn't be able to use
an indexer to access the items.

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redBirds.

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Nope.

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But you could still use
the ElementAt method.

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redBirds.ElementAt(0).

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And just like the other
methods of this type,

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we can use OrDefault to avoid an exception
if the element doesn't exist.

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So redBirds.ElementAt, we'll do 99.

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But if we used OrDefault,

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just null.

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Let's recap these methods.

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Single, use when
the element must be unique.

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Exception if more than one,
or none exists.

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First, use if there could be more than
one, but you only need the first.

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Exception if no match.

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Last, use if there could be more than one,
but you only need the last.

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Exception if no match.

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ElementAt, use if you know
the exact position of the element.

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Exception if no match.

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SingleOrDefault, use only when the element
must be unique, or it may not exist.

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FirstOrDefault, use if there could
be more than one or none, but

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you only need the first.

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LastOrDefault, use if there
could be more than one or none,

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but you only need the last.

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And ElementAtOrDefault, use if you know
the exact position of the element, but

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it might not be there.