1
00:00:00,150 --> 00:00:02,148
In the split method we just coded,

2
00:00:02,148 --> 00:00:06,130
we checked to make sure that
this parameter isn't null.

3
00:00:06,130 --> 00:00:09,850
Sometimes we want the parameter
to be able to be null.

4
00:00:09,850 --> 00:00:15,060
For example there's a method called
is null or empty in the string class.

5
00:00:15,060 --> 00:00:16,810
Let's go take a look at that.

6
00:00:16,810 --> 00:00:22,630
Here's the is null or empty method in the
string class and notice that it's static.

7
00:00:22,630 --> 00:00:28,190
The reason it's static is
because if the string is null,

8
00:00:28,190 --> 00:00:33,350
you can't call a method on it, otherwise
you'll get a null reference exception,

9
00:00:33,350 --> 00:00:38,000
but with extension methods you can call
a method on something that is null.

10
00:00:38,000 --> 00:00:43,900
So let's write our own version of
the is null or empty method for string.

11
00:00:43,900 --> 00:00:46,670
So say public static and

12
00:00:46,670 --> 00:00:50,100
notice that I'm doing this in the string
extension static class again.

13
00:00:51,490 --> 00:00:57,460
In return bool and
the name is null or empty.

14
00:00:59,180 --> 00:01:02,010
It takes, this extend string.

15
00:01:03,070 --> 00:01:07,699
We'll just call it this, and inside here

16
00:01:07,699 --> 00:01:12,727
we'll just call string.is null or
empty and

17
00:01:12,727 --> 00:01:17,640
pass in this and
we'll want to return that.

18
00:01:19,980 --> 00:01:22,120
Now let's go over to program.cs and
try it out.

19
00:01:23,210 --> 00:01:25,700
So here we have our null myString.

20
00:01:26,750 --> 00:01:33,400
So I can just call
myString.IsNullOrEmpty like so.

21
00:01:33,400 --> 00:01:37,380
Now, one thing you may not know
about the string class is that

22
00:01:37,380 --> 00:01:41,730
the string class actually implements
the IEnumerable interface.

23
00:01:43,060 --> 00:01:43,580
Let's take a look.

24
00:01:48,100 --> 00:01:53,080
As you can see,
a string is an I numerable of char.

25
00:01:53,080 --> 00:01:58,060
So it could actually move our is null or
empty extension method into

26
00:01:58,060 --> 00:02:03,700
the I Enumerable extensions class and
extend all I Enumerables.

27
00:02:03,700 --> 00:02:05,130
Let's see how that would look.

28
00:02:05,130 --> 00:02:07,958
First I'll create a new class to
contain this extension method.

29
00:02:13,882 --> 00:02:17,437
I'll call it IEnumerable extensions.

30
00:02:22,904 --> 00:02:23,860
Make it public.

31
00:02:24,870 --> 00:02:25,530
Make it static.

32
00:02:29,410 --> 00:02:30,300
And we'll copy.

33
00:02:33,830 --> 00:02:35,670
And I'm not gonna delete it from here.

34
00:02:35,670 --> 00:02:43,507
I'm actually just gonna copy it from here.

35
00:02:43,507 --> 00:02:50,286
We'll copy that over here.

36
00:02:50,286 --> 00:02:52,871
We will change this so that is null or

37
00:02:52,871 --> 00:02:55,980
empty can be called on
any I numerable of T.

38
00:02:57,230 --> 00:03:01,216
So say this I

39
00:03:01,216 --> 00:03:06,810
numerable of T.

40
00:03:06,810 --> 00:03:08,560
And instead of calling string.

41
00:03:08,560 --> 00:03:13,333
isnullorempty we'll

42
00:03:13,333 --> 00:03:18,356
return @this = null or

43
00:03:18,356 --> 00:03:23,140
!@this.Any().

44
00:03:23,140 --> 00:03:30,170
The Any method is actually a extension
method provided by system.link.

45
00:03:30,170 --> 00:03:33,790
If you're familiar with
the link you may know that link

46
00:03:33,790 --> 00:03:39,130
relies heavily on extension methods and
any is one of those extension methods.

47
00:03:39,130 --> 00:03:42,160
There is something else subtle
happening here, this or

48
00:03:42,160 --> 00:03:48,480
condition is actually taking advantage
of a feature called short circuiting.

49
00:03:48,480 --> 00:03:52,180
Remember calling Any on this variable

50
00:03:52,180 --> 00:03:56,730
will throw a null reference
exception if this is null, but

51
00:03:56,730 --> 00:04:02,390
if this is null then the first part
of this condition will be true.

52
00:04:02,390 --> 00:04:06,170
Short circuiting means that
the first condition of an or

53
00:04:06,170 --> 00:04:12,120
that evaluates to true will cause
the entire expression to be true.

54
00:04:12,120 --> 00:04:15,940
If that happens,
any of the later conditions or

55
00:04:15,940 --> 00:04:19,560
expressions in the condition,
are no longer evaluated,

56
00:04:19,560 --> 00:04:24,200
because C Sharp already knows that
the expression should be true.

57
00:04:24,200 --> 00:04:28,000
So if this is null, then this code
right here will never be called.

58
00:04:29,140 --> 00:04:32,790
We're now in a very
interesting situation now.

59
00:04:32,790 --> 00:04:39,420
We now have two methods that are named the
same that can be called on the same type.

60
00:04:39,420 --> 00:04:40,950
Here I'll demonstrate to you.

61
00:04:43,660 --> 00:04:47,630
Here we have myString.IsNullOrEmpty.

62
00:04:47,630 --> 00:04:50,462
Let's see what happens if we comment out

63
00:04:50,462 --> 00:04:55,200
the isNullorempty method in
the string extensions class.

64
00:04:57,500 --> 00:05:01,530
If we go back to Programm.cs we
don't see any red squiggly lines.

65
00:05:02,820 --> 00:05:05,075
And this will compile just fine.

66
00:05:07,163 --> 00:05:08,720
See the build succeeded.

67
00:05:09,890 --> 00:05:14,790
So I wonder which is null or
empty method will be called.

68
00:05:14,790 --> 00:05:18,580
To test this out, let's go back here and
uncomment this method.

69
00:05:18,580 --> 00:05:20,410
And then let's put a break
point inside of it.

70
00:05:22,760 --> 00:05:26,720
Let's also put a break point inside
the IEnumerable extensions version.

71
00:05:28,610 --> 00:05:29,430
Let's go ahead and run this.

72
00:05:32,050 --> 00:05:34,180
Notice that the is null or

73
00:05:34,180 --> 00:05:39,300
empty method inside of the string
extensions class was called.

74
00:05:39,300 --> 00:05:41,650
Instead, of the is null or

75
00:05:41,650 --> 00:05:45,650
the empty method inside of
the IEnumerable extensions class.

76
00:05:45,650 --> 00:05:49,510
So how did C Sharp know which
method should be called?

77
00:05:49,510 --> 00:05:54,320
Well, C Sharp knows that my
string is of type stream.

78
00:05:54,320 --> 00:05:57,190
It's also of type IEnumerable of char.

79
00:05:59,030 --> 00:06:02,334
But string is a more specific type.

80
00:06:02,334 --> 00:06:08,140
So when C Sharp determines which
extension method should be called,

81
00:06:08,140 --> 00:06:12,300
it always calls the one that deals
with the more specific type.

82
00:06:13,870 --> 00:06:17,870
Meaning it's the farthest down
in the inheritance chain.

83
00:06:17,870 --> 00:06:21,073
We can see another
example of this in link.

84
00:06:22,539 --> 00:06:27,587
Let's go back to the browser,
And search for

85
00:06:27,587 --> 00:06:32,274
the system.collections.generic.

86
00:06:33,913 --> 00:06:36,409
.list documentation.

87
00:06:42,915 --> 00:06:47,290
List contains a method called contains,

88
00:06:47,290 --> 00:06:50,915
which takes a parameter of type T.

89
00:06:50,915 --> 00:06:54,890
But link also provides
a method called contains.

90
00:06:56,430 --> 00:06:59,180
Here these are all of
the methods provided by

91
00:06:59,180 --> 00:07:02,000
link that can be used on the list class.

92
00:07:02,000 --> 00:07:06,631
These are all extension methods,
the contains method provided by link and

93
00:07:06,631 --> 00:07:11,370
the contains method provided by the list
class have identical signatures.

94
00:07:11,370 --> 00:07:16,230
The C-sharp knows that it should call
contains on a list when list is passed in

95
00:07:16,230 --> 00:07:20,090
and it should call links contains
when anything else is passed in.

96
00:07:21,110 --> 00:07:24,960
Let's create another extension method for
IEnumerable.

97
00:07:24,960 --> 00:07:28,970
A very common extension method that
many people use on IEnumerable

98
00:07:28,970 --> 00:07:33,250
that isn't provided by link
is the is empty method.

99
00:07:33,250 --> 00:07:37,354
So say public static bool is empty.

100
00:07:39,083 --> 00:07:43,311
This IEnumerable of type

101
00:07:43,311 --> 00:07:48,650
T You need to provide
the type parameter here.

102
00:07:54,240 --> 00:08:00,356
And all this will do is
return if there are not any

103
00:08:00,356 --> 00:08:05,880
items in the IEnumerable.

104
00:08:05,880 --> 00:08:10,250
Now it isn't unimaginable that
Microsoft could in the future

105
00:08:10,250 --> 00:08:12,990
add this identical method to .net.

106
00:08:12,990 --> 00:08:15,920
After all it is a pretty
obvious method to have.

107
00:08:15,920 --> 00:08:21,380
If Microsoft added this method and
we upgraded to that new version of .net,

108
00:08:21,380 --> 00:08:25,830
and we were using our own
extension method called is empty.

109
00:08:25,830 --> 00:08:29,798
We get a compiler error telling us
that there are ambiguous methods and

110
00:08:29,798 --> 00:08:33,185
the compiler doesn't
know which one to use.

111
00:08:33,185 --> 00:08:39,425
In that case, all we need to do is go
back and remove our is empty method.

112
00:08:39,425 --> 00:08:43,015
Extension methods should have
a very straightforward and

113
00:08:43,015 --> 00:08:44,805
obvious implementation.

114
00:08:44,805 --> 00:08:47,125
This makes them more reusable.

115
00:08:47,125 --> 00:08:49,925
But it also helps protect
against the situation

116
00:08:49,925 --> 00:08:54,270
where a method with the same name is
added to the class in the future.

117
00:08:54,270 --> 00:08:57,750
If you implement them the way you
would expect them to be implemented

118
00:08:57,750 --> 00:09:00,950
it's more likely that they will
have the same behavior and

119
00:09:00,950 --> 00:09:04,220
you can just remove your implementation
of the extension method.