When planning out my
programs, I often start with a chain of thought like
so:
A football
team is just a list of football players. Therefore, I should represent it
with:var football_team = new
List();
The ordering of this list represent the order in which the players are listed
in the
roster.
But
I realize later that teams also have other properties, besides the mere list of players,
that must be recorded. For example, the running total of scores this season, the current
budget, the uniform colors, a string
representing the name of
the team, etc..
So then I
think:
Okay, a
football team is just like a list of players, but additionally, it has a name (a
string
) and a running total of scores (an
int
). .NET does not provide a class for storing football teams,
so I will make my own class. The most similar and relevant existing structure is
List
, so I will inherit from
it:class FootballTeam :
List
{
public string TeamName;
public int RunningTotal
}
But
it turns out that a
guideline says you shouldn't inherit from
List
. I'm thoroughly confused by this
guideline in two respects.
Why
not?
Apparently href="https://stackoverflow.com/a/5376358/1042555">List
is
somehow optimized for performance. How so? What performance problems will I
cause if I extend List
? What exactly will
break?
Another reason I've seen is that
List
is provided by Microsoft, and I have no control over it,
so href="https://stackoverflow.com/questions/5376203/inherit-listt#comment6077237_5376343">I
cannot change it later, after exposing a "public API". But I struggle to
understand this. What is a public API and why should I care? If my current project does
not and is not likely to ever have this public API, can I safely ignore this guideline?
If I do inherit from List
and it turns out
I need a public API, what difficulties will I
have?
Why does it even matter? A list is a list.
What could possibly change? What could I possibly want to
change?
And lastly, if Microsoft did not want me
to inherit from List
, why didn't they make the class
sealed
?
What else am I
supposed to use?
Apparently, for
custom collections, Microsoft has provided a Collection
class
which should be extended instead of List
. But this class is
very bare, and does not have many useful things, href="https://stackoverflow.com/questions/1474863/addrange-to-a-collection">such as
AddRange
, for instance. href="https://stackoverflow.com/a/12039943/1042555">jvitor83's answer
provides a performance rationale for that particular method, but how is a slow
AddRange
not better than no
AddRange
?
Inheriting
from Collection
is way more work than inheriting from
List
, and I see no benefit. Surely Microsoft wouldn't tell me
to do extra work for no reason, so I can't help feeling like I am somehow
misunderstanding something, and inheriting Collection
is
actually not the right solution for my
problem.
I've seen suggestions such as
implementing IList
. Just no. This is dozens of lines of
boilerplate code which gains me nothing.
Lastly,
some suggest wrapping the List
in something:
class FootballTeam
{
public List Players;
}
There are
two problems with
this:
It makes my
code needlessly verbose. I must now callmy_team.Players.Count
instead of justmy_team.Count
. Thankfully, with C# I can define
indexers to make indexing transparent, and forward all the methods of the internal
List
... But that's a lot of code! What do I get for all that
work?It just plain doesn't make any
sense. A football team doesn't "have" a list of players. It is the
list of players. You don't say "John McFootballer has joined SomeTeam's players". You
say "John has joined SomeTeam". You don't add a letter to "a string's characters", you
add a letter to a string. You don't add a book to a library's books, you add a book to a
library.
I
realize that what happens "under the hood" can be said to be "adding X to Y's internal
list", but this seems like a very counter-intuitive way of thinking about the
world.
My question
(summarized)
What is the correct C# way of
representing a data structure, which, "logically" (that is to say, "to the human mind")
is just a list
of things
with a few
bells and whistles?
Is inheriting from
List
always unacceptable? When is it acceptable?
Why/why not? What must a programmer consider, when deciding whether to inherit from
List
or not?
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