Why not inherit from List T

Question

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 lt FootballPlayer gt           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 lt FootballPlayer gt   so I will inherit from it   class FootballTeam   List lt FootballPlayer gt          public string TeamName       public int RunningTotal       But it turns out that a guideline says you shouldn t inherit from List lt T gt   I m thoroughly confused by this guideline in two respects   Why not   Apparently 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 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  such as AddRange  for instance  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 lt FootballPlayer gt  Players       There are two problems with this    It makes my code needlessly verbose  I must now call my team Players Count instead of just my 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 lt T gt  always unacceptable  When is it acceptable  Why why not  What must a programmer consider  when deciding whether to inherit from List lt T gt  or not

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A football team is not a list of football players  A football team is composed of a list of football players    This is logically wrong   class FootballTeam   List lt FootballPlayer gt          public string TeamName       public int RunningTotal      and this is correct   class FootballTeam         public List lt FootballPlayer gt  players     public string TeamName       public int RunningTotal

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Just because I think the other answers pretty much go off on a tangent of whether a football team  is-a  List lt FootballPlayer gt  or  has-a  List lt FootballPlayer gt   which really doesn t answer this question as written   The OP chiefly asks for clarification on guidelines for inheriting from List lt T gt       A guideline says that you shouldn t inherit from List lt T gt   Why not    Because List lt T gt  has no virtual methods  This is less of a problem in your own code  since you can usually switch out the implementation with relatively little pain - but can be a much bigger deal in a public API      What is a public API and why should I care    A public API is an interface you expose to 3rd party programmers  Think framework code  And recall that the guidelines being referenced are the   NET Framework Design Guidelines  and not the   NET Application Design Guidelines   There is a difference  and - generally speaking - public API design is a lot more strict      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    Pretty much  yeah  You may want to consider the rationale behind it to see if it applies to your situation anyway  but if you re not building a public API then you don t particularly need to worry about API concerns like versioning  of which  this is a subset    If you add a public API in the future  you will either need to abstract out your API from your implementation  by not exposing your List lt T gt  directly  or violate the guidelines with the possible future pain that entails      Why does it even matter  A list is a list  What could possibly change  What could I possibly want to change    Depends on the context  but since we re using FootballTeam as an example - imagine that you can t add a FootballPlayer if it would cause the team to go over the salary cap  A possible way of adding that would be something like    class FootballTeam   List lt FootballPlayer gt         override void Add FootballPlayer player            if  this Sum p   gt  p Salary    player Salary  gt  SALARY CAP               throw new InvalidOperationException  Would exceed salary cap                           Ah   but you can t override Add because it s not virtual  for performance reasons     If you re in an application  which  basically  means that you and all of your callers are compiled together  then you can now change to using IList lt T gt  and fix up any compile errors    class FootballTeam   IList lt FootballPlayer gt         private List lt FootballPlayer gt  Players   get  set          override void Add FootballPlayer player            if  this Players Sum p   gt  p Salary    player Salary  gt  SALARY CAP               throw new InvalidOperationException  Would exceed salary cap                              boiler plate for rest of IList         but  if you ve publically exposed to a 3rd party you just made a breaking change that will cause compile and or runtime errors   TL DR - the guidelines are for public APIs  For private APIs  do what you want

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This is a classic example of composition vs inheritance   In this specific case   Is the team a list of players with added behavior  or  Is the team an object of its own that happens to contain a list of players   By extending List you are limiting yourself in a number of ways    You cannot restrict access  for example  stopping people changing the roster   You get all the List methods whether you need want them all or not  What happens if you want to have lists of other things as well  For example  teams have coaches  managers  fans  equipment  etc  Some of those might well be lists in their own right  You limit your options for inheritance  For example you might want to create a generic Team object  and then have BaseballTeam  FootballTeam  etc  that inherit from that  To inherit from List you need to do the inheritance from Team  but that then means that all the various types of team are forced to have the same implementation of that roster    Composition - including an object giving the behavior you want inside your object   Inheritance - your object becomes an instance of the object that has the behavior you want   Both have their uses  but this is a clear case where composition is preferable

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What is the correct C  way of representing a data structure      Remeber   All models are wrong  but some are useful   -George E  P  Box  There is no a  correct way   only a useful one   Choose one that is useful to you and your users  That s it  Develop economically  don t over-engineer  The less code you write  the less code you will need to debug   read the following editions    -- Edited  My best answer would be    it depends  Inheriting from a List would expose the clients of this class to methods that may be should not be exposed  primarily because FootballTeam looks like a business entity    -- Edition 2  I sincerely don t remember to what I was referring on the    don t over-engineer    comment  While I believe the KISS mindset is a good guide  I want to emphasize that inheriting a business class from List would create more problems than it resolves  due abstraction leakage   On the other hand  I believe there are a limited number of cases where simply to inherit from List is useful  As I wrote in the previous edition  it depends  The answer to each case is heavily influenced by both knowledge  experience and personal preferences   Thanks to  kai for helping me to think more precisely about the answer

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This reminds me of the  Is a  versus  has a  tradeoff   Sometimes it is easier and makesmore sense to inherit directly from a super class   Other times it makes more sense to create a standalone class and include the class you would have inherited from as a member variable   You can still access the functionality of the class but are not bound to the interface or any other constraints that might come from inheriting from the class   Which do you do   As with a lot of things   it depends on the context   The guide I would use is that in order to inherit from another class there truly should be an  is a  relationship   So if you a writing a class called BMW  it could inherit from Car because a BMW truly is a car   A Horse class can inherit from the Mammal class because a horse actually is a mammal in real life and any Mammal functionality should be relevant to Horse   But can you say that a team is a list   From what I can tell  it does not seem like a Team really  is a  List   So in this case  I would have a List as a member variable

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Let me rewrite your question  so you might see the subject from a different perspective   When I need to represent a football team  I understand that it is basically a name  Like   The Eagles   string team   new string      Then later I realized teams also have players   Why can t I just extend the string type so that it also holds a list of players   Your point of entry into the problem is arbitrary  Try to think what does a team have  properties   not what it is   After you do that  you could see if it shares properties with other classes  And think about inheritance

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As everyone has pointed out  a team of players is not a list of players  This mistake is made by many people everywhere  perhaps at various levels of expertise  Often the problem is subtle and occasionally very gross  as in this case  Such designs are bad because these violate the  Liskov Substitution Principle  The internet has many good articles explaining this concept e g   http   en wikipedia org wiki Liskov substitution principle   In summary  there are two rules to be preserved in a Parent Child relationship among classes    a Child should require no characteristic less than what completely defines the Parent  a Parent should require no characteristic in addition to what completely defines the Child    In other words  a Parent is a necessary definition of a child  and a child is a sufficient definition of a Parent   Here is a way to think through ones solution and apply the above principle that should help one avoid such a mistake  One should test ones hypothesis by verifying if all the operations of a parent class are valid for the derived class both structurally and semantically    Is a football team a list of football players    Do all properties of a list apply to a team in the same meaning    Is a team a collection of homogenous entities  Yes  team is a collection of Players Is the order of inclusion of players descriptive of the state of the team and does the team ensure that the sequence is preserved unless explicitly changed  No  and No Are players expected to be included dropped based on their sequencial position in the team  No    As you see  only the first characteristic of a list is applicable to a team  Hence a team is not a list  A list would be a implementation detail of how you manage your team  so it should only be used to store the player objects and be manipulated with methods of Team class   At this point I d like to remark that a Team class should  in my opinion  not even be implemented using a List  it should be implemented using a Set data structure  HashSet  for example  in most cases

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If your class users need all the methods and properties   List has  you should derive your class from it  If they don t need them  enclose the List and make wrappers for methods your class users actually need    This is a strict rule  if you write a public API  or any other code that will be used by many people  You may ignore this rule if you have a tiny app and no more than 2 developers  This will save you some time    For tiny apps  you may also consider choosing another  less strict language  Ruby  JavaScript - anything that allows you to write less code

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First of all  it has to do with usability  If you use inheritance  the Team class will expose behavior  methods  that are designed purely for object manipulation  For example  AsReadOnly   or CopyTo obj  methods make no sense for the team object  Instead of the AddRange items  method you would probably want a more descriptive AddPlayers players  method   If you want to use LINQ  implementing a generic interface such as ICollection lt T gt  or IEnumerable lt T gt  would make more sense   As mentioned  composition is the right way to go about it  Just implement a list of players as a private variable

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Problems with serializing One aspect is missing  Classes that inherit from List can t be serialized correctly using XmlSerializer  In that case DataContractSerializer must be used instead  or an own serializing implementation is needed  public class DemoList   List lt Demo gt           using XmlSerializer this properties won t be seralized        There is no error  the data is simply not there      string AnyPropertyInDerivedFromList   get  set            public class Demo          this properties will be seralized     string AnyPropetyInDemo   get  set         Further reading  When a class is inherited from List lt  gt   XmlSerializer doesn  39 t serialize other attributes

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I think I don t agree with your generalization   A team isn t just a collection of players   A team has so much more information about it - name  emblem  collection of management admin staff  collection of coaching crew  then collection of players  So properly  your FootballTeam class should have 3 collections and not itself be a collection  if it is to properly model the real world   You could consider a PlayerCollection class which like the Specialized StringCollection offers some other facilities - like validation and checks before objects are added to or removed from the internal store   Perhaps  the notion of a PlayerCollection betters suits your preferred approach   public class PlayerCollection   Collection lt Player gt          And then the FootballTeam can look like this   public class FootballTeam         public string Name   get  set        public string Location   get  set         public ManagementCollection Management   get  protected set      new ManagementCollection         public CoachingCollection CoachingCrew   get  protected set      new CoachingCollection         public PlayerCollection Players   get  protected set      new PlayerCollection

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It depends on the behaviour of your  team  object   If it behaves just like a collection  it might be OK to represent it first with a plain List   Then you might start to notice that you keep duplicating code that iterates on the list  at this point you have the option of creating a FootballTeam object that wraps the list of players   The FootballTeam class becomes the home for all the code that iterates on the list of players      It makes my code needlessly verbose  I must now call my team Players Count instead of just my 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    Encapsulation   Your clients need not know what goes on inside of FootballTeam   For all your clients know  it might be implemented by looking the list of players up in a database   They don t need to know  and this improves your design      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    Exactly    you will say footballTeam Add john   not footballTeam List Add john    The internal list will not be visible

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Wow  your post has an entire slew of questions and points  Most of the reasoning you get from Microsoft is exactly on point  Let s start with everything about List lt T gt   List lt T gt  is highly optimized  Its main usage is to be used as a private member of an object  Microsoft did not seal it because sometimes you might want to create a class that has a friendlier name  class MyList lt T  TX gt    List lt CustomObject lt T  Something lt TX gt  gt           Now it s as easy as doing var list   new MyList lt int  string gt      CA1002  Do not expose generic lists  Basically  even if you plan to use this app as the sole developer  it s worthwhile to develop with good coding practices  so they become instilled into you and second nature  You are still allowed to expose the list as an IList lt T gt  if you need any consumer to have an indexed list  This lets you change the implementation within a class later on  Microsoft made Collection lt T gt  very generic because it is a generic concept    the name says it all  it is just a collection  There are more precise versions such as SortedCollection lt T gt   ObservableCollection lt T gt   ReadOnlyCollection lt T gt   etc  each of which implement IList lt T gt  but not List lt T gt   Collection lt T gt  allows for members  i e  Add  Remove  etc   to be overridden because they are virtual  List lt T gt  does not  The last part of your question is spot on  A Football team is more than just a list of players  so it should be a class that contains that list of players  Think Composition vs Inheritance  A Football team has a list of players  a roster   it isn t a list of players    If I were writing this code  the class would probably look something like so  public class FootballTeam          Football team rosters are generally 53 total players      private readonly List lt T gt   roster   new List lt T gt  53        public IList lt T gt  Roster               get   return  roster                  Yes  I used LINQ here  This is so I don t have to worry about         roster Length vs  roster Count vs anything else      public int PlayerCount               get   return  roster Count                    Any additional members you want to expose wrap

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There are a lot excellent answers here  but I want to touch on something I didn t see mentioned  Object oriented design is about empowering objects   You want to encapsulate all your rules  additional work and internal details inside an appropriate object  In this way other objects interacting with this one don t have to worry about it all  In fact  you want to go a step further and actively prevent other objects from bypassing these internals   When you inherit from List  all other objects can see you as a List  They have direct access to the methods for adding and removing players  And you ll have lost your control  for example   Suppose you want to differentiate when a player leaves by knowing whether they retired  resigned or were fired  You could implement a RemovePlayer method that takes an appropriate input enum  However  by inheriting from List  you would be unable to prevent direct access to Remove  RemoveAll and even Clear  As a result  you ve actually disempowered your FootballTeam class     Additional thoughts on encapsulation    You raised the following concern      It makes my code needlessly verbose  I must now call my team Players Count instead of just my team Count    You re correct  that would be needlessly verbose for all clients to use you team  However  that problem is very small in comparison to the fact that you ve exposed List Players to all and sundry so they can fiddle with your team without your consent   You go on to say      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 re wrong about the first bit  Drop the word  list   and it s actually obvious that a team does have players  However  you hit the nail on the head with the second  You don t want clients calling ateam Players Add       You do want them calling ateam AddPlayer       And your implemention would  possibly amongst other things  call Players Add      internally     Hopefully you can see how important encapsulation is to the objective of empowering your objects  You want to allow each class to do its job well without fear of interference from other objects

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What if the FootballTeam has a reserves team along with the main team   class FootballTeam       List lt FootballPlayer gt  Players   get  set        List lt FootballPlayer gt  ReservePlayers   get  set        How would you model that with    class FootballTeam   List lt FootballPlayer gt          public string TeamName       public int RunningTotal      The relationship is clearly has a and not is a   or RetiredPlayers   class FootballTeam       List lt FootballPlayer gt  Players   get  set        List lt FootballPlayer gt  ReservePlayers   get  set        List lt FootballPlayer gt  RetiredPlayers   get  set        As a rule of thumb  if you ever want to inherit from a collection  name the class SomethingCollection    Does your SomethingCollection semantically make sense  Only do this if your type is a collection of Something   In the case of FootballTeam it doesn t sound right  A Team is more than a Collection  A Team can have coaches  trainers  etc as the other answers have pointed out   FootballCollection sounds like a collection of footballs or maybe a collection of football paraphernalia  TeamCollection  a collection of teams    FootballPlayerCollection sounds like a collection of players which would be a valid name for a class that inherits from List lt FootballPlayer gt  if you really wanted to do that    Really List lt FootballPlayer gt  is a perfectly good type to deal with  Maybe IList lt FootballPlayer gt  if you are returning it from a method   In summary  Ask yourself   Is X a Y  or Has X a Y  Do my class names mean what they are

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There are some good answers here  I would add to them the following points      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    Ask any ten non-computer-programmer people who are familiar with the existence of football to fill in the blank      A football team is a particular kind of         Did anyone say  list of football players with a few bells and whistles   or did they all say  sports team  or  club  or  organization    Your notion that a football team is a particular kind of list of players is in your human mind and your human mind alone    List lt T gt  is a mechanism   Football team is a business object -- that is  an object that represents some concept that is in the business domain of the program  Don t mix those  A football team is a kind of team  it has a roster  a roster is a list of players  A roster is not a particular kind of list of players  A roster is a list of players  So make a property called Roster that is a List lt Player gt   And make it ReadOnlyList lt Player gt  while you re at it  unless you believe that everyone who knows about a football team gets to delete players from the roster      Is inheriting from List lt T gt  always unacceptable     Unacceptable to who  Me  No      When is it acceptable     When you re building a mechanism that extends the List lt T gt  mechanism      What must a programmer consider  when deciding whether to inherit from List lt T gt  or not    Am I building a mechanism or a business object       But that s a lot of code  What do I get for all that work    You spent more time typing up your question that it would have taken you to write forwarding methods for the relevant members of List lt T gt  fifty times over  You re clearly not afraid of verbosity  and we are talking about a very small amount of code here  this is a few minutes work   UPDATE  I gave it some more thought and there is another reason to not model a football team as a list of players  In fact it might be a bad idea to model a football team as having a list of players too  The problem with a team as having a list of players is that what you ve got is a snapshot of the team at a moment in time  I don t know what your business case is for this class  but if I had a class that represented a football team I would want to ask it questions like  how many Seahawks players missed games due to injury between 2003 and 2013   or  What Denver player who previously played for another team had the largest year-over-year increase in yards ran   or  Did the Piggers go all the way this year     That is  a football team seems to me to be well modeled as a collection of historical facts such as when a player was recruited  injured  retired  etc  Obviously the current player roster is an important fact that should probably be front-and-center  but there may be other interesting things you want to do with this object that require a more historical perspective

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I just wanted to add that Bertrand Meyer  the inventor of Eiffel and design by contract  would have Team inherit from List lt Player gt  without so much as batting an eyelid   In his book  Object-Oriented Software Construction  he discusses the implementation of a GUI system where rectangular windows can have child windows  He simply has Window inherit from both Rectangle and Tree lt Window gt  to reuse the implementation   However  C  is not Eiffel  The latter supports multiple inheritance and renaming of features  In C   when you subclass  you inherit both the interface and the implemenation  You can override the implementation  but the calling conventions are copied directly from the superclass  In Eiffel  however  you can modify the names of the public methods  so you can rename Add and Remove to Hire and Fire in your Team  If an instance of Team is upcast back to List lt Player gt   the caller will use Add and Remove to modify it  but your virtual methods Hire and Fire will be called

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When they say List lt T gt  is  optimized  I think they want to mean that it doesn t have features like virtual methods which are bit more expensive  So the problem is that once you expose List lt T gt  in your public API  you loose ability to enforce business rules or customize its functionality later  But if you are using this inherited class as internal within your project  as opposed to potentially exposed to thousands of your customers partners other teams as API  then it may be OK if it saves your time and it is the functionality you want to duplicate  The advantage of inheriting from List lt T gt  is that you eliminate lot of dumb wrapper code that is just never going to be customized in foreseeable future  Also if you want your class to explicitly have exact same semantics as List lt T gt  for the life of your APIs then also it may be OK   I often see lot of people doing tons of extra work just because of FxCop rule says so or someone s blog says it s a  bad  practice  Many times  this turns code in to design pattern palooza weirdness  As with lot of guideline  treat it as guideline that can have exceptions

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class FootballTeam   List lt FootballPlayer gt          public string TeamName       public int RunningTotal      Previous code means  a bunch of guys from the street playing football  and they happen to have a name  Something like     Anyway  this code  from m-y s answer    public class FootballTeam          A team s name     public string TeamName           Football team rosters are generally 53 total players      private readonly List lt T gt   roster   new List lt T gt  53        public IList lt T gt  Roster               get   return  roster               public int PlayerCount               get   return  roster Count                    Any additional members you want to expose wrap      Means  this is a football team which has management  players  admins  etc  Something like     This is how is your logic presented in pictures

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It depends on the context  When you consider your team as a list of players  you are projecting the  idea  of a foot ball team down to one aspect  You reduce the  team  to the people you see on the field  This projection is only correct in a certain context  In a different context  this might be completely wrong  Imagine you want to become a sponsor of the team  So you have to talk to the managers of the team  In this context the team is projected to the list of its managers  And these two lists usually don t overlap very much  Other contexts are the current versus the former players  etc   Unclear semantics  So the problem with considering a team as a list of its players is that its semantic depends on the context and that it cannot be extended when the context changes  Additionally it is hard to express  which context you are using   Classes are extensible  When you using a class with only one member  e g  IList activePlayers   you can use the name of the member  and additionally its comment  to make the context clear  When there are additional contexts  you just add an additional member   Classes are more complex  In some cases it might be overkill to create an extra class  Each class definition must be loaded through the classloader and will be cached by the virtual machine  This costs you runtime performance and memory  When you have a very specific context it might be OK to consider a football team as a list of players  But in this case  you should really just use a  IList   not a class derived from it   Conclusion   Considerations  When you have a very specific context  it is OK to consider a team as a list of players  For example inside a method it is completely OK to write   IList lt Player gt  footballTeam         When using F   it can even be OK to create a type abbreviation   type FootballTeam   IList lt Player gt    But when the context is broader or even unclear  you should not do this  This is especially the case when you create a new class whose context in which it may be used in the future is not clear  A warning sign is when you start to add additional attributes to your class  name of the team  coach  etc    This is a clear sign that the context where the class will be used is not fixed and will change in the future  In this case you cannot consider the team as a list of players  but you should model the list of the  currently active  not injured  etc   players as an attribute of the team

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What the guidelines say is that the public API should not reveal the internal design decision of whether you are using a list  a set  a dictionary  a tree or whatever  A  team  is not necessarily a list  You may implement it as a list but users of your public API should use you class on a need to know basis  This allows you to change your decision and use a different data structure without affecting the public interface

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Does allowing people to say  myTeam subList 3  5     make any sense at all  If not then it shouldn t be a List

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While I don t have a complex comparison as most of these answers do  I would like to share my method for handling this situation  By extending IEnumerable lt T gt   you can allow your Team class to support Linq query extensions  without publicly exposing all the methods and properties of List lt T gt    class Team   IEnumerable lt Player gt        private readonly List lt Player gt  playerList       public Team                 playerList   new List lt Player gt                public Enumerator GetEnumerator                 return playerList GetEnumerator                      class Player

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When is it acceptable    To quote Eric Lippert      When you re building a mechanism that extends the List lt T gt  mechanism    For example  you are tired of the absence of the AddRange method in IList lt T gt    public interface IMoreConvenientListInterface lt T gt    IList lt T gt        void AddRange IEnumerable lt T gt  collection      public class MoreConvenientList lt T gt    List lt T gt   IMoreConvenientListInterface lt T gt

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Prefer Interfaces over Classes  Classes should avoid deriving from classes and instead implement the minimal interfaces necessary    Inheritance breaks Encapsulation  Deriving from classes breaks encapsulation     exposes internal details about how your collection is implemented declares an interface  set of public functions and properties  that may not be appropriate   Among other things this makes it harder to refactor your code    Classes are an Implementation Detail  Classes are an implementation detail that should be hidden from other parts of your code   In short a System List is a specific implementation of an abstract data type  that may or may not be appropriate now and in the future    Conceptually the fact that the System List data type is called  list  is a bit of a red-herring  A System List lt T gt  is a mutable ordered collection that supports amortized O 1  operations for adding  inserting  and removing elements  and O 1  operations for retrieving the number of elements or getting and setting element by index   The Smaller the Interface the more Flexible the Code  When designing a data structure  the simpler the interface is  the more flexible the code is  Just look at how powerful LINQ is for a demonstration of this   How to Choose Interfaces  When you think  list  you should start by saying to yourself   I need to represent a collection of baseball players   So let s say you decide to model this with a class  What you should do first is decide what the minimal amount of interfaces that this class will need to expose    Some questions that can help guide this process    Do I need to have the count  If not consider implementing IEnumerable lt T gt  Is this collection going to change after it has been initialized  If not consider IReadonlyList lt T gt    Is it important that I can access items by index  Consider ICollection lt T gt  Is the order in which I add items to the collection important  Maybe it is an ISet lt T gt   If you indeed want these thing then go ahead and implement IList lt T gt      This way you will not be coupling other parts of the code to implementation details of your baseball players collection and will be free to change how it is implemented as long as you respect the interface   By taking this approach you will find that code becomes easier to read  refactor  and reuse    Notes about Avoiding Boilerplate  Implementing interfaces in a modern IDE should be easy  Right click and choose  Implement Interface   Then forward all of the implementations to a member class if you need to    That said  if you find you are writing lots of boilerplate  it is potentially because you are exposing more functions than you should be  It is the same reason you shouldn t inherit from a class   You can also design smaller interfaces that make sense for your application  and maybe just a couple of helper extension functions to map those interfaces to any others that you need  This is the approach I took in my own IArray interface for the LinqArray library

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Design   Implementation  What methods and properties you expose is a design decision  What base class you inherit from is an implementation detail  I feel it s worth taking a step back to the former   An object is a collection of data and behaviour   So your first questions should be    What data does this object comprise in the model I m creating  What behaviour does this object exhibit in that model  How might this change in future    Bear in mind that inheritance implies an  isa   is a  relationship  whereas composition implies a  has a   hasa  relationship  Choose the right one for your situation in your view  bearing in mind where things might go as your application evolves   Consider thinking in interfaces before you think in concrete types  as some people find it easier to put their brain in  design mode  that way   This isn t something everyone does consciously at this level in day to day coding  But if you re mulling this sort of topic  you re treading in design waters  Being aware of it can be liberating   Consider Design Specifics  Take a look at List lt T gt  and IList lt T gt  on MSDN or Visual Studio  See what methods and properties they expose  Do these methods all look like something someone would want to do to a FootballTeam in your view   Does footballTeam Reverse   make sense to you  Does footballTeam ConvertAll lt TOutput gt    look like something you want    This isn t a trick question  the answer might genuinely be  yes   If you implement inherit List lt Player gt  or IList lt Player gt   you re stuck with them  if that s ideal for your model  do it   If you decide yes  that makes sense  and you want your object to be treatable as a collection list of players  behaviour   and you therefore want to implement ICollection or IList  by all means do so  Notionally   class FootballTeam       ICollection lt Player gt                If you want your object to contain a collection list of players  data   and you therefore want the collection or list to be a property or member  by all means do so  Notionally   class FootballTeam           public ICollection lt Player gt  Players   get               You might feel that you want people to be able to only enumerate the set of players  rather than count them  add to them or remove them  IEnumerable lt Player gt  is a perfectly valid option to consider   You might feel that none of these interfaces are useful in your model at all  This is less likely  IEnumerable lt T gt  is useful in many situations  but it s still possible   Anyone who attempts to tell you that one of these it is categorically and definitively wrong in every case is misguided  Anyone who attempts to tell you it is categorically and definitively right in every case is misguided   Move on to Implementation  Once you ve decided on data and behaviour  you can make a decision about implementation  This includes which concrete classes you depend on via inheritance or composition   This may not be a big step  and people often conflate design and implementation since it s quite possible to run through it all in your head in a second or two and start typing away   A Thought Experiment  An artificial example  as others have mentioned  a team is not always  just  a collection of players  Do you maintain a collection of match scores for the team  Is the team interchangable with the club  in your model  If so  and if your team isa collection of players  perhaps it also isa collection of staff and or a collection of scores  Then you end up with   class FootballTeam       ICollection lt Player gt                             ICollection lt StaffMember gt                            ICollection lt Score gt                 Design notwithstanding  at this point in C  you won t be able to implement all of these by inheriting from List lt T gt  anyway  since C   only  supports single inheritance   If you ve tried this malarky in C    you may consider this a Good Thing   Implementing one collection via inheritance and one via composition is likely to feel dirty  And properties such as Count become confusing to users unless you implement ILIst lt Player gt  Count and IList lt StaffMember gt  Count etc  explicitly  and then they re just painful rather than confusing  You can see where this is going  gut feeling whilst thinking down this avenue may well tell you it feels wrong to head in this direction  and rightly or wrongly  your colleagues might also if you implemented it this way    The Short Answer  Too Late   The guideline about not inheriting from collection classes isn t C  specific  you ll find it in many programming languages  It is received wisdom not a law  One reason is that in practice composition is considered to often win out over inheritance in terms of comprehensibility  implementability and maintainability  It s more common with real world   domain objects to find useful and consistent  hasa  relationships than useful and consistent  isa  relationships unless you re deep in the abstract  most especially as time passes and the precise data and behaviour of objects in code changes  This shouldn t cause you to always rule out inheriting from collection classes  but it may be suggestive

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My dirty secret  I don t care what people say  and I do it   NET Framework is spread with  XxxxCollection   UIElementCollection for top of my head example    So what stops me saying   team Players ByName  Nicolas     When I find it better than  team ByName  Nicolas     Moreover  my PlayerCollection might be used by other class  like  Club  without any code duplication   club Players ByName  Nicolas     Best practices of yesterday  might not be the one of tomorrow  There is no reason behind most best practices  most are only wide agreement among the community  Instead of asking the community if it will blame you when you do that ask yourself  what is more readable and maintainable   team Players ByName  Nicolas      or  team ByName  Nicolas     Really  Do you have any doubt  Now maybe you need to play with other technical constraints that prevent you to use List lt T  in your real use case  But don t add a constraint that should not exist  If Microsoft did not document the why  then it is surely a  best practice  coming from nowhere

[c#] Why not inherit from List<T>?

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