When to use IList and when to use List

Question

I know that IList is the interface and List is the concrete type but I still don t know when to use each one  What I m doing now is if I don t need the Sort or FindAll methods I use the interface  Am I right  Is there a better way to decide when to use the interface or the concrete type

User · Answer

You are most often better of using the most general usable type, in this case the IList or even better the IEnumerable interface, so that you can switch the implementation conveniently at a later time.

However, in .NET 2.0, there is an annoying thing - IList does not have a Sort() method. You can use a supplied adapter instead:

ArrayList.Adapter(list).Sort()

User · Answer

It s always best to use the lowest base type possible  This gives the implementer of your interface  or consumer of your method  the opportunity to use whatever they like behind the scenes   For collections you should aim to use IEnumerable where possible  This gives the most flexibility but is not always suited

User · Answer

If you re working within a single method  or even in a single class or assembly in some cases  and no one outside is going to see what you re doing  use the fullness of a List  But if you re interacting with outside code  like when you re returning a list from a method  then you only want to declare the interface without necessarily tying yourself to a specific implementation  especially if you have no control over who compiles against your code afterward  If you started with a concrete type and you decided to change to another one  even if it uses the same interface  you re going to break someone else s code unless you started off with an interface or abstract base type

User · Answer

There are two rules I follow    Accept the most basic type that will work Return the richest type your user will need   So when writing a function or method that takes a collection  write it not to take a List  but an IList lt T gt   an ICollection lt T gt   or IEnumerable lt T gt    The generic interfaces will still work even for heterogenous lists because System Object can be a T too   Doing this will save you headache if you decide to use a Stack or some other data structure further down the road   If all you need to do in the function is foreach through it  IEnumerable lt T gt  is really all you should be asking for   On the other hand  when returning an object out of a function  you want to give the user the richest possible set of operations without them having to cast around   So in that case  if it s a List lt T gt  internally  return a copy as a List lt T gt

User · Answer

IEnumerable You should try and use the least specific type that suits your purpose  IEnumerable is less specific than IList  You use IEnumerable when you want to loop through the items in a collection     IList IList implements IEnumerable  You should use IList when you need access by index to your collection  add and delete elements  etc     List List implements IList

User · Answer

You should use the interface only if you need it  e g   if your list is casted to an IList implementation other than List  This is true when  for example  you use NHibernate  which casts ILists into an NHibernate bag object when retrieving data   If List is the only implementation that you will ever use for a certain collection  feel free to declare it as a concrete List implementation

User · Answer

There s an important thing that people always seem to overlook   You can pass a plain array to something which accepts an IList lt T gt  parameter  and then you can call IList Add   and will receive a runtime exception   Unhandled Exception  System NotSupportedException  Collection was of a fixed size   For example  consider the following code   private void test IList lt int gt  list        list Add 1       If you call that as follows  you will get a runtime exception   int   array   new int 0   test array     This happens because using plain arrays with IList lt T gt  violates the Liskov substitution principle   For this reason  if you are calling IList lt T gt  Add   you may want to consider requiring a List lt T gt  instead of an IList lt T gt

User · Answer

Microsoft guidelines as checked by FxCop discourage use of List lt T gt  in public APIs - prefer IList lt T gt    Incidentally  I now almost always declare one-dimensional arrays as IList lt T gt   which means I can consistently use the IList lt T gt  Count property rather than Array Length   For example   public interface IMyApi       IList lt int gt  GetReadOnlyValues       public class MyApiImplementation   IMyApi       public IList lt int gt  GetReadOnlyValues                 List lt int gt  myList   new List lt int gt                 populate list         return myList AsReadOnly            public class MyMockApiImplementationForUnitTests   IMyApi       public IList lt int gt  GetReadOnlyValues                 IList lt int gt  testValues   new int     1  2  3            return testValues

User · Answer

AList object allows you to create a list  add things to it  remove it  update it  index into it and etc  List is used whenever you just want a generic List where you specify object type in it and that s it   IList on the other hand is an Interface  Basically  if you want to create your own type of List  say a list class called BookList  then you can use the Interface to give you basic methods and structure to your new class  IList is for when you want to create your own  special sub-class that implements List   Another difference is   IList is an Interface and cannot be instantiated  List is a class and can be instantiated  It means   IList lt string gt  MyList   new IList lt string gt      List lt string gt  MyList   new List lt string gt

User · Answer

In situations I usually come across  I rarely use IList directly   Usually I just use it as an argument to a method    void ProcessArrayData IList almostAnyTypeOfArray           Do some stuff with the IList array     This will allow me to do generic processing on almost any array in the  NET framework  unless it uses IEnumerable and not IList  which happens sometimes   It really comes down to the kind of functionality you need  I d suggest using the List class in most cases  IList is best for when you need to make a custom array that could have some very specific rules that you d like to encapsulate within a collection so you don t repeat yourself  but still want  NET to recognize it as a list

User · Answer

I don t think there are hard and fast rules for this type of thing  but I usually go by the guideline of using the lightest possible way until absolutely necessary   For example  let s say you have a Person class and a Group class  A Group instance has many people  so a List here would make sense  When I declare the list object in Group I will use an IList lt Person gt  and instantiate it as a List   public class Group     private IList lt Person gt  people     public Group         this people   new List lt Person gt             And  if you don t even need everything in IList you can always use IEnumerable too  With modern compilers and processors  I don t think there is really any speed difference  so this is more just a matter of style

User · Answer

I would agree with Lee s advice for taking parameters  but not returning   If you specify your methods to return an interface that means you are free to change the exact implementation later on without the consuming method ever knowing  I thought I d never need to change from a List lt T  but had to later change to use a custom list library for the extra functionality it provided  Because I d only returned an IList lt T  none of the people that used the library had to change their code   Of course that only need apply to methods that are externally visible  i e  public methods   I personally use interfaces even in internal code  but as you are able to change all the code yourself if you make breaking changes it s not strictly necessary

[c#] When to use IList and when to use List

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