List or IList [closed]

Answer 1

IList<T> is an interface so you can inherit another class and still implement IList<T> while inheriting List<T> prevents you to do so.

For example if there is a class A and your class B inherits it then you can't use List<T>

class A : B, IList<T> { ... }

Answer 2

The interface ensures that you at least get the methods you are expecting; being aware of the definition of the interface ie. all abstract methods that are there to be implemented by any class inheriting the interface. so if some one makes a huge class of his own with several methods besides the ones he inherited from the interface for some addition functionality, and those are of no use to you, its better to use a reference to a subclass (in this case the interface) and assign the concrete class object to it.

additional advantage is that your code is safe from any changes to concrete class as you are subscribing to only few of the methods of concrete class and those are the ones that are going to be there as long as the concrete class inherits from the interface you are using. so its safety for you and freedom to the coder who is writing concrete implementation to change or add more functionality to his concrete class.

Answer 3

If you are exposing your class through a library that others will use, you generally want to expose it via interfaces rather than concrete implementations. This will help if you decide to change the implementation of your class later to use a different concrete class. In that case the users of your library won't need to update their code since the interface doesn't change.

If you are just using it internally, you may not care so much, and using List<T> may be ok.

Answer 4

The less popular answer is programmers like to pretend their software is going to be re-used the world over, when infact the majority of projects will be maintained by a small amount of people and however nice interface-related soundbites are, you're deluding yourself.

Architecture Astronauts. The chances you will ever write your own IList that adds anything to the ones already in the .NET framework are so remote that it's theoretical jelly tots reserved for "best practices".

Obviously if you are being asked which you use in an interview, you say IList, smile, and both look pleased at yourselves for being so clever. Or for a public facing API, IList. Hopefully you get my point.

Answer 5

List<T> is a specific implementation of IList<T>, which is a container that can be addressed the same way as a linear array T[] using an integer index. When you specify IList<T> as the type of the method's argument, you only specify that you need certain capabilities of the container.

For example, the interface specification does not enforce a specific data structure to be used. The implementation of List<T> happens to the same performance for accessing, deleting and adding elements as a linear array. However, you could imagine an implementation that is backed by a linked list instead, for which adding elements to the end is cheaper (constant-time) but random-access much more expensive. (Note that the .NET LinkedList<T> does not implement IList<T>.)

This example also tells you that there may be situations when you need to specify the implementation, not the interface, in the argument list: In this example, whenever you require a particular access performance characteristic. This is usually guaranteed for a specific implementation of a container (List<T> documentation: "It implements the IList<T> generic interface using an array whose size is dynamically increased as required.").

Additionally, you might want to consider exposing the least functionality you need. For example. if you don't need to change the content of the list, you should probably consider using IEnumerable<T>, which IList<T> extends.

Answer 6

You can look at this argument from several angles including the one of a purely OO approach which says to program against an Interface not an implementation. With this thought, using IList follows the same principal as passing around and using Interfaces that you define from scratch. I also believe in the scalability and flexibility factors provided by an Interface in general. If a class implmenting IList<T> needs to be extended or changed, the consuming code does not have to change; it knows what the IList Interface contract adheres to. However using a concrete implementation and List<T> on a class that changes, could cause the calling code to need to be changed as well. This is because a class adhering to IList<T> guarantees a certain behavior that is not guaranteed by a concrete type using List<T>.

Also having the power to do something like modify the default implementation of List<T> on a class Implementing IList<T> for say the .Add, .Remove or any other IList method gives the developer a lot of flexibility and power, otherwise predefined by List<T>