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The answers to questions like this: List<T> or IList<T> always seem to agree that returning an interface is better than returning a concrete implementation of a collection. But I'm struggling with this. Instantiating an interface is impossible, so if your method is returning an interface, it's actually still returning a specific implementation. I was experimenting a bit with this by writing 2 small methods:
public static IList<int> ExposeArrayIList()
{
return new[] { 1, 2, 3 };
}
public static IList<int> ExposeListIList()
{
return new List<int> { 1, 2, 3 };
}
And use them in my test program:
static void Main(string[] args)
{
IList<int> arrayIList = ExposeArrayIList();
IList<int> listIList = ExposeListIList();
//Will give a runtime error
arrayIList.Add(10);
//Runs perfectly
listIList.Add(10);
}
In both cases when I try to add a new value, my compiler gives me no errors, but obviously the method which exposes my array as an IList<T> gives a runtime error when I try to add something to it.
So people who don't know what's happening in my method, and have to add values to it, are forced to first copy my IList to a List to be able to add values without risking errors. Of course they can do a typecheck to see if they're dealing with a List or an Array, but if they don't do that, and they want to add items to the collection they have no other choice to copy the IList to a List, even if it already is a List. Should an array never be exposed as IList?
Another concern of mine is based upon the accepted answer of the linked question (emphasis mine):
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 may be ok.
Imagine someone actually used my IList<T> they got from my ExposeListIlist() method just like that to add/remove values. Everything works fine. But now like the answer suggests, because returning an interface is more flexible I return an array instead of a List (no problem on my side!), then they're in for a treat...
TLDR:
1) Exposing an interface causes unnecessary casts? Does that not matter?
2) Sometimes if users of the library don't use a cast, their code can break when you change your method, even though the method remains perfectly fine.
I am probably overthinking this, but I don't get the general consensus that returning an interface is to be preferred over returning an implementation.
Maybe this is not directly answering your question, but in .NET 4.5+, I prefer to follow these rules when designing public or protected APIs:
do return IEnumerable<T>, if only enumeration is available;
do return IReadOnlyCollection<T> if both enumeration and items count are available;
do return IReadOnlyList<T>, if enumeration, items count and indexed access are available;
do return ICollection<T> if enumeration, items count and modification are available;
do return IList<T>, if enumeration, items count, indexed access and modification are available.
Last two options assume, that method must not return array as IList<T> implementation.
No, because the consumer should know what exactly IList is:
IList is a descendant of the ICollection interface and is the base
interface of all non-generic lists. IList implementations fall into
three categories: read-only, fixed-size, and variable-size. A
read-only IList cannot be modified. A fixed-size IList does not allow
the addition or removal of elements, but it allows the modification of
existing elements. A variable-size IList allows the addition, removal,
and modification of elements.
You can check for IList.IsFixedSize and IList.IsReadOnly and do what you want with it.
I think IList is an example of a fat interface and it should have been split into multiple smaller interfaces and it also violates Liskov substitution principle when you return an array as an IList.
Read more if you want to make decision about returning interface
UPDATE
Digging more and I found that IList<T> does not implement IList and IsReadOnly is accessible through base interface ICollection<T> but there is no IsFixedSize for IList<T>. Read more about why generic IList<> does not inherit non-generic IList?
As with all "interface versus implementation" question, you'll have to realise what exposing a public member means: it defines the public API of this class.
If you expose a List<T> as a member (field, property, method, ...), you tell the consumer of that member: the type obtained by accessing this method is a List<T>, or something derived of that.
Now if you expose an interface, you hide the "implementation detail" of your class using a concrete type. Of course you can't instantiate IList<T>, but you can use an Collection<T>, List<T>, derivations thereof or your own type implementing IList<T>.
The actual question is "Why does Array implement IList<T>", or "Why has the IList<T> interface so many members".
It also depends on what you want the consumers of that member to do. If you actually return an internal member through your Expose... member, you'll want to return a new List<T>(internalMember) anyway, as otherwise the consumer can try and cast them to IList<T> and modify your internal member through that.
If you just expect consumers to iterate the results, expose IEnumerable<T> or IReadOnlyCollection<T> instead.
Be careful with blanket quotes that are taken out of context.
Returning an interface is better than returning a concrete implementation
This quote only makes sense if it's used in the context of the SOLID principles. There are 5 principles but for the purposes of this discussion we'll just talk about the last 3.
Dependency inversion principle
one should “Depend upon Abstractions. Do not depend upon concretions.”
In my opinion, this principle is the most difficult to understand. But if you look at the quote carefully it looks a lot like your original quote.
Depend on interfaces (abstractions). Do no depend on concrete implementations (concretions).
This is still a little confusing but if we start applying the other principles together it starts to make a lot more sense.
Liskov substitution principle
“objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.”
As you pointed out, returning an Array is clearly different behavior to returning a List<T> even though they both implement IList<T>. This is most certainly a violation of LSP.
The important thing to realize is that interfaces are about the consumer. If you're returning an interface, you've created a contract that any methods or properties on that interface can be used without changing the behavior of the program.
Interface segregation principle
“many client-specific interfaces are better than one general-purpose interface.”
If you're returning an interface, you should return the most client specific interface your implementation supports. In other words, if you're not expecting the client to call the Add method you shouldn't return an interface with an Add method on it.
Unfortunately, the interfaces in the .NET framework (particularly the early versions) are not always ideal client specific interfaces. Although as #Dennis pointed out in his answer, there are a lot more choices in .NET 4.5+.
Returning an interface is not necessarily better than returning a concrete implementation of a collection. You should always have a good reason to use an interface instead of a concrete type. In your example it seems pointless to do so.
Valid reasons to use an interface could be:
You do not know what the implementation of the methods returning the interface will look like and there may be many, developed over time. It may be other people writing them, from other companies. So you just want to agree on the bare necessities and leave it up to them how to implement the functionality.
You want to expose some common functionality independent from your class hierarchy in a type-safe way. Objects of different base types that should offer the same methods would implement your interface.
One could argue that 1 and 2 are basically the same reason. They are two different scenarios that ultimately lead to the same need.
"It's a contract". If the contract is with yourself and your application is closed in both functionality and time, there is often no point in using an interface.
I have spent quite a few hours pondering the subject of exposing list members. In a similar question to mine, Jon Skeet gave an excellent answer. Please feel free to have a look.
ReadOnlyCollection or IEnumerable for exposing member collections?
I am usually quite paranoid to exposing lists, especially if you are developing an API.
I have always used IEnumerable for exposing lists, as it is quite safe, and it gives that much flexibility. Let me use an example here:
public class Activity
{
private readonly IList<WorkItem> workItems = new List<WorkItem>();
public string Name { get; set; }
public IEnumerable<WorkItem> WorkItems
{
get
{
return this.workItems;
}
}
public void AddWorkItem(WorkItem workItem)
{
this.workItems.Add(workItem);
}
}
Anyone who codes against an IEnumerable is quite safe here. If I later decide to use an ordered list or something, none of their code breaks and it is still nice. The downside of this is IEnumerable can be cast back to a list outside of this class.
For this reason, a lot of developers use ReadOnlyCollection for exposing a member. This is quite safe since it can never be cast back to a list. For me I prefer IEnumerable since it provides more flexibility, should I ever want to implement something different than a list.
I have come up with a new idea I like better. Using IReadOnlyCollection:
public class Activity
{
private readonly IList<WorkItem> workItems = new List<WorkItem>();
public string Name { get; set; }
public IReadOnlyCollection<WorkItem> WorkItems
{
get
{
return new ReadOnlyCollection<WorkItem>(this.workItems);
}
}
public void AddWorkItem(WorkItem workItem)
{
this.workItems.Add(workItem);
}
}
I feel this retains some of the flexibility of IEnumerable and is encapsulated quite nicely.
I posted this question to get some input on my idea. Do you prefer this solution to IEnumerable? Do you think it is better to use a concrete return value of ReadOnlyCollection? This is quite a debate and I want to try and see what are the advantages/disadvantages that we all can come up with.
EDIT
First of all thank you all for contributing so much to the discussion here. I have certainly learned a ton from each and every one and would like to thank you sincerely.
I am adding some extra scenarios and info.
There are some common pitfalls with IReadOnlyCollection and IEnumerable.
Consider the example below:
public IReadOnlyCollection<WorkItem> WorkItems
{
get
{
return this.workItems;
}
}
The above example can be casted back to a list and mutated, even though the interface is readonly. The interface, despite it's namesake does not guarantee immutability. It is up to you to provide an immutable solution, therefore you should return a new ReadOnlyCollection. By creating a new list (a copy essentially), the state of your object is safe and sound.
Richiban says it best in his comment: a interface only guarantees what something can do, not what it cannot do.
See below for an example:
public IEnumerable<WorkItem> WorkItems
{
get
{
return new List<WorkItem>(this.workItems);
}
}
The above can be casted and mutated, but your object is still immutable.
Another outside the box statement would be collection classes. Consider the following:
public class Bar : IEnumerable<string>
{
private List<string> foo;
public Bar()
{
this.foo = new List<string> { "123", "456" };
}
public IEnumerator<string> GetEnumerator()
{
return this.foo.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return this.GetEnumerator();
}
}
The class above can have methods for mutating foo the way you want it to be, but your object can never be casted to a list of any sort and mutated.
Carsten Führmann makes a fantastic point about yield return statements in IEnumerables.
One important aspect seems to be missing from the answers so far:
When an IEnumerable<T> is returned to the caller, they must consider the possibility that the returned object is a "lazy stream", e.g. a collection built with "yield return". That is, the performance penalty for producing the elements of the IEnumerable<T> may have to be paid by the caller, for each use of the IEnumerable. (The productivity tool "Resharper" actually points this out as a code smell.)
By contrast, an IReadOnlyCollection<T> signals to the caller that there will be no lazy evaluation. (The Count property, as opposed to the Count extension method of IEnumerable<T> (which is inherited by IReadOnlyCollection<T> so it has the method as well), signals non-lazyness. And so does the fact that there seem to be no lazy implementations of IReadOnlyCollection.)
This is also valid for input parameters, as requesting an IReadOnlyCollection<T> instead of IEnumerable<T> signals that the method needs to iterate several times over the collection. Sure the method could create its own list from the IEnumerable<T> and iterate over that, but as the caller may already have a loaded collection at hand it would make sense to take advantage of it whenever possible. If the caller only has an IEnumerable<T> at hand, he only needs to add .ToArray() or .ToList() to the parameter.
What IReadOnlyCollection does not do is prevent the caller to cast to some other collection type. For such protection, one would have to use the class ReadOnlyCollection<T>.
In summary, the only thing IReadOnlyCollection<T> does relative to IEnumerable<T> is add a Count property and thus signal that no lazyness is involved.
Talking about class libraries, I think IReadOnly* is really useful, and I think you're doing it right :)
It's all about immutable collection... Before there were just immutables and to enlarge arrays was a huge task, so .net decided to include in the framework something different, mutable collection, that implement the ugly stuff for you, but IMHO they didn't give you a proper direction for immutable that are extremely useful, especially in a high concurrency scenario where sharing mutable stuff is always a PITA.
If you check other today languages, such as objective-c, you will see that in fact the rules are completely inverted! They quite always exchange immutable collection between different classes, in other words the interface expose just immutable, and internally they use mutable collection (yes, they have it of course), instead they expose proper methods if they want let the outsiders change the collection (if the class is a stateful class).
So this little experience that I've got with other languages pushes me to think that .net list are so powerful, but the immutable collection were there for some reason :)
In this case is not a matter of helping the caller of an interface, to avoid him to change all the code if you're changing internal implementation, like it is with IList vs List, but with IReadOnly* you're protecting yourself, your class, to being used in not a proper way, to avoid useless protection code, code that sometimes you couldn't also write (in the past in some piece of code I had to return a clone of the complete list to avoid this problem).
My take on concerns of casting and IReadOnly* contracts, and 'proper' usages of such.
If some code is being “clever” enough to perform an explicit cast and break the interface contract, then it is also “clever” enough to use reflection or otherwise do nefarious things such as access the underlying List of a ReadOnlyCollection wrapper object. I don’t program against such “clever” programmers.
The only thing that I guarantee is that after said IReadOnly*-interface objects are exposed, then my code will not violate that contract and will not modified the returned collection object.
This means that I write code that returns List-as-IReadOnly*, eg., and rarely opt for an actual read-only concrete type or wrapper. Using IEnumerable.ToList is sufficient to return an IReadOnly[List|Collection] - calling List.AsReadOnly adds little value against “clever” programmers who can still access the underlying list that the ReadOnlyCollection wraps.
In all cases, I guarantee that the concrete types of IReadOnly* return values are eager. If I ever write a method that returns an IEnumerable, it is specifically because the contract of the method is that which “supports streaming” fsvo.
As far as IReadOnlyList and IReadOnlyCollection, I use the former when there is 'an' implied stable ordering established that is meaningful to index, regardless of purposeful sorting. For example, arrays and Lists can be returned as an IReadOnlyList while a HashSet would better be returned as an IReadOnlyCollection. The caller can always assign the I[ReadOnly]List to an I[ReadOnly]Collection as desired: this choice is about the contract exposed and not what a programmer, “clever” or otherwise, will do.
It seems that you can just return an appropriate interface:
...
private readonly List<WorkItem> workItems = new List<WorkItem>();
// Usually, there's no need the property to be virtual
public virtual IReadOnlyList<WorkItem> WorkItems {
get {
return workItems;
}
}
...
Since workItems field is in fact List<T> so the natural idea IMHO is to expose the most wide interface which is IReadOnlyList<T> in the case
!! IEnumerable vs IReadOnlyList !!
IEnumerable has been with us from the beginning of time. For many years, it was a de facto standard way to represent a read-only collection. Since .NET 4.5, however, there is another way to do that: IReadOnlyList.
Both collection interfaces are useful.
<>
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Can anyone explain to me why I would want to use IList over List in C#?
Related question: Why is it considered bad to expose List<T>
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.
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.
Interface is a promise (or a contract).
As it is always with the promises - smaller the better.
Some people say "always use IList<T> instead of List<T>".
They want you to change your method signatures from void Foo(List<T> input) to void Foo(IList<T> input).
These people are wrong.
It's more nuanced than that. If you are returning an IList<T> as part of the public interface to your library, you leave yourself interesting options to perhaps make a custom list in the future. You may not ever need that option, but it's an argument. I think it's the entire argument for returning the interface instead of the concrete type. It's worth mentioning, but in this case it has a serious flaw.
As a minor counterargument, you may find every single caller needs a List<T> anyway, and the calling code is littered with .ToList()
But far more importantly, if you are accepting an IList as a parameter you'd better be careful, because IList<T> and List<T> do not behave the same way. Despite the similarity in name, and despite sharing an interface they do not expose the same contract.
Suppose you have this method:
public Foo(List<int> a)
{
a.Add(someNumber);
}
A helpful colleague "refactors" the method to accept IList<int>.
Your code is now broken, because int[] implements IList<int>, but is of fixed size. The contract for ICollection<T> (the base of IList<T>) requires the code that uses it to check the IsReadOnly flag before attempting to add or remove items from the collection. The contract for List<T> does not.
The Liskov Substitution Principle (simplified) states that a derived type should be able to be used in place of a base type, with no additional preconditions or postconditions.
This feels like it breaks the Liskov substitution principle.
int[] array = new[] {1, 2, 3};
IList<int> ilist = array;
ilist.Add(4); // throws System.NotSupportedException
ilist.Insert(0, 0); // throws System.NotSupportedException
ilist.Remove(3); // throws System.NotSupportedException
ilist.RemoveAt(0); // throws System.NotSupportedException
But it doesn't. The answer to this is that the example used IList<T>/ICollection<T> wrong. If you use an ICollection<T> you need to check the IsReadOnly flag.
if (!ilist.IsReadOnly)
{
ilist.Add(4);
ilist.Insert(0, 0);
ilist.Remove(3);
ilist.RemoveAt(0);
}
else
{
// what were you planning to do if you were given a read only list anyway?
}
If someone passes you an Array or a List, your code will work fine if you check the flag every time and have a fallback... But really; who does that? Don't you know in advance if your method needs a list that can take additional members; don't you specify that in the method signature? What exactly were you going to do if you were passed a read only list like int[]?
You can substitute a List<T> into code that uses IList<T>/ICollection<T> correctly. You cannot guarantee that you can substitute an IList<T>/ICollection<T> into code that uses List<T>.
There's an appeal to the Single Responsibility Principle / Interface Segregation Principle in a lot of the arguments to use abstractions instead of concrete types - depend on the narrowest possible interface. In most cases, if you are using a List<T> and you think you could use a narrower interface instead - why not IEnumerable<T>? This is often a better fit if you don't need to add items. If you need to add to the collection, use the concrete type, List<T>.
For me IList<T> (and ICollection<T>) is the worst part of the .NET framework. IsReadOnly violates the principle of least surprise. A class, such as Array, which never allows adding, inserting or removing items should not implement an interface with Add, Insert and Remove methods. (see also https://softwareengineering.stackexchange.com/questions/306105/implementing-an-interface-when-you-dont-need-one-of-the-properties)
Is IList<T> a good fit for your organisation? If a colleague asks you to change a method signature to use IList<T> instead of List<T>, ask them how they'd add an element to an IList<T>. If they don't know about IsReadOnly (and most people don't), then don't use IList<T>. Ever.
Note that the IsReadOnly flag comes from ICollection<T>, and indicates whether items can be added or removed from the collection; but just to really confuse things, it does not indicate whether they can be replaced, which in the case of Arrays (which return IsReadOnlys == true) can be.
For more on IsReadOnly, see msdn definition of ICollection<T>.IsReadOnly
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.
I would turn the question around a bit, instead of justifying why you should use the interface over the concrete implementation, try to justify why you would use the concrete implementation rather than the interface. If you can't justify it, use the interface.
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> { ... }
public void Foo(IList<Bar> list)
{
// Do Something with the list here.
}
In this case you could pass in any class which implements the IList<Bar> interface. If you used List<Bar> instead, only a List<Bar> instance could be passed in.
The IList<Bar> way is more loosely coupled than the List<Bar> way.
A principle of TDD and OOP generally is programming to an interface not an implementation.
In this specific case since you're essentially talking about a language construct, not a custom one it generally won't matter, but say for example that you found List didn't support something you needed. If you had used IList in the rest of the app you could extend List with your own custom class and still be able to pass that around without refactoring.
The cost to do this is minimal, why not save yourself the headache later? It's what the interface principle is all about.
The most important case for using interfaces over implementations is in the parameters to your API. If your API takes a List parameter, then anyone who uses it has to use List. If the parameter type is IList, then the caller has much more freedom, and can use classes you never heard about, which may not even have existed when your code was written.
Supprising that none of these List vs IList questions (or answers) mentions the signature difference. (Which is why I searched for this question on SO!)
So here's the methods contained by List that are not found in IList, at least as of .NET 4.5 (circa 2015)
AddRange
AsReadOnly
BinarySearch
Capacity
ConvertAll
Exists
Find
FindAll
FindIndex
FindLast
FindLastIndex
ForEach
GetRange
InsertRange
LastIndexOf
RemoveAll
RemoveRange
Reverse
Sort
ToArray
TrimExcess
TrueForAll
What if .NET 5.0 replaces System.Collections.Generic.List<T> to System.Collection.Generics.LinearList<T>. .NET always owns the name List<T> but they guarantee that IList<T> is a contract. So IMHO we (atleast I) are not supposed to use someone's name (though it is .NET in this case) and get into trouble later.
In case of using IList<T>, the caller is always guareented things to work, and the implementer is free to change the underlying collection to any alternative concrete implementation of IList
All concepts are basically stated in most of the answers above regarding why use interface over concrete implementations.
IList<T> defines those methods (not including extension methods)
IList<T> MSDN link
Add
Clear
Contains
CopyTo
GetEnumerator
IndexOf
Insert
Remove
RemoveAt
List<T> implements those nine methods (not including extension methods), on top of that it has about 41 public methods, which weighs in your consideration of which one to use in your application.
List<T> MSDN link
You would because defining an IList or an ICollection would open up for other implementations of your interfaces.
You might want to have an IOrderRepository that defines a collection of orders in either a IList or ICollection. You could then have different kinds of implementations to provide a list of orders as long as they conform to "rules" defined by your IList or ICollection.
IList<> is almost always preferable as per the other poster's advice, however note there is a bug in .NET 3.5 sp 1 when running an IList<> through more than one cycle of serialization / deserialization with the WCF DataContractSerializer.
There is now a SP to fix this bug : KB 971030
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.
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>
Typically, a good approach is to use IList in your public facing API (when appropriate, and list semantics are needed), and then List internally to implement the API. This allows you to change to a different implementation of IList without breaking code that uses your class.
The class name List may be changed in next .net framework but the interface is never going to change as interface is contract.
Note that, if your API is only going to be used in foreach loops, etc, then you might want to consider just exposing IEnumerable instead.
Often you have to implement a collection because it is not present among those of the .NET Framework. In the examples that I find online, often the new collection is built based on another collection (for example, List<T>): in this way it is possible to avoid the management of the resizing of the collection.
public class CustomCollection<T>
{
private List<T> _baseArray;
...
public CustomCollection(...)
{
this._baseArray = new List<T>(...);
}
}
What are the disadvantages of following this approach? Only lower performance because of the method calls to the base collection? Or the compiler performs some optimization?
Moreover, in some cases the field relating to the base collection (for example the above _baseArray) is declared as readonly. Why?
The main disadvantage is the fact that if you want to play nice you'll have to implement a lot of interfaces by hand (ICollection, IEnumerable, possibly IList... both generic and non-generic), and that's quite a bit of code. Not complex code, since you're just relaying the calls, but still code. The extra call to the inner list shouldn't make too big of a difference in most cases.
It's to enforce the fact that once the inner list is set, it cannot be changed into another list.
Usually it's best to inherit from one of the many built-in collection classes to make your own collection, instead of doing it the hard way. Collection<T> is a good starting point, and nobody is stopping you from inheriting List<T> itself.
For #2: if the private member is only assigned to in the constructor or when declared, it can be readonly. This is usually true if you only have one underlying collection and don't ever need to recreate it.
I'd say a pretty large disadvantage of this approach is that you can't use LINQ on your custom collection unless you implement IEnumerable. A better approach might be to subclass and force new implementation on methods as necessary, ex:
public class FooList<T> : List<T>
{
public new void Add(T item)
{
// any FooList-specific logic regarding adding items
base.Add(item);
}
}
As for the readonly keyword, it means that you can only set the variable in the constructor.
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Can anyone explain to me why I would want to use IList over List in C#?
Related question: Why is it considered bad to expose List<T>
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.
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.
Interface is a promise (or a contract).
As it is always with the promises - smaller the better.
Some people say "always use IList<T> instead of List<T>".
They want you to change your method signatures from void Foo(List<T> input) to void Foo(IList<T> input).
These people are wrong.
It's more nuanced than that. If you are returning an IList<T> as part of the public interface to your library, you leave yourself interesting options to perhaps make a custom list in the future. You may not ever need that option, but it's an argument. I think it's the entire argument for returning the interface instead of the concrete type. It's worth mentioning, but in this case it has a serious flaw.
As a minor counterargument, you may find every single caller needs a List<T> anyway, and the calling code is littered with .ToList()
But far more importantly, if you are accepting an IList as a parameter you'd better be careful, because IList<T> and List<T> do not behave the same way. Despite the similarity in name, and despite sharing an interface they do not expose the same contract.
Suppose you have this method:
public Foo(List<int> a)
{
a.Add(someNumber);
}
A helpful colleague "refactors" the method to accept IList<int>.
Your code is now broken, because int[] implements IList<int>, but is of fixed size. The contract for ICollection<T> (the base of IList<T>) requires the code that uses it to check the IsReadOnly flag before attempting to add or remove items from the collection. The contract for List<T> does not.
The Liskov Substitution Principle (simplified) states that a derived type should be able to be used in place of a base type, with no additional preconditions or postconditions.
This feels like it breaks the Liskov substitution principle.
int[] array = new[] {1, 2, 3};
IList<int> ilist = array;
ilist.Add(4); // throws System.NotSupportedException
ilist.Insert(0, 0); // throws System.NotSupportedException
ilist.Remove(3); // throws System.NotSupportedException
ilist.RemoveAt(0); // throws System.NotSupportedException
But it doesn't. The answer to this is that the example used IList<T>/ICollection<T> wrong. If you use an ICollection<T> you need to check the IsReadOnly flag.
if (!ilist.IsReadOnly)
{
ilist.Add(4);
ilist.Insert(0, 0);
ilist.Remove(3);
ilist.RemoveAt(0);
}
else
{
// what were you planning to do if you were given a read only list anyway?
}
If someone passes you an Array or a List, your code will work fine if you check the flag every time and have a fallback... But really; who does that? Don't you know in advance if your method needs a list that can take additional members; don't you specify that in the method signature? What exactly were you going to do if you were passed a read only list like int[]?
You can substitute a List<T> into code that uses IList<T>/ICollection<T> correctly. You cannot guarantee that you can substitute an IList<T>/ICollection<T> into code that uses List<T>.
There's an appeal to the Single Responsibility Principle / Interface Segregation Principle in a lot of the arguments to use abstractions instead of concrete types - depend on the narrowest possible interface. In most cases, if you are using a List<T> and you think you could use a narrower interface instead - why not IEnumerable<T>? This is often a better fit if you don't need to add items. If you need to add to the collection, use the concrete type, List<T>.
For me IList<T> (and ICollection<T>) is the worst part of the .NET framework. IsReadOnly violates the principle of least surprise. A class, such as Array, which never allows adding, inserting or removing items should not implement an interface with Add, Insert and Remove methods. (see also https://softwareengineering.stackexchange.com/questions/306105/implementing-an-interface-when-you-dont-need-one-of-the-properties)
Is IList<T> a good fit for your organisation? If a colleague asks you to change a method signature to use IList<T> instead of List<T>, ask them how they'd add an element to an IList<T>. If they don't know about IsReadOnly (and most people don't), then don't use IList<T>. Ever.
Note that the IsReadOnly flag comes from ICollection<T>, and indicates whether items can be added or removed from the collection; but just to really confuse things, it does not indicate whether they can be replaced, which in the case of Arrays (which return IsReadOnlys == true) can be.
For more on IsReadOnly, see msdn definition of ICollection<T>.IsReadOnly
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.
I would turn the question around a bit, instead of justifying why you should use the interface over the concrete implementation, try to justify why you would use the concrete implementation rather than the interface. If you can't justify it, use the interface.
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> { ... }
public void Foo(IList<Bar> list)
{
// Do Something with the list here.
}
In this case you could pass in any class which implements the IList<Bar> interface. If you used List<Bar> instead, only a List<Bar> instance could be passed in.
The IList<Bar> way is more loosely coupled than the List<Bar> way.
A principle of TDD and OOP generally is programming to an interface not an implementation.
In this specific case since you're essentially talking about a language construct, not a custom one it generally won't matter, but say for example that you found List didn't support something you needed. If you had used IList in the rest of the app you could extend List with your own custom class and still be able to pass that around without refactoring.
The cost to do this is minimal, why not save yourself the headache later? It's what the interface principle is all about.
The most important case for using interfaces over implementations is in the parameters to your API. If your API takes a List parameter, then anyone who uses it has to use List. If the parameter type is IList, then the caller has much more freedom, and can use classes you never heard about, which may not even have existed when your code was written.
Supprising that none of these List vs IList questions (or answers) mentions the signature difference. (Which is why I searched for this question on SO!)
So here's the methods contained by List that are not found in IList, at least as of .NET 4.5 (circa 2015)
AddRange
AsReadOnly
BinarySearch
Capacity
ConvertAll
Exists
Find
FindAll
FindIndex
FindLast
FindLastIndex
ForEach
GetRange
InsertRange
LastIndexOf
RemoveAll
RemoveRange
Reverse
Sort
ToArray
TrimExcess
TrueForAll
What if .NET 5.0 replaces System.Collections.Generic.List<T> to System.Collection.Generics.LinearList<T>. .NET always owns the name List<T> but they guarantee that IList<T> is a contract. So IMHO we (atleast I) are not supposed to use someone's name (though it is .NET in this case) and get into trouble later.
In case of using IList<T>, the caller is always guareented things to work, and the implementer is free to change the underlying collection to any alternative concrete implementation of IList
All concepts are basically stated in most of the answers above regarding why use interface over concrete implementations.
IList<T> defines those methods (not including extension methods)
IList<T> MSDN link
Add
Clear
Contains
CopyTo
GetEnumerator
IndexOf
Insert
Remove
RemoveAt
List<T> implements those nine methods (not including extension methods), on top of that it has about 41 public methods, which weighs in your consideration of which one to use in your application.
List<T> MSDN link
You would because defining an IList or an ICollection would open up for other implementations of your interfaces.
You might want to have an IOrderRepository that defines a collection of orders in either a IList or ICollection. You could then have different kinds of implementations to provide a list of orders as long as they conform to "rules" defined by your IList or ICollection.
IList<> is almost always preferable as per the other poster's advice, however note there is a bug in .NET 3.5 sp 1 when running an IList<> through more than one cycle of serialization / deserialization with the WCF DataContractSerializer.
There is now a SP to fix this bug : KB 971030
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.
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>
Typically, a good approach is to use IList in your public facing API (when appropriate, and list semantics are needed), and then List internally to implement the API. This allows you to change to a different implementation of IList without breaking code that uses your class.
The class name List may be changed in next .net framework but the interface is never going to change as interface is contract.
Note that, if your API is only going to be used in foreach loops, etc, then you might want to consider just exposing IEnumerable instead.