When to use IList and when to use List


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?

8/19/2008 11:09:54 PM

Accepted 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<T>, an ICollection<T>, or IEnumerable<T>. 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<T> 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<T> internally, return a copy as a List<T>.

8/20/2008 1:58:57 AM

Microsoft guidelines as checked by FxCop discourage use of List<T> in public APIs - prefer IList<T>.

Incidentally, I now almost always declare one-dimensional arrays as IList<T>, which means I can consistently use the IList<T>.Count property rather than Array.Length. For example:

public interface IMyApi
    IList<int> GetReadOnlyValues();

public class MyApiImplementation : IMyApi
    public IList<int> GetReadOnlyValues()
        List<int> myList = new List<int>();
        ... populate list
        return myList.AsReadOnly();
public class MyMockApiImplementationForUnitTests : IMyApi
    public IList<int> GetReadOnlyValues()
        IList<int> testValues = new int[] { 1, 2, 3 };
        return testValues;

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