Is it better to use Enumerable.Empty<T>() as opposed to new List<T>() to initialize an IEnumerable<T>?

169
votes

Suppose you have a class Person : 

public class Person
{
   public string Name { get; set;}
   public IEnumerable<Role> Roles {get; set;}
}

I should obviously instantiate the Roles in the constructor. Now, I used to do it with a List like this : 

public Person()
{
   Roles = new List<Role>();
}

But I discovered this static method in the System.Linq namespace

IEnumerable<T> Enumerable.Empty<T>();

From MSDN:

The Empty(TResult)() method caches an empty sequence of type TResult. When the object it returns is enumerated, it yields no elements.

In some cases, this method is useful for passing an empty sequence to a user-defined method that takes an IEnumerable(T). It can also be used to generate a neutral element for methods such as Union. See the Example section for an example of this use of

So is it better to write the constructor like that? Do you use it? Why? or if not, Why not? 

public Person()
{
   Roles = Enumerable.Empty<Role>();
}
6
c#linqconstructor
This is a Data class. I intend to use this class as a Model class when implementing a repository pattern with Entity Framework 4.0 (playing around...). so I think it's fine to have a public setter here, isn't it? – Stéphane
serbech, how will you Add a role (inside Person) when other code can installed any kind of IEnumerable derived class for the list? What will you cast it to? – Henk Holterman
I see your point, I should probably have a IList internally, or even a List here? And that would just remove my original problem... – Stéphane
I use Enumerable.Empty<T> in unit testing to indicate unhappy path tests to help communicate the intent. As many have already pointed it out the bonus is no allocation on the GC which helps when your unit tests number in the hundreds or more. – jjhayter
Now one can also use Array.Empty<Role>() which is explicitly an array and hence an IList<T>. In Core it's in fact the same array, with only the type exposed differing. – Jon Hanna

6 Answers

208
votes

I think most postings missed the main point. Even if you use an empty array or empty list, those are objects and they are stored in memory. The Garbage Collector has to take care of them. If you are dealing with a high throughput application, it could be a noticeable impact.

Enumerable.Empty does not create an object per call thus putting less load on the GC.

If the code is in low-throughput location, then it boils down to aesthetic considerations though.

107
votes

I think Enumerable.Empty<T> is better because it is more explicit: your code clearly indicates your intentions. It might also be a bit more efficient, but that's only a secondary advantage.

31
votes

On the performance front, let's see how Enumerable.Empty<T> is implemented.

It returns EmptyEnumerable<T>.Instance, which is defined as:

internal class EmptyEnumerable<T>
{
    public static readonly T[] Instance = new T[0];
}

Static fields on generic types are allocated per generic type parameter. This means that the runtime can lazily create these empty arrays only for the types user code needs, and reuse the instances as many times as needed without adding any pressure on the garbage collector.

To wit:

Debug.Assert(ReferenceEquals(Enumerable.Empty<int>(), Enumerable.Empty<int>()));
13
votes

Assuming you actually want to populate the Roles property somehow, then encapsulate that by making it's setter private and initialising it to a new list in the constructor:

public class Person
{
    public string Name { get; set; }
    public IList<Role> Roles { get; private set; }

    public Person()
    {
        Roles = new List<Role>();
    }
}

If you really really want to have the public setter, leave Roles with a value of null and avoid the object allocation.

7
votes

The problem with your approach is that you can't add any items to the collection - I would have a private structure like list and then expose the items as an Enumerable:

public class Person
{
    private IList<Role> _roles;

    public Person()
    {
        this._roles = new List<Role>();
    }

    public string Name { get; set; }

    public void AddRole(Role role)
    {
        //implementation
    }

    public IEnumerable<Role> Roles
    {
        get { return this._roles.AsEnumerable(); }
    }
}

If you intend some other class to create the list of roles (which I wouldn't recommend) then I wouldn't initialise the enumerable at all in Person.

7
votes

The typical problem with exposing the private List as an IEnumerable is that the client of your class can mess with it by casting. This code would work:

  var p = new Person();
  List<Role> roles = p.Roles as List<Role>;
  roles.Add(Role.Admin);

You can avoid this by implementing an iterator:

public IEnumerable<Role> Roles {
  get {
    foreach (var role in mRoles)
      yield return role;
  }
}