How can I overcome the overhead of creating a List<T> from an IEnumerable<T>?

Question

I am using some of the LINQ select stuff to create some collections, which return IEnumerable<T>.

In my case I need a List<T>, so I am passing the result to List<T>'s constructor to create one.

I am wondering about the overhead of doing this. The items in my collections are usually in the millions, so I need to consider this.

I assume, if the IEnumerable<T> contains ValueTypes, it's the worst performance.

Am I right? What about Ref Types? Either way there is also the cost of calling, List<T>.Add a million times, right?

Any way to solve this? Like can I "overload" methods like LINQ Select using extension methods)?

Answer 1

It would be best to avoid the need for a list. If you can keep your caller using IEnumerable<T>, you will save yourself some headaches.

LINQ's ToList() will take your enumerable, and just construct a new List<T> directly from it, using the List<T>(IEnumerable<T>) constructor. This will be the same as making the list yourself, performance wise (although LINQ does a null check, as well).

If you're adding the elements yourself, use the AddRange method instead of the Add. ToList() is very similar to AddRange (since it's using the constructor which takes IEnumerable<T>), which typically will be your best bet, performance wise, in this case.

Answer 2

Don't pass an IEnumerable to the List constructor. IEnumerable has a ToList() method, which can't possibly do worse than that, and has nicer syntax (IMHO).

That said, that only changes the answer to your question to "it depends" - in particular, it depends on what the IEnumerable actually is behind the scenes. If it happens to be a List already, then ToList will effectively be free, of course will go much faster than if it were another type. It's still not super-fast.

The best way to solve this, of course, is to try to figure out how to do your processing on an IEnumerable rather than a List. That may not be possible.

---

Edit: Some people in the comments are debating whether or not ToList() will actually be any faster when called on a List than if not, and whether ToList() will be any faster than the list constructor. At this point, speculating is getting pointless, so here's some code:

using System;
using System.Linq;
using System.Collections.Generic;

public static class ToListTest
{
    public static int Main(string[] args)
    {
        List<int> intlist = new List<int>();
        for (int i = 0; i < 1000000; i++)
            intlist.Add(i);

        IEnumerable<int> intenum = intlist;

        for (int i = 0; i < 1000; i++)
        {
            List<int> foo = intenum.ToList();
        }

        return 0;
    }
}

Running this code with an IEnumerable that's really a List goes about 6-10 times faster than if I replace it with a LinkedList or Stack (on my pokey 2.4 GHz P4, using Mono 1.2.6). Conceivably this could be due to some unfortunate interaction between ToList() and the particular implementations of LinkedList or Stack's enumerations, but at least the point remains: speed will depend on the underlying type of the IEnumerable. That said, even with a List as the source, it still takes 6 seconds for me to make 1000 ToList() calls, so it's far from free.

The next question is whether ToList() is any more intelligent than the List constructor. The answer to that turns out to be no: the List constructor is just as fast as ToList(). In hindsight, Jon Skeet's reasoning makes sense - I was just forgetting that ToList() was an extension method. I still (much) prefer ToList() syntactically, but there's no performance reason to use it.

So the short version is that the best answer is still "don't convert to a List if you can avoid it". Barring that, actual performance will depend drastically on what the IEnumerable actually is, but at best it'll be sluggish, as opposed to glacial. I've amended my original answer to reflect this.

Answer 3

No, there's no particular penalty for the element type being value types, assuming you're using IEnumerable<T> instead of IEnumerable. You won't get any boxing going on.

If you actually know the size of the result beforehand (which the result of Select probably won't) you might want to consider creating the list with that size of buffer, then using AddRange to add the values. Otherwise the list will have to resize its buffer every time it fills it.

For instance, instead of doing:

Foo[] foo = new Foo[100];
IEnumerable<string> query = foo.Select(foo => foo.Name);
List<string> queryList = new List<string>(query);

you might do:

Foo[] foo = new Foo[100];
IEnumerable<string> query = foo.Select(x => x.Name);
List<string> queryList = new List<string>(foo.Length);
queryList.AddRange(query);

You know that calling Select will produce a sequence of the same length as the original query source, but nothing in the execution environment has that information as far as I'm aware.

Answer 4

Generally speaking, a method returning IEnumerable doesn't have to evaluate any of the items before the item is actually needed. So, theoretically, when you return an IEnumerable none of you items need to exist at that time.

So creating a list means that you will really need to evaluate items, get them and place them somewhere in memory (at least their references). There is nothing that can be done about this - if you really need to have a list.

Answer 5

A number of other responders have already provided ideas for how to improve the performance of copying an IEnumerable<T> into a List<T> - I don't think that much can be added on that front.

However, based on what you have described you need to do with the results, and the fact that you get rid of the list when you're done (which I presume means that the intermediate results are not interesting) - you may want to consider whether you really need to materialize a List<T>.

Rather than creating a List<T> and operating on the contents of that list - consider writing a lazy extension method for IEnumerable<T> that performs the same processing logic. I've done this myself in a number of cases, and writing such logic in C# is not so bad when using the [yield return][1] syntax supported by the compiler.

This approach works well if all you're trying to do is visit each item in the results and collection some information from it. Often, what you need to do is just visit each element in the collection on demand, do some processing with it, and then move on. This approach is generally more scalable and performant that creating a copy of the collection just to iterate over it.

Now, this advice may not work for you for other reasons, but it's worth considering as an alternative to finding the most efficient way to materialize a very large list.

Answer 6

From reading the various comments and the question I get the following requirements

for a collection of data you need to run through that collection, filter out some objects and then perform some transformation on the remaining objects. If thats the case you can do something like this:

var result = from item in collection
             where item.Id > 10 //or some more sensible condition
             select Operation(item);

and if you need to the perform more filtering and transformation you can nest your LINQ queries like

var result = from filteredItem in (from item in collection
                                  where item.Id > 10 //or some more sensible condition
                                  select Operation(item))
                 where filteredItem.SomePropertyAvailableAfterFirstTransformation == "new"
                 select SecondTransfomation(filteredItem);