Array versus List: When to use which?

Question

MyClass[] array;
List<MyClass> list;

What are the scenarios when one is preferable over the other? And why?

Answer 1

Most of the times, using a List would suffice. A List uses an internal array to handle its data, and automatically resizes the array when adding more elements to the List than its current capacity, which makes it more easy to use than an array, where you need to know the capacity beforehand.

See http://msdn.microsoft.com/en-us/library/ms379570(v=vs.80).aspx#datastructures20_1_topic5 for more information about Lists in C# or just decompile System.Collections.Generic.List<T>.

If you need multidimensional data (for example using a matrix or in graphics programming), you would probably go with an array instead.

As always, if memory or performance is an issue, measure it! Otherwise you could be making false assumptions about the code.

Answer 2

Populating a list is easier than an array. For arrays, you need to know the exact length of data, but for lists, data size can be any. And, you can convert a list into an array.

List<URLDTO> urls = new List<URLDTO>();

urls.Add(new URLDTO() {
    key = "wiki",
    url = "https://...",
});

urls.Add(new URLDTO()
{
    key = "url",
    url = "http://...",
});

urls.Add(new URLDTO()
{
    key = "dir",
    url = "https://...",
});

// convert a list into an array: URLDTO[]
return urls.ToArray();

Answer 3

Lists in .NET are wrappers over arrays, and use an array internally. The time complexity of operations on lists is the same as would be with arrays, however there is a little more overhead with all the added functionality / ease of use of lists (such as automatic resizing and the methods that come with the list class). Pretty much, I would recommend using lists in all cases unless there is a compelling reason not to do so, such as if you need to write extremely optimized code, or are working with other code that is built around arrays.

Answer 4

They may be unpopular, but I am a fan of Arrays in game projects. - Iteration speed can be important in some cases, foreach on an Array has significantly less overhead if you are not doing much per element - Adding and removing is not that hard with helper functions - Its slower, but in cases where you only build it once it may not matter - In most cases, less extra memory is wasted (only really significant with Arrays of structs) - Slightly less garbage and pointers and pointer chasing

That being said, I use List far more often than Arrays in practice, but they each have their place.

It would be nice if List where a built in type so that they could optimize out the wrapper and enumeration overhead.

Answer 5

Unless you are really concerned with performance, and by that I mean, "Why are you using .Net instead of C++?" you should stick with List<>. It's easier to maintain and does all the dirty work of resizing an array behind the scenes for you. (If necessary, List<> is pretty smart about choosing array sizes so it doesn't need to usually.)

Answer 6

Really just answering to add a link which I'm surprised hasn't been mentioned yet: Eric's Lippert's blog entry on "Arrays considered somewhat harmful."

You can judge from the title that it's suggesting using collections wherever practical - but as Marc rightly points out, there are plenty of places where an array really is the only practical solution.