ArrayList vs List<object>

Question

I saw this reply from Jon on Initialize generic object with unknown type:

If you want a single collection to contain multiple unrelated types of values, however, you will have to use List<object>

I'm not comparing ArrayList vs List<>, but ArrayList vs List<object>, as both will be exposing elements of type object. What would be the benefit of using either one in this case?

EDIT: It's no concern for type safety here, since both class is exposing object as its item. One still needs to cast from object to the desired type. I'm more interested in anything other than type safety.

EDIT: Thanks Marc Gravell and Sean for the answer. Sorry, I can only pick 1 as answer, so I'll up vote both.

Solution 1

You'll be able to use the LINQ extension methods directly with List<object>, but not with ArrayList, unless you inject a Cast<object>() / OfType<object> (thanks to IEnumerable<object> vs IEnumerable). That's worth quite a bit, even if you don't need type safety etc.

The speed will be about the same; structs will still be boxed, etc - so there isn't much else to tell them apart. Except that I tend to see ArrayList as "oops, somebody is writing legacy code again..." ;-p

Solution 2

One big benefit to using List<object> is that these days most code is written to use the generic classes/interfaces. I suspect that these days most people would write a method that takes a IList<object> instead of an IList. Since ArrayList doesn't implement IList<object> you wouldn't be able to use an array list in these scenarios.

I tend to think of the non-generic classes/interfaces as legacy code and avoid them whenever possible.

Solution 3

In this case, ArrayList vs. List<Object> then you won't notice any differences in speed. There might be some differences in the actual methods available on each of these, particular in .NET 3.5 and counting extension methods, but that has more to do with ArrayList being somewhat deprecated than anything else.

Solution 4

Yes, besides being typesafe, generic collections might be actually faster.

From the MSDN (http://msdn.microsoft.com/en-us/library/system.collections.generic.aspx)

The System.Collections.Generic namespace contains interfaces and classes that define generic collections, which allow users to create strongly typed collections that provide better type safety and performance than non-generic strongly typed collections.

Solution 5

Do some benchmarking and you will know what performs best. I guestimate that the difference is very small.

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Comments

[faulty]

Obviously I'm not referring to type safety here

[shen]

actually im not sure the question is about performance but that is a good point. my guess is arraylist perf would be worse due to additional boxing, unboxing.

[Brian]

List<object> doesn't give you much in the way of improvements to type safety and performance over using an ArrayList Most of these improvements are specific to List<T> where T is anything except object.

[Brian]

@Anonymous: What additional boxing? ArrayList and List<object> should have the same amount of boxing/unboxing.

[shen]

"a single collection to contain multiple unrelated types of values" - my understanding was that when adding different types to a List collection object there was boxing/unboxing occuring, whereas with the arraylist there was not. I guess I am wrong about this.