I want to know what is the initial size of ArrayList in C#?
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5This may be stating the obvious, but unless you're on .NET 1.x you should really use List<T> instead.Brian Rasmussen– Brian Rasmussen2009-10-14 08:37:53 +00:00Commented Oct 14, 2009 at 8:37
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1Your app should'n depend on that. If you know you have an upper limit of elements and performance is an issue, initialize the container with that limit, otherwise rely on its auto sizing capabilities.Frank Bollack– Frank Bollack2009-10-14 08:58:59 +00:00Commented Oct 14, 2009 at 8:58
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6 Answers
0. See below.
16. (I have to add characters to this answer, since 18 characters are minimum)
Edit, Oops - the initial capacity is 16. Initial size is of course 0, because it's empty. Have to learn how to read. Or you have to learn how to form your questions. ;)
Edit again; Initial capacity of an ArrayList in .NET 1.0 is 16. In 2.0 it was 4, and now - with .NET 3.5 - the initial capacty has been lowered to 0. I don't have an explanation of why, thou.
When adding a first element to the list, the capacity will be set to 4. There after, every time when arraylist.Count eq arraylist.Capacity, the capacity will double.
4 Comments
Joey
That explains my differing observations here. Weird.
Dylan Beattie
Do you have a source / link for the .NET 2.0 and 3.5 capacity sizes? Be curious to know more about this... I thought it was still 16.
Björn
No, can't find it in MSDN or any MS explanation for this.
RCIX
I'm guessing that way it doesn't have to allocate any memory for a list you may not use?
The ArrayList starts with Size = 0 (because it's empty) and Capacity = 16.
The capacity is doubled as necessary, and capacity-doubling is a O(n) operation where n is the new capacity. So if you're inserting 5,000 elements into your list, the framework's going to be doubling the ArrayList capacity nine times - and each doubling operation is twice as expensive as the previous one.
In other words - if you know you're going to be putting 5,000 elements in a list, you're much better off explicitly initializing it to hold 5,000 elements.
You can explicitly set the Capacity of an existing arraylist if you know you're about to insert a large number of elements. You can also decrease Capacity explicitly, but if you set Capacity < Count, you'll get an ArgumentOutOfRange exception.
1 Comment
Eric Lippert
Though each doubling operation is twice as expensive as the previous one, it happens half as often. That makes the amortized cost of each insertion constant, and therefore inserting n elements is O(n) overall. Yes, you are better off allocating the thing to the right size in the first place if its large, but even if you do not, the total additional cost is pretty small. Another cost which you are neglecting is that on average, the list wastes a quarter of its space capacity.
NOTE: The following seemingly only holds true for .NET 3.5; in previous versions of the framework the values were different.
According to my tests here both the initial size and capacity are zero:
PS> $a = new-object system.collections.arrayList
PS> $a.Capacity
0
PS> $a.count
0
Also, looking at the source code in Reflector, the same holds true:
public virtual int Capacity
{
get
{
return this._items.Length;
}
...
}
And _items gets set to an empty object[] in the ctor.
1 Comment
Björn
You're right, as from .NET 3.5 the initial capacity was lowered to 0. In .NET 1.0 it was 16. I have updated my answer and will upvote yours. ;)
The ArrayList is empty when you have created it, so the size is zero.
Unless you are stuck with framework 1, you should not use the ArrayList class at all. Use the strongly typed generic List<T> class instead.
Comments
ArrayList list = new ArrayList();
size = 0 before adding items to the arrayList, means no items are there.
Comments
Try yourself.
int capacity = (new ArrayList()).Capacity;
should give you the initial capacity.
