List C# capacity is always 0 even though using a constructor with capacity ? -

given simple synchronization code :

using system; using system.collections; using system.collections.generic; using system.linq; using system.text; using system.threading; using system.threading.tasks;  namespace mutex {     class calcarraythreads     {         private static system.threading.mutex mutex = new system.threading.mutex();         private const int threadsnumber = 10;         private static list<int> list = new list<int>(10);         public static void sumpartialarray(object index)         {             mutex.waitone();             int indexboxed = (int) index;             int sum = 0;             (int = indexboxed; < indexboxed + 9; i++)             {                 sum += i;             }              console.writeline(string.format("thread : {0} calculated value : {1}", thread.currentthread.name, sum));              // list.add(sum);             list[indexboxed] = sum;             mutex.releasemutex();                     // console.writeline(list.count());         }         static void main(string[] args)         {             (int = 0; < threadsnumber; i++)             {                 thread mythread = new thread(new parameterizedthreadstart(sumpartialarray));                 mythread.name = string.format("thread{0}", + 1);                 mythread.start(i);             }             console.read();         }     } } 

when use line :

list[indexboxed] = sum; 

i :

unhandled exception: system.argumentoutofrangeexception: index out of range. must non-negative , less size of collection.

even though capacity of list 10 .

why ?

if did following array, it'd work expected, setting second element:

int[] list = new int[10];  list[2] = 5;  // second element 5, no exception thrown 

looking @ list<t> constructor, when pass in capacity, it's doing similar using internal array:

this._items = new t[capacity]; 

so seems should work then, when try access element less capacity, here's how indexer implemented:

public t this[int index] {         {         if (index >= this._size)             throw new argumentoutofrangeexception("...");         return this._items[index];     }     set     {         if (index >= this._size)             throw new argumentoutofrangeexception("...");         this._items[index] = value;     } } 

it's checking _size variable first, , throwing exception if index you're requesting larger it. if weren't check, it'd work expect.

_size initialized 0 unless pass non-empty collection constructor, , changes value when use add, removeat, clear, etc methods. internally, it's still using array store elements, , if _size greater capacity (say, after trying add 1 more element), allocates new array , copies elements older (smaller) array it.

i see 2 solutions consider using:

either use array, this:

private static int[] list = new int[10]; 

or supply collection of default value (here, bunch of zeroes) via constructor:

private static list<int> list = new list<int>(enumerable.repeat(0, 10));