Dictionary is not like a array

As the title, everybody knows that, a dictionary is not a array, and not a arraylist, neither two array or two arraylist.

Then there is a problem:

I use this to iterate the array(arr_tmp):

for(int i = 0; i < arr.length; i++){
    sum += arr[i];
}

I use this to iterate the arraylist(arrlst_tmp):

for(int i = 0; i < arrlst_tmp.count; i++){
    sum += arrlst_tmp[i];
}

and I use the same thing to iterate the dictionary(dic_tmp):

for(int i = 0; i < dic_tmp.count; i++){
    sum += dic_tmp.ElementAt(i).Value;
}

But one day I read some thing from another one, it tells about the run time and the efficiency of the iterate takes, then he recommended this:

foreach (KeyValuePair<int, int> pair in dic_tmp)
{
    sum += pair.Value;
}

TKS that you tell me I'm foolish, and it will take time and will take long time for me to be a programmer.