Performance: List.Count vs checking a stored variable

Question

I wonder if this makes any difference:

 for (int i = 0; i < values.Count; i++)
        {
            //
        }

vs

int num = values.Count;

for(int=0; i<num; i++)
{

}

I think the second approach is better because you don't need to count all the items in each iteration. But I may be wrong. Could someone illuminate me?

Answer 1

The list already stores its Count internally. The comparison you are doing is related to code style, not performance. Since the compiler will optimize the retrieval of 'Count'

Answer 2

Well you can look at the .NET Source code here http://referencesource.microsoft.com/#mscorlib/system/collections/generic/list.cs#aa7e01fcb80a917e

  public int Count {
        get {
            Contract.Ensures(Contract.Result<int>() >= 0);
            return _size; 
        }
    }

It would appear that .Count property on list does a quick internal check and then returns _size. So it should be pretty close to the performance of you storing the value yourself.