I need to iterate and count. What is fastest or preferred: ToArray() or ToList()? [duplicate]

Answer 1

If you are really concerned about performance, you should loop over the IEnumerable and count it as you go. This avoids having to create a new collection altogether, and the intersection only has to be iterated once:

void Foobar(string[] arr, Dictionary)
{
   var t = arr.Intersect(dic.Keys);
   int count = 0;
   foreach(var item in t)
   {
      count++;
      ..
   }

   var j = count;
}

But like someone else said: this smells of micro-optimization. If performance really matters in this situation, at least do performance profiling to find out which method is really the fastest for you.