Question

how can i implement this foreach loop with for loop? because i heard that for loop is much faster than foreach loop.

   foreach (RV item in gridview.SelectedItems.ToList())
   {
        R req= db.Requests.Single(x => x.Id == item.Id);
        ...
   }

Answer 1

You heard incorrectly.
for loops are not much faster than foreach loops.
In fact, for some collections, foreach loops are much faster than for loops.

ToList() is slower than either of them.

If you really want to make your code faster, you should use an O(n) LINQ join instead of the O(n²) Single() call.