Question

Here I have a simple example to find an item in a list of strings. Normally I use a for loop or anonymous delegate to do it like this:

int GetItemIndex(string search)
{
   int found = -1;
   if ( _list != null )
   {
     foreach (string item in _list) // _list is an instance of List<string>
     {
        found++;
        if ( string.Equals(search, item) )
        {
           break;
        }
      }
      /* Use an anonymous delegate
      string foundItem = _list.Find( delegate(string item) {
         found++;
         return string.Equals(search, item);
      });
      */
   }
   return found;
}

LINQ is new for me. Can I use LINQ to find an item in the list? If it is possible, how?

Answer 1

If you want the index of the element, this will do it:

int index = list.Select((item, i) => new { Item = item, Index = i })
                .First(x => x.Item == search).Index;

// or
var tagged = list.Select((item, i) => new { Item = item, Index = i });
int index = (from pair in tagged
            where pair.Item == search
            select pair.Index).First();

You can’t get rid of the lambda in the first pass.

Note that this will throw if the item doesn’t exist. This solves the problem by resorting to nullable ints:

var tagged = list.Select((item, i) => new { Item = item, Index = (int?)i });
int? index = (from pair in tagged
            where pair.Item == search
            select pair.Index).FirstOrDefault();

If you want the item:

// Throws if not found
var item = list.First(item => item == search);
// or
var item = (from item in list
            where item == search
            select item).First();

// Null if not found
var item = list.FirstOrDefault(item => item == search);
// or
var item = (from item in list
            where item == search
            select item).FirstOrDefault();

If you want to count the number of items that match:

int count = list.Count(item => item == search);
// or
int count = (from item in list
            where item == search
            select item).Count();

If you want all the items that match:

var items = list.Where(item => item == search);
// or
var items = from item in list
            where item == search
            select item;

And don’t forget to check the list for null in any of these cases.

Or use (list ?? Enumerable.Empty<string>()) instead of list.