Question

Say I have the following list:

 List<int> integerList = new List<int> {
    1, 10, 13, 5, 7, 123, 47, 69, 22, 77, 94, 201, 120, 73, 98, 99, 101, 4, 6, 9, 19, 21, 24, 221, 909, 45, 27, 28, 29, 30,
   };

What Linq query will retrieve for me the index of a certain value in the list? In on other words, I’d like a query that would return to me an index value of 5 if I pass it a value of ‘123’.

Thank you.

Answer 1

Well the easiest approach is not to use LINQ 🙂

int index = integerList.IndexOf(123);

However, if you must:

var index = integerList.Select((value, index) => new { value, index })
                       .Where(x => x.value == desiredValue)
                       .Select(x => (int?) x.index)
                       .FirstOrDefault();

That will give you an int? with a “null” value if the value can’t be found, or the first matching index otherwise. You can turn it into the regular “use -1 as not found” using the null coalescing operator:

var index = integerList.Select((value, index) => new { value, index })
                       .Where(x => x.value == desiredValue)
                       .Select(x => (int?) x.index)
                       .FirstOrDefault() ?? -1;