Find an item in List by LINQ?

There's a few ways (note this is not a complete list).

1) Single will return a single result, but will throw an exception if it finds none or more than one (which may or may not be what you want):

string search = "lookforme";
List<string> myList = new List<string>();
string result = myList.Single(s => s == search);

Note SingleOrDefault() will behave the same, except it will return null for reference types, or the default value for value types, instead of throwing an exception.

2) Where will return all items which match your criteria, so you may get an IEnumerable with one element:

IEnumerable<string> results = myList.Where(s => s == search);

3) First will return the first item which matches your criteria:

string result = myList.First(s => s == search);

Note FirstOrDefault() will behave the same, except it will return null for reference types, or the default value for value types, instead of throwing an exception.

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.

Thanks to Pavel for helping out in the comments.

If it really is a List<string> you don't need LINQ, just use:

int GetItemIndex(string search)
{
    return _list == null ? -1 : _list.IndexOf(search);
}

If you are looking for the item itself, try:

string GetItem(string search)
{
    return _list == null ? null : _list.FirstOrDefault(s => s.Equals(search));
}