Get List<> Element Position in C# Using Linq

How to get index using LINQ?

An IEnumerable is not an ordered set.

Although most IEnumerables are ordered, some (such as Dictionary or HashSet) are not.

Therefore, LINQ does not have an IndexOf method.

However, you can write one yourself:

///<summary>Finds the index of the first item matching an expression in an enumerable.</summary>
///<param name="items">The enumerable to search.</param>
///<param name="predicate">The expression to test the items against.</param>
///<returns>The index of the first matching item, or -1 if no items match.</returns>
public static int FindIndex<T>(this IEnumerable<T> items, Func<T, bool> predicate) {
    if (items == null) throw new ArgumentNullException("items");
    if (predicate == null) throw new ArgumentNullException("predicate");

    int retVal = 0;
    foreach (var item in items) {
        if (predicate(item)) return retVal;
        retVal++;
    }
    return -1;
}
///<summary>Finds the index of the first occurrence of an item in an enumerable.</summary>
///<param name="items">The enumerable to search.</param>
///<param name="item">The item to find.</param>
///<returns>The index of the first matching item, or -1 if the item was not found.</returns>
public static int IndexOf<T>(this IEnumerable<T> items, T item) { return items.FindIndex(i => EqualityComparer<T>.Default.Equals(item, i)); }

Get index of object in a list using Linq

You don't need to use LINQ, you can use FindIndex of List<T>:

int index = customers.FindIndex(c => c.ID == 150);

Get List element position in c# using LINQ

var list = new List<int> { 3, 1, 0, 5 };
int pos = list.IndexOf(list.Min()); // returns 2

How can I get the index of an item in a list in a single step?

How about the List.FindIndex Method:

int index = myList.FindIndex(a => a.Prop == oProp);

This method performs a linear search; therefore, this method is an
O(n) operation, where n is Count.

If the item is not found, it will return -1

linq find in which position is my object in List

Use the IndexOf method:

kriteriji.IndexOf(doc);

C# Linq Find all indexes of item in Listint within another Listint

var ResultList = List1.Select(x => List2.IndexOf(x));

Find an item in a list by LINQ

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.

LINQ way to get items between two indexes in a List

You can use list.Skip(startIndex).Take(endIndex - startIndex) construct.

Where

startIndex : is an index of the first item to select

endIndex : is and index of the last item to select

Use LINQ to move item to top of list

LINQ is strong in querying collections, creating projections over existing queries or generating new queries based on existing collections. It is not meant as a tool to re-order existing collections inline. For that type of operation it's best to use the type at hande.

Assuming you have a type with a similar definition as below

class Item {
  public int Id { get; set; }
  ..
}

Then try the following

List<Item> list = GetTheList();
var index = list.FindIndex(x => x.Id == 12);
var item = list[index];
list[index] = list[0];
list[0] = item;

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