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List<T>.IndexOf() method with example in C#

C# List<T>.IndexOf() method: Here, we are going to learn about the IndexOf() method of List with example.
Submitted by IncludeHelp, on March 16, 2019

C# List<T>.IndexOf() Method

List<T>.IndexOf() method is used to get the index of first occurrence of an element in the list.

Syntax:

    int List<T>.IndexOf(T item);
    int List<T>.IndexOf(T item, int start_index);
    int List<T>.IndexOf(T item, int start_index, int count);

Parameter:

• item is an element of type T, whose first occurrence will be returned, if item found.
• start_index is the starting position from where you want to find the element in the list.
• count is the total elements from "start_index" to search the element in the list.

Return value: It returns index of the element, if element founds in the list between specified indexes, if element does not found in the list – it returns -1.

Example:

    int list declaration:
    List<int> a = new List<int>();

    adding elements:
    a.Add(10);
    a.Add(20);
    a.Add(30);
    a.Add(40);
    a.Add(50);
    a.Add(10);
    a.Add(20);
    a.Add(30);
    a.Add(40);
    a.Add(50);
    
    Method calls:
    a.IndexOf(20)   //output: 1
    a.IndexOf(100)   //output: -1
    a.IndexOf(20, 1)   //output: 1
    a.IndexOf(100, 1)   //output: -1
    a.IndexOf(20, 1, 3)   //output: 1
    a.IndexOf(20, 0, 1)   //output: -1

C# Example to get the index an element in the list using List<T>.IndexOf() Method

using System;
using System.Text;
using System.Collections.Generic;

namespace Test
{
    class Program
    {
        static void printList(List<int> lst)
        {
            //printing elements
            foreach (int item in lst)
            {
                Console.Write(item + " ");
            }
            Console.WriteLine();
        }

        static void Main(string[] args)
        {
            //integer list
            List<int> a = new List<int>();

            //adding elements
            a.Add(10);
            a.Add(20);
            a.Add(30);
            a.Add(40);
            a.Add(50);
            a.Add(10);
            a.Add(20);
            a.Add(30);
            a.Add(40);
            a.Add(50);

            //print the list
            Console.WriteLine("list elements...");
            printList(a);

            //using List.IndexOf(T item)
            //finding 20
            int index = a.IndexOf(20);
            if (index != -1)
                Console.WriteLine("20 found at " + index + " position.");
            else
                Console.WriteLine("20 does not found in the list");

            //finding 100
            index = a.IndexOf(100);
            if (index != -1)
                Console.WriteLine("100 found at " + index + " position.");
            else
                Console.WriteLine("100 does not found in the list");

            //using List.IndexOf(T item, int index)
            //finding 20
            index = a.IndexOf(20, 1); //start index is 1
            if (index != -1)
                Console.WriteLine("20 found at " + index + " position.");
            else
                Console.WriteLine("20 does not found in the list");

            //finding 100
            index = a.IndexOf(100, 1); //start index is 1
            if (index != -1)
                Console.WriteLine("100 found at " + index + " position.");
            else
                Console.WriteLine("100 does not found in the list");

            //using List.IndexOf(T item, int start_index, int count)
            //finding 20
            //search will perform from index 1 to next 3 elements
			index = a.IndexOf(20, 1, 3); 
            if (index != -1)
                Console.WriteLine("20 found at " + index + " position.");
            else
                Console.WriteLine("20 does not found in the list");

            //finding 20
			//search will perform from index 0 to next 1 element
            index = a.IndexOf(20, 0, 1); 
            if (index != -1)
                Console.WriteLine("20 found at " + index + " position.");
            else
                Console.WriteLine("20 does not found in the list");

            //hit ENTER to exit
            Console.ReadLine();
        }
    }
}

Output

list elements...
10 20 30 40 50 10 20 30 40 50
20 found at 1 position.
100 does not found in the list
20 found at 1 position.
100 does not found in the list
20 found at 1 position.
20 does not found in the list

Reference: List<T>.IndexOf Method