C# | Searching the index of specified object in Collection<T>
Last Updated :
11 Jul, 2025
Improve
Collection<T>.IndexOf(T) method is used to search for the specified object and returns the zero-based index of the first occurrence within the entire Collection<T>.
Syntax:
CSHARP
Output:
CSHARP
Output:
public int IndexOf (T item);Here, item is the object to locate in the List<T>. The value can be null for reference types. Return Value: This method returns the zero-based index of the first occurrence of item within the entire Collection<T>, if found, otherwise, -1. Below given are some examples to understand the implementation in a better way: Example 1:
// C# code to search for the specified
// object and returns the zero-based
// index of the first occurrence within
// the entire Collection
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
class GFG {
// Driver code
public static void Main()
{
// Creating a collection of strings
Collection<string> myColl = new Collection<string>();
// Adding elements in Collection myColl
myColl.Add("A");
myColl.Add("B");
myColl.Add("C");
myColl.Add("D");
myColl.Add("D");
myColl.Add("E");
// Displaying the elements in myColl
foreach(string str in myColl)
{
Console.WriteLine(str);
}
// Searching for the specified object
// and returns the zero-based index of
// the first occurrence within the entire
// Collection. If the object doesn't contain the
// object, then -1 is returned
Console.WriteLine("Index : " + myColl.IndexOf("D"));
}
}
// C# code to search for the specified// object and returns the zero-based// index of the first occurrence within// the entire Collectionusing System;using System.Collections.Generic;using System.Collections.ObjectModel;class GFG { // Driver code public static void Main() { // Creating a collection of strings Collection<string> myColl = new Collection<string>(); // Adding elements in Collection myColl myColl.Add("A"); myColl.Add("B"); myColl.Add("C"); myColl.Add("D"); myColl.Add("D"); myColl.Add("E"); // Displaying the elements in myColl foreach(string str in myColl) { Console.WriteLine(str); } // Searching for the specified object // and returns the zero-based index of // the first occurrence within the entire // Collection. If the object doesn't contain the // object, then -1 is returned Console.WriteLine("Index : " + myColl.IndexOf("D")); }}A B C D D E Index : 3Example 2:
// C# code to search for the specified
// object and returns the zero-based
// index of the first occurrence within
// the entire Collection
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
class GFG {
// Driver code
public static void Main()
{
// Creating a collection of ints
Collection<int> myColl = new Collection<int>();
// Adding elements in Collection myColl
myColl.Add(2);
myColl.Add(3);
myColl.Add(4);
myColl.Add(5);
// Displaying the elements in myColl
foreach(int i in myColl)
{
Console.WriteLine(i);
}
// Searching for the specified object
// and returns the zero-based index of
// the first occurrence within the entire
// Collection. If the object doesn't contain the
// object, then -1 is returned
Console.WriteLine("Index : " + myColl.IndexOf(7));
}
}
// C# code to search for the specified// object and returns the zero-based// index of the first occurrence within// the entire Collectionusing System;using System.Collections.Generic;using System.Collections.ObjectModel;class GFG { // Driver code public static void Main() { // Creating a collection of ints Collection<int> myColl = new Collection<int>(); // Adding elements in Collection myColl myColl.Add(2); myColl.Add(3); myColl.Add(4); myColl.Add(5); // Displaying the elements in myColl foreach(int i in myColl) { Console.WriteLine(i); } // Searching for the specified object // and returns the zero-based index of // the first occurrence within the entire // Collection. If the object doesn't contain the // object, then -1 is returned Console.WriteLine("Index : " + myColl.IndexOf(7)); }}2 3 4 5 Index : -1Note:
- The Collection<T> is searched forward starting at the first element and ending at the last element.
- This method determines equality using the default equality comparer EqualityComparer<T>.Default for T, the type of values in the list.
- This method performs a linear search. Therefore, this method is an O(n) operation, where n is Count.
