C# program to find the index of an element in a List

In C#, you can find the index of an element in a List<T> using the IndexOf() method. This method returns the zero-based index of the first occurrence of the specified element, or -1 if the element is not found.

Syntax

Following is the syntax for the IndexOf() method −

int IndexOf(T item)
int IndexOf(T item, int startIndex)
int IndexOf(T item, int startIndex, int count)

Parameters

• item: The element to locate in the list.
• startIndex: The zero-based starting index of the search (optional).
• count: The number of elements to search (optional).

Return Value

The method returns an int representing the zero-based index of the element. If the element is not found, it returns -1.

Using IndexOf() to Find Element Index

Example

Set a list and add elements −

List<int> val = new List<int>();

// integer elements
val.Add(35);
val.Add(55);
val.Add(68);

Let's say now we need to find the index of element 68. For that, use the IndexOf() method −

int index = val.IndexOf(68);

Here is the complete code −

using System;
using System.Collections.Generic;

public class Demo {
    public static void Main() {
        List<int> val = new List<int>();
        
        // integer elements
        val.Add(35);
        val.Add(55);
        val.Add(68);
        
        // finding the index of element 68
        int index = val.IndexOf(68);
        Console.WriteLine("Index of 68: " + index);
        
        // finding index of non-existing element
        int notFound = val.IndexOf(100);
        Console.WriteLine("Index of 100: " + notFound);
    }
}

The output of the above code is −

Index of 68: 2
Index of 100: -1

Using IndexOf() with String List

Example

using System;
using System.Collections.Generic;

public class Demo {
    public static void Main() {
        List<string> fruits = new List<string> {"apple", "banana", "cherry", "banana"};
        
        // Find first occurrence of "banana"
        int index1 = fruits.IndexOf("banana");
        Console.WriteLine("First occurrence of 'banana': " + index1);
        
        // Find "banana" starting from index 2
        int index2 = fruits.IndexOf("banana", 2);
        Console.WriteLine("'banana' starting from index 2: " + index2);
        
        // Element not found
        int index3 = fruits.IndexOf("orange");
        Console.WriteLine("Index of 'orange': " + index3);
    }
}

The output of the above code is −

First occurrence of 'banana': 1
'banana' starting from index 2: 3
Index of 'orange': -1

Using IndexOf() with Custom Objects

Example

using System;
using System.Collections.Generic;

public class Person {
    public string Name { get; set; }
    public int Age { get; set; }
    
    public override bool Equals(object obj) {
        if (obj is Person other) {
            return Name == other.Name && Age == other.Age;
        }
        return false;
    }
    
    public override int GetHashCode() {
        return Name.GetHashCode() ^ Age.GetHashCode();
    }
}

public class Demo {
    public static void Main() {
        List<Person> people = new List<Person> {
            new Person { Name = "John", Age = 25 },
            new Person { Name = "Jane", Age = 30 },
            new Person { Name = "Bob", Age = 35 }
        };
        
        Person searchPerson = new Person { Name = "Jane", Age = 30 };
        int index = people.IndexOf(searchPerson);
        
        Console.WriteLine("Index of Jane (30): " + index);
    }
}

The output of the above code is −

Index of Jane (30): 1

Conclusion

The IndexOf() method in C# Lists provides a simple way to find the zero-based index of any element. It returns -1 when the element is not found, making it easy to check for element existence. For custom objects, you need to override Equals() and GetHashCode() methods for proper comparison.