C# | Searching the index of specified object in Collection<T>

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:

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"));
    }
}

Output:

A
B
C
D
D
E
Index : 3

Example 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));
    }
}

Output:

2
3
4
5
Index : -1

Note:

• 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.

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.objectmodel.collection-1.indexof?view=netframework-4.7.2