C# | How to check whether a List contains the elements that match the specified conditions

List<T>.Exists(Predicate<T>) Method is used to check whether the List<T> contains elements which match the conditions defined by the specified predicate. Properties of List:

• It is different from the arrays. A list can be resized dynamically but arrays cannot.
• List class can accept null as a valid value for reference types and it also allows duplicate elements.
• If the Count becomes equals to Capacity then the capacity of the List increases automatically by reallocating the internal array. The existing elements will be copied to the new array before the addition of the new element.

Syntax:

public bool Exists (Predicate<T> match);

Parameter:

match: It is the Predicate<T> delegate which defines the conditions of the elements to search for.

Return Value: This method returns True if the List<T> contains one or more elements that match the conditions defined by the specified predicate otherwise it returns False. Exception: This method will give ArgumentNullException if the match is null. Below programs illustrate the use List<T>.Exists(Predicate<T>) Method: Example 1:

CSharp

// C# Program to check whether a List contains
// the elements that match the specified conditions
// defined by the predicate
using System;
using System.Collections;
using System.Collections.Generic;

class Geeks {

    // function which checks whether an
    // element is even or not. Or you can
    // say it is the specified condition
    private static bool isEven(int i)
    {
        return ((i % 2) == 0);
    }

    // Main Method
    public static void Main(String[] args)
    {

        // Creating an List<T> of Integers
        List<int> firstlist = new List<int>();

        // Adding elements to List
        for (int i = 1; i <= 10; i++) {
            firstlist.Add(i);
        }

        Console.WriteLine("Elements Present in List:\n");

        // Displaying the elements of List
        foreach(int k in firstlist)
        {
            Console.WriteLine(k);
        }

        Console.WriteLine(" ");

        Console.Write("Result: ");

        // Check the elements of firstlist that
        // match the conditions defined by predicate
        Console.WriteLine(firstlist.Exists(isEven));
    }
}

Output:

Elements Present in List:

1
2
3
4
5
6
7
8
9
10
 
Result: True

Example 2:

CSharp

// C# Program to check whether a List contains
// the elements that match the specified conditions
// defined by the predicate
using System;
using System.Collections;
using System.Collections.Generic;

class Geeks {

    // function which checks whether an
    // element is even or not. Or you can
    // say it is the specified condition
    private static bool isEven(int i)
    {
        return ((i % 2) == 0);
    }

    // Main Method
    public static void Main(String[] args)
    {

        // Creating an List<T> of Integers
        List<int> firstlist = new List<int>();

        // Adding elements to List
        firstlist.Add(5);
        firstlist.Add(7);
        firstlist.Add(9);
        firstlist.Add(11);
        firstlist.Add(3);
        firstlist.Add(17);
        firstlist.Add(19);

        Console.WriteLine("Elements Present in List:\n");

        // Displaying the elements of List
        foreach(int k in firstlist)
        {
            Console.WriteLine(k);
        }

        Console.WriteLine(" ");

        Console.Write("Result: ");

        // Check the elements of firstlist that
        // match the conditions defined by predicate
        Console.WriteLine(firstlist.Exists(isEven));
    }
}

Output:

Elements Present in List:

5
7
9
11
3
17
19
 
Result: False

Time complexity: O(n) for Exists method

Space complexity: O(n) where n is the size of the list

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.list-1.exists?view=netframework-4.7.2