What is List.Contains() method in C#?

The List<T> class in C# is a list of objects having a specific type T. This class is a basic component of the .NET framework’s generic collection types and provides flexibility and convenience for managing collections of objects. We’ll learn about a very useful method of List<T> class, List.Contains(), which helps in the search functionality of many real-world applications.

The `List.Contains()` method

It contains a method List.Contains() to check the existence of specific elements within lists. It helps us to check the presence or absence of a particular element in the list. We’ll go through the details of List.Contains() method and some code examples.

Syntax

It accepts a single parameter of the same type as the elements in the list and returns a boolean value.

Here’s a breakdown of the syntax:

Access modifier: public indicates that the method is accessible from outside the class.
Return type: bool specifies that the method returns a boolean value. It returns true if the element is found and false if it is not in the list.
Parameter: T item represents the item of type T to be searched in the list.

Example: Checking book availability

Let’s understand the usage of List.Contains() method with the help of a basic code example. We’ll use this method to check if a student exists in a class list through the student’s name.

using System;
using System.Collections.Generic;
class Program
{
    static void Main()
    {
        // Creating a list of student names
        List<string> studentNames = new List<string> { "Alice", "Bob", "Charlie", "David", "Emma" };
        // Student name to search for
        string studentToFind1 = "Bob";
        string studentToFind2 = "Alexa";
        // Checking if the list contains the specified student “studentToFind1”
        if (studentNames.Contains(studentToFind1))
        {
            Console.WriteLine($"The list contains the student: {studentToFind1}.");
        }
        else
        {
            Console.WriteLine($"The list does not contain the student: {studentToFind1}");
        }
        // Checking if the list contains the specified student “studentToFind2”
        if (studentNames.Contains(studentToFind2))
        {
            Console.WriteLine($"The list contains the student: {studentToFind2}.");
        }
        else
        {
            Console.WriteLine($"The list does not contain the student: {studentToFind2}");
        }
    }
}

Code explanation

Here is the explanation of the above code example:

Line 9: We create a list of type string named as studentNames and populate the list with a few names of the students.
Lines 12–13: We create two string variables, studentToFind1 and studentToFind2 , to store the student’s name we want to search in the list.
Lines 16–23: In the if else block, we use the Contains() method of the List<T> class to check if the studentNames list contains the student studentToFind1 and print the message on the console.
Lines 26–33: In the if else block, we use the Contains() method of the List<T> class to check if the studentNames list contains the student studentToFind2 and print the message on the console.

Let’s look at another example of List.Contains() method usage.

Example: Book existence verification

We’ll use this method to check whether a book exists in the library or not using the book and author’s name.

using System;
using System.Collections.Generic;
class Program
{
    static void Main(string[] args)
    {
         // Creating a list of book names
        List<Book> library = new List<Book>();
        library.Add(new Book("1984", "George Orwell"));
        library.Add(new Book("To Kill a Mockingbird", "Harper Lee"));
        library.Add(new Book("The Great Gatsby", "F. Scott Fitzgerald"));
        // Checking if the book exists in the library or not  
        if (library.Contains(new Book("To Kill a Mockingbird", "Harper Lee"))) {
            Console.WriteLine("Already Exists");
        }
        else {
            Console.WriteLine("Doesn't Exit");
        }
    }
}
public class Book : IEquatable<Book>
{
    public Book(string title, string author)
    {
        this.Title = title;
        this.Author = author;
    }
    public string Title { get; set; }
    public string Author { get; set; }
    public bool Equals(Book book)
    {
        // Checking the equality on the basis of title and author 
        if (book == null) return false;
        return this.Title == book.Title && this.Author == book.Author;
    }
}

Code explanation

Here is the explanation of the above code example:

Lines 9–13: We create a list of type Book named as Library and populate the list with a few names of the books and authors.
Lines 15–20: In the if else block, we use the Contains() method of the List<T> class to check if the Library list contains the book and print the message on the console.
Lines 24–42: The Book class has two properties to represent a book: title and author. It implements the IEquatable<Book> interface for equality check. It overrides the IEquatable<T>.Equals method and checks the equality of books based on title and author. It helps us to use the Contains method by just passing the title and author of the book.

Conclusion

The List.Contains() method efficiently checks the presence or absence of an element in the list using a very simple and straightforward approach. It also ensures code safety by verifying the presence of elements before accessing or manipulating them. This makes it an indispensable tool for developers working with collections in various applications.

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What is List.Contains() method in C#?

The List.Contains() method

Syntax

Example: Checking book availability

Code explanation

Example: Book existence verification

Code explanation

Conclusion

The `List.Contains()` method