The 'object' Base Class

Part of the Object Oriented Programming section of Coddy's C# journey — lesson 23 of 70.

In C#, every class you create automatically inherits from a special class called object. Even when you don't explicitly specify a base class, your class still has object as its ultimate ancestor. This means every type in C# shares a common set of methods.

The object class provides several virtual methods that you can override in your own classes:

public class Person
{
    public string Name { get; set; }
    
    // Override from object
    public override string ToString()
    {
        return $"Person: {Name}";
    }
    
    public override bool Equals(object obj)
    {
        if (obj is Person other)
            return Name == other.Name;
        return false;
    }
    
    public override int GetHashCode()
    {
        return Name?.GetHashCode() ?? 0;
    }
}

The three most commonly overridden methods are ToString() for string representation, Equals() for comparing objects, and GetHashCode() for hash-based collections. By default, ToString() returns the type name, and Equals() checks reference equality.

Because everything inherits from object, you can store any type in an object variable:

object item1 = new Person { Name = "Alice" };
object item2 = 42;
object item3 = "Hello";

Console.WriteLine(item1.ToString());  // Person: Alice

This universal base class is what makes features like collections that hold mixed types possible, and it's why every object in C# has access to methods like ToString() and GetType().

Challenge

Easy

Let's build a book comparison system that demonstrates how overriding methods from the object base class gives your custom types meaningful behavior for string representation and equality checking.

You'll create two files to organize your code:

Book.cs: Define a Book class in the Library namespace. Your book should have Title (string) and Author (string) properties, along with a constructor that sets both values. Override the three key methods inherited from object:
- ToString() should return "{Title} by {Author}"
- Equals(object obj) should return true if the other object is a Book with the same Title and Author
- GetHashCode() should combine the hash codes of both properties
Program.cs: In your main file, create two books using input values. Display each book using ToString(), then check if they're equal and print the result. Finally, demonstrate that you can store a Book in an object variable and still get meaningful output when calling ToString().

You will receive four inputs:

First book's title
First book's author
Second book's title
Second book's author

Print the output in this format:

Book 1: {ToString result}
Book 2: {ToString result}
Are they equal? {True/False}
Stored as object: {ToString result of first book stored in object variable}

For example, if the inputs are 1984, George Orwell, 1984, and George Orwell, the output should be:

Book 1: 1984 by George Orwell
Book 2: 1984 by George Orwell
Are they equal? True
Stored as object: 1984 by George Orwell

If the inputs are 1984, George Orwell, Brave New World, and Aldous Huxley, the output should be:

Book 1: 1984 by George Orwell
Book 2: Brave New World by Aldous Huxley
Are they equal? False
Stored as object: 1984 by George Orwell

Notice how your overridden methods give Book objects meaningful behavior - they display nicely as strings and can be compared by their content rather than just their memory references!

Cheat sheet

In C#, every class automatically inherits from the object class, which provides several virtual methods that can be overridden:

Common Object Methods to Override

ToString() - Returns a string representation of the object (default returns the type name)
Equals(object obj) - Compares objects for equality (default checks reference equality)
GetHashCode() - Returns a hash code for use in hash-based collections

Overriding Object Methods

public class Person
{
    public string Name { get; set; }
    
    // Override ToString for custom string representation
    public override string ToString()
    {
        return $"Person: {Name}";
    }
    
    // Override Equals for content-based comparison
    public override bool Equals(object obj)
    {
        if (obj is Person other)
            return Name == other.Name;
        return false;
    }
    
    // Override GetHashCode for hash-based collections
    public override int GetHashCode()
    {
        return Name?.GetHashCode() ?? 0;
    }
}

Universal Base Class

Any type can be stored in an object variable:

object item1 = new Person { Name = "Alice" };
object item2 = 42;
object item3 = "Hello";

Console.WriteLine(item1.ToString());  // Person: Alice

This universal inheritance enables collections that hold mixed types and ensures every object has access to methods like ToString() and GetType().

Try it yourself

using System;
using Library;

class Program
{
    public static void Main(string[] args)
    {
        // Read input for two books
        string title1 = Console.ReadLine();
        string author1 = Console.ReadLine();
        string title2 = Console.ReadLine();
        string author2 = Console.ReadLine();

        // TODO: Create two Book objects using the input values

        // TODO: Print Book 1 using ToString()
        // Format: "Book 1: {ToString result}"

        // TODO: Print Book 2 using ToString()
        // Format: "Book 2: {ToString result}"

        // TODO: Check if the books are equal and print the result
        // Format: "Are they equal? {True/False}"

        // TODO: Store the first book in an object variable and print it
        // Format: "Stored as object: {ToString result}"
    }
}