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Discover the superclass of all data types: the Object class.

The Object Class

The `Equals()` and `ToString()` methods in `Object` are `virtual`, so they can be overridden.

For example, we can override `ToString()` in the `Diary` class:

```cs
class Diary
{
  /* other members omitted */

  public override string ToString()
  {
    return $"This Diary is currently on page {CurrentPage}."; 
  }
}
```

Now any `Diary` instance will use this version of the method:

```cs
Diary dy = new Diary(7);
Console.WriteLine(dy.ToString());
// Output: "This Diary is currently on page 7."
```

 Overriding Object Members

Good job! In this lesson you learned that:


* Every type, eventually, inherits from `Object`
* `Object` members include `Equals()`, `ToString()`, and `GetType()`
* The `ToString()` method allows `Console.WriteLine()` to work for all types


Review

Every class is derived from `Object` &mdash; even classes that we define ourselves!

When you create a class, C# implicitly makes it inherit `Object`. So when we write this code:
```cs
class Book
{}
```

C# assumes we mean:
```cs
class Book : Object
{}
```

Even if we already declared a superclass, like...
```cs
class Book : Media
{}
```

...`Object` will be at the top of the family tree. Maybe `Media` directly inherits from `Object`, or its base class inherits from `Object`, etc.

We can prove this to ourselves by attempting to convert any reference to an `Object`. None of this code will throw an error because each type &mdash; at some point &mdash; inherits from `Object`:
```cs
Object o1 = new Dissertation();
Object o2 = new Diary();
Object o3 = new Random();
Object o4 = new Forest("Amazon");
```

Value types and strings also inherit from `Object`:
```cs
Object o5 = 21;
Object o6 = false;
Object o7 = "Hello you!";
```


 The Object Type


`Object` has a few useful members and they're accessible by every type. Here are some important ones:

* `Equals(Object)` &mdash; returns `true` if the current instance and the argument are equal (using value equality for value types and referential equality for reference types)
* `GetType()` &mdash; returns the type of the object
* `ToString()` &mdash; returns a string describing the object

You can see each method in action here:
```cs
Object o1 = new Object();
// t is System.Object
Type t = o1.GetType();

string s = o1.ToString();
// Prints "System.Object"
Console.WriteLine(s);

Object o2 = o1;
// Equals true
bool b = o1.Equals(o2);
```
 
Remember that we can access inherited members from a derived class. In this case, every type inherits from `Object`, so every type can access these members!


For the full list of `Object` members read the [Microsoft documentation](https://docs.microsoft.com/en-us/dotnet/api/system.object).




 Object Members


In C# there is one type of reference that can be used for all objects. It's aptly called `Object`. 

Every class is derived from `Object`. Whether it's the class' superclass or the superclass' superclass' superclass, `Object` is at the top of the class' inheritance hierarchy.

Since references can be upcast to any type in its inheritance hierarchy, then all types can by referenced as `Object`s:
```cs
Object o1 = new Dissertation();
Object o2 = new Diary();
Object o3 = new Random();
Object o4 = new Forest("Amazon");
```
If that's so, why not use `Object` references for everything? Because the functionality of an object is limited by its reference type. We lose all of a specific type's specific functionality when we reference it as an `Object` type. We would also lose the automatic type-checking that saves us from type errors.

When we do use them, `Object` references can be very useful! For example, if we're not sure what type a variable is, we can safely store it as an `Object`. We can also assume that any object has access to the standard `Object` members for basic manipulation.

In this lesson, you'll learn:
* How every type inherits from `Object`
* The useful members in `Object`


Object is at the top of every inheritance hierarchy.

 Introduction to Object

At the very beginning of your C# journey, you learned about `Console.WriteLine()`. You used this tool with nearly every type, from `int` and `bool` to `Dissertation` and `Diary`:
```cs
bool b = true;
Diary d = new Diary();
Console.WriteLine(b);
Console.WriteLine(d);
```

We can use this same tool with every type because every type inherits from `Object`!

Under the hood, `Console.WriteLine()` uses `ToString()`, which is defined in `Object`. Every object needs some kind of `string` representation to be printed in text. These two lines are equivalent:
```cs
Console.WriteLine(b);
Console.WriteLine(b.ToString());
```
