Description

It provides a way of attaching new state and behaviour to an object dynamically.
The object does not know it is being “decorated”.
Decorators both inherit the original class and contain an instantiation of it.

When to use it

When you have:

An existing component class that may be unavailable for subclassing.

You want to:

Attach additional state or behavior to an object dynamically.
Make changes to some objects in a class without affecting others.
Avoid subclassing because too many classes could result.

But consider using instead:

The Adapter pattern, which sets up an interface between different classes.
The Composite pattern, which aggregates an object without also inheriting its interface.
The Proxy pattern, which specifically controls access to objects.
The Strategy pattern, which changes the original object rather than wrapping it.

Exemple

IUser.cs

public interface IUser
{
    string Name { get; set; }
}

User.cs

public class User : IUser
{
    public string Name { get; set; }

    public User(string name)
    {
        this.Name = name;
    }
}

UserTitle

public class UserTitle : IUser
{
    private IUser user;

    public string Name
    {
        get => $"{user.Name} ({Title})";
        set => user.Name = value;
    }

    public string Title { get; set; }

    public UserTitle(IUser user, string title)
    {
        this.user = user;
        this.Title = title;
    }
}

UserEmail.cs

public class UserEmail : IUser
{
    private IUser user;

    public string Name
    {
        get => $"{user.Name} - {Email}";
        set => user.Name = value;
    }

    public string Email { get; set; }

    public UserEmail(IUser user, string email)
    {
        this.user = user;
        this.Email = email;
    }
}

var user = new User("Nicolas");
Console.WriteLine(user.Name);  // Nicolas

var userWithTitle = new UserTitle(user, "Big Boss");
Console.WriteLine(userWithTitle.Name);  // Nicolas (Big Boss)

var userWithEmail = new UserEmail(user, "nicolas@world-company.net");
Console.WriteLine(userWithEmail.Name);  // Nicolas (Big Boss) - nicolas@world-company.net