Description

Attach additional responsibilities to an object dynamically.
Decorators provide a flexible alternative to subclassing for extending functionality

Add new functionalities dynamically to existing objects
The object does not know it is being « decorated »
Decorators both inherit the original class and contain an instantiation of it
It is an alternative to subclass to add new functionalities

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.cs

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