Introduce Null Object

• Problema: Verificaciones repetidas de null.
• Solución: Crear un objeto "nulo" que implemente la misma interfaz.
• Code Smells que resuelve: Comprobaciones Repetidas de Null, Envidia de Características

public void ProcessCustomer(Customer customer)
{
    if (customer != null)
    {
        string name = customer.Name;
        if (name != null)
        {
            Console.WriteLine("Processing customer: " + name);
        }
        else
        {
            Console.WriteLine("Processing unnamed customer");
        }
        
        if (customer.HasDiscount())
        {
            ApplyDiscount(customer);
        }
    }
    else
    {
        Console.WriteLine("No customer to process");
    }
}

public interface ICustomer
{
    string Name { get; }
    bool HasDiscount();
    void ApplyDiscount(double amount);
}

public class Customer : ICustomer
{
    public string Name { get; set; }
    
    public bool HasDiscount()
    {
        // Implementación real
    }
    
    public void ApplyDiscount(double amount)
    {
        // Implementación real
    }
}

public class NullCustomer : ICustomer
{
    public string Name => "Guest";
    
    public bool HasDiscount()
    {
        return false;
    }
    
    public void ApplyDiscount(double amount)
    {
        // No hace nada
    }
}

// Uso:
public void ProcessCustomer(ICustomer customer)
{
    // No se necesita verificar null
    Console.WriteLine("Processing customer: " + customer.Name);
    
    if (customer.HasDiscount())
    {
        ApplyDiscount(customer);
    }
}

// Al llamar:
ICustomer customer = GetCustomer() ?? new NullCustomer();
ProcessCustomer(customer);