Validate if a CPF already exists in the List before inserting C#

Question

Good evening, I am creating a sales system (college work) that has a simple registration of Clients (name and Cpf), we are working with the Project in layers, I am having difficulty because I am trying to validate if the CPF already exists in the list using FOREACH, but it doesn’t seem to be working because it’s accepting all the CPF’s and at the time I ask for a re-turn of the list, it’s not returning me anything... what’s wrong with the validation of Foreach? Thank you in advance!.

Code in C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using VendasOsorioA.Model;

namespace VendasOsorioA.DAL
{
    class ClienteDAO
    {

        private static List<Cliente> clientes = new List<Cliente>(); 
        public static void CadastrarCliente(Cliente c) 
        {
            foreach (Cliente ClienteCadastrado in clientes)
            {
                if (c.Cpf.Equals(ClienteCadastrado.cpf))
                {
                    Console.WriteLine("CPF JÁ EXISTE NA BASE, CADASTRO RECUSADO.");
                }
                else
                {
                    clientes.Add(c);
                    Console.WriteLine("CLIENTE CADASTRADO COM SUCESSO.");
                }
            }    
        }
        public static List<Cliente> RetornarClientes()
        {
            return clientes;
        }
    }

    }

Answer 1

Hello! The problem is the logic you used. Your code is going through the list of registered customers, which is initially empty. How about this version?

        public static void CadastrarCliente(Cliente c)
        {
            foreach (Cliente ClienteCadastrado in clientes)
            {
                if (c.Cpf.Equals(ClienteCadastrado.Cpf))
                {
                    Console.WriteLine("CPF JÁ EXISTE NA BASE, CADASTRO RECUSADO.");
                    return;
                }
            }
            clientes.Add(c);
            Console.WriteLine("CLIENTE CADASTRADO COM SUCESSO.");
        }

Answer 2

I would recommend changing your code by doing a LINQ instead of a foreach(). This allows you to solve your problem and also simplifies the analysis of your code.

Do it like this:

private static List<Cliente> clientes = new List<Cliente>(); 

if (clientes.Any(cliente => cliente.cpf == cpf))
{
    clientes.Add(c);
    Console.WriteLine("CLIENTE CADASTRADO COM SUCESSO.");
}
else
{
    Console.WriteLine("CPF JÁ EXISTE NA BASE, CADASTRO RECUSADO.");
}

Answer 3

One of the most efficient ways to do this is by using a HashSet and not a Lista. The HashSetis an implementation of a data structure that ensures no duplicated data in time O(1), unlike the Time List O(n) since in the worst case it is necessary to go through the list until the end to know whether or not the element is in it.

However you need to implement the method Equals and GetHashCode in the kind you keep in your HashSet, in this case would be in the type Cliente.

public class Cliente{
    //... Métodos, campos, propriedades, construtores etc

    public override bool Equals(object obj)
    {
        //sua implementação
    }

    public override int GetHashCode()
    {
        //sua implementação
    }
}

private static HashSet<Cliente> clientes = new HashSet<Cliente>(); 
public static void CadastrarCliente(Cliente c) 
{
    if(clientes.Add(c)){
        Console.WriteLine("CLIENTE CADASTRADO COM SUCESSO.");
    }else{
        Console.WriteLine("CPF JÁ EXISTE NA BASE, CADASTRO RECUSADO.");
    } 
}

Another thing that is wrong in your code is that you are changing the list within the foreach, normally doing so results in an exception.