'Arraylist' does not contain a definition for 'add'

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C#

.NET

I am getting this error:
Arraylist' does not contain a definition for 'Add' and no accessible extension method 'Add' accepting a first argument of type 'Arraylist' could be found (are you missing a using directive or an assembly reference?)

Here is the code where the error is popping up:

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

using EntitiesLayer;

namespace DataLayer
{
    public class D_Dashboard
    {
        readonly SqlConnection connect = new SqlConnection(ConfigurationManager.ConnectionStrings["SistDB"].ConnectionString);

        SqlCommand command;
        SqlDataReader dataReader;

        public void PreferedClients(E_Dashboard entityObject)
        {
            command = new SqlCommand("SPClientes_Preferidos", connect) 
            {
                CommandType = CommandType.StoredProcedure
            };

            connect.Open();
            dataReader = command.ExecuteReader();

            while (dataReader.Read()) 
            {
                entityObject.Profesionales.Add(dataReader.GetString(0));
                entityObject.CantFacturado.Add(dataReader.GetInt32(1)); //Here is where the error pops up
            }

            dataReader.Close();
            connect.Close();
        }
    }
}

As there is references to the E_Dashboard class, here's the class' code:

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

namespace EntitiesLayer
{
    public class E_Dashboard
    {
        public ArrayList Profesionales { get; set; } = new ArrayList();

        public ArrayList CantClientes { get; set; } = new ArrayList();

        public ArrayList Clientes { get; set; } = new ArrayList();

        public Arraylist CantFacturado { get; set; } = new ArrayList();

        public static decimal TOTALVENTASDIA { get; set; }

        public int TotalClientes { get; set; }

        public int TotalProfesionales { get; set; }

    }
}

What I have tried:

I tried checking which .Net framework version was the project using, which is the newest available for me (4.7.2). I have a few days checking the internet and my code without getting to a solution, all this because I really don't know what else to do. Thanks in advance.