How to pass an object as a parameter to another class method

Question

I am developing an object-oriented agenda and recording in a text file.

I’m finding the following problem:

when saved a contact it is passing to another object as if it were a pointer

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


namespace AgendaConsole
{
    class Program
    {
        private static bool sair = false;

        static void Main(string[] args)
        {
            Contato C = new Contato();
            Agenda A = new Agenda();

            if (A.Inicia())
            {
                Console.WriteLine("Agenda Carregou!");
            }

            do
            {
                string resposta;
                Console.Clear();
                Console.WriteLine("***AGENDA***");
                Console.WriteLine("1- Cadastrar");
                Console.WriteLine("2- Consultar");
                Console.WriteLine("3- Alterar");
                Console.WriteLine("4- Excluir");
                Console.WriteLine("5- Sair");
                resposta = Console.ReadLine();
                switch (resposta)
                {
                    case "1":

                        Console.Clear();
                        Console.WriteLine("Nome:");
                        C.setNome(Console.ReadLine());
                        Console.WriteLine("Email:");
                        C.setEmail(Console.ReadLine());
                        Console.WriteLine("Telefone:");
                        C.setTelefone(Console.ReadLine());
                        A.adicionaContato(C);

                        Console.WriteLine("Pressione qualquer tecla para voltar ao menu.");
                        Console.ReadKey();
                        break;
                    case "5":
                        sair = true;
                        break;

                }
            } while (!sair);
        }
    }
}

In line A.addendContact(C);

 public void adicionaContato(Contato C)
        {
            this.Contatos[posicao] = C;
            posicao++;
        }

always end up replicating and adding at position 0 and in which should even add duplicate contact, in the added callContact it is already with object in the agenda.

Updating ...

classe Agenda

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

namespace AgendaConsole
{
    class Agenda
    {
        private Contato[] Contatos = new Contato[100];
        GravaArquivo G = new GravaArquivo();
        int posicao = 0;

        public bool Inicia()
        {
            for (int i = 0; i < 100; i++)
            {
                this.Contatos = G.CarregaAgenda();
            }
            return true;
        }

        public Contato Busca(string b)
        {
            Contato C = new Contato();
            for (int i = 0; i < 100; i++)
            {
                if (b.Equals(this.Contatos[i].GetNome()))
                {
                    C = Contatos[i];
                }
            }
            return C;
        }

        public bool Salvar()
        {
            G.GravaAgenda(Contatos);

            return true;
        }

        public int buscaPosicao()
        {
            int cont = 0;
            for (int i = 0; i < 100; i++)
            {
                if (this.Contatos[i].GetNome().Equals(""))
                {
                    return cont;
                }
                else
                {
                    cont++;
                }
            }
            return cont;
        }

        public void adicionaContato(Contato C)
        {
            posicao=buscaPosicao();
            this.Contatos[posicao] = C;            
        }
    }
}