Error trying to create session using a C# class (Object reference not set to an instance of an object)

Question

I wonder if it is possible to record a session using a simple C # class. Action consists in:

1. Access the URL (WCF).
2. Saves session.
3. Return User.

---

I have the following class

namespace BLL.Rules
{
public class UsuarioRules
{

    public Usuarios login(string Senha)
    {
        try 
        {           
            UsuariosDAL ud = new UsuariosDAL();

            GenerateHash helperMD5 = new GenerateHash();

            var senhaMD5 = helperMD5.CreateHashMD5(Senha.ToString());

            var u = ud.ListarPorSenha(senhaMD5);

            if (u != null)
            {
                string teste = "teste";
                HttpContext.Current.Session["Id"] = teste;

            }
            else
            {
                HttpContext.Current.Session.Abandon();
            }
            return u;
        }
        catch (Exception ex)
        {
            string exString = ex.ToString();

        }

    }


}

This class is accessed by service

namespace WCF
{
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class Usuario : IUsuario
{


    public Usuarios login(string senha)
    {
        try
        {
            UsuarioRules ur = new UsuarioRules();
            Usuarios u = ur.login(senha);

            return u;
        }
        catch (Exception ex)
        {

            throw new Exception("Erro ao realizar login: " + ex);
        }

    }
}
}

The time that will create the session returns the error.

• Object reference not set to an instance of an object.

Please help, thank you.