SqlConnection Cn = new SqlConnection(ConfigurationManager.ConnectionStrings["MyConnection"].ToString());

    public DataTable D_listado()
    {
        SqlCommand CMD = new SqlCommand("sp_listar", Cn)
        {
            CommandType = CommandType.StoredProcedure
        };
        SqlDataAdapter da = new SqlDataAdapter(CMD);
        DataTable dt = new DataTable();
        da.Fill(dt);
        return dt;
    }
}