private void Form_Load(object sender, EventArgs e)  //загрузка формы
        {
            new Thread(InsData).Start();  //в потоке вызваю метод
        }

private void InsData()  //метод который нужно вызывать в потоке
        {
            try
            {
                string sql = "SELECT name FROM fraction WHERE id>0";
                //Открываем соединение
                con.Open();
                using (SqlCommand command1 = new SqlCommand(sql, con))
                {
                    SqlDataReader dataReader1 = command1.ExecuteReader();
                    while (dataReader1.Read())
                    {
                        fraction.BeginInvoke(new MethodInvoker(() => fraction.Items.Add(dataReader1["name"])));
                    }

                    dataReader1.Close();
                }


                sql = "SELECT name FROM marka WHERE id>0";
                using (SqlCommand command1 = new SqlCommand(sql, con))
                {
                    SqlDataReader dataReader1 = command1.ExecuteReader();
                    while (dataReader1.Read())
                    {
                        this.BeginInvoke(new MethodInvoker(() => marka.Items.Add(dataReader1["name"])));
                       // marka.Items.Add(dataReader1["name"]);
                    }
                    dataReader1.Close();
                }

                sql = "SELECT name FROM metcategory WHERE id>0";
                using (SqlCommand command1 = new SqlCommand(sql, con))
                {
                    SqlDataReader dataReader1 = command1.ExecuteReader();
                    while (dataReader1.Read())
                    {
                        this.BeginInvoke(new MethodInvoker(() => metCategory.Items.Add(dataReader1["name"])));
                        //metCategory.Items.Add(dataReader1["name"]);
                    }
                    dataReader1.Close();
                }

                if (con.State == ConnectionState.Open) con.Close();
            }
            catch (Exception ex)
            { MessageBox.Show(ex.Message); }
        }