private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (comboBox1.SelectedValue == null) return;
    string SeaID = comboBox1.SelectedValue.ToString();
    string queryString2 = String.Format("SELECT Id, Area FROM dbo.Area WHERE seaID ='" + SeaID + "'");
    using (SqlConnection connection = new SqlConnection(@"Data Source=(LocalDB)MSSQLLocalDB;AttachDbFilename=|DataDirectory|ПАК_ИП_ОЭБ_Data.mdf;Integrated Security=True"))
    {
        try
        {
            SqlCommand cmd2 = new SqlCommand(queryString2, connection);
            if (cmd2 != null)
            {
                DataTable tbl2 = new DataTable();
                SqlDataAdapter da2 = new SqlDataAdapter(cmd2);
                da2.Fill(tbl2);
                this.comboBox2.DataSource = tbl2;
                this.comboBox2.DisplayMember = "Area";// столбец для отображения
                this.comboBox2.ValueMember = "Id";//столбец с id
                comboBox2.SelectedIndex = -1;
                comboBox2.Enabled = true;
            }
            else
            {
                MessageBox.Show("1");
                comboBox2.SelectedIndex = -1;
                comboBox2.Enabled = false;
            }
        }
        finally
        {
            if (connection != null)
            {
                connection.Close();
            }
        }

    }

else
            {
                MessageBox.Show("1");
                comboBox2.SelectedIndex = -1;
                comboBox2.Enabled = false;
            }