ComboBox.Items[i]

            NpgsqlConnection conn23 = new NpgsqlConnection(Conection_string);
            conn23.Open();
            string SQL1 = "SELECT DISTINCT nominal_size FROM tryba where tryba."+kolonka+"='"+comboBox3.Text+"' ;";
            NpgsqlDataAdapter da11 = new NpgsqlDataAdapter(SQL1, conn23);
            DataSet ds11 = new DataSet();
            da11.Fill(ds11);
            comboBox4.DataSource = ds11.Tables[0];
            comboBox4.DisplayMember = "nominal_size";
            conn23.Close();
 
            for (int i = 0; i <=comboBox4.Items.Count; i++)
            {
                arr[i] = (double)comboBox4.Items[i];
            }

             comboBox4.Items.Clear();
            for (int i = 0; i <= arr.Length; i++)
            {
                comboBox4.Items.Add(Convert.ToString((double)arr[i] * 10));
            }

arr[i] = (double)comboBox4.Items[i];

            for (int i = 0; i < comboBox4.Items.Count; i++)
            {
                arr[i] = float.Parse(comboBox4.Items[i].ToString().Replace(".",System.Globalization.CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator));
            }

NpgsqlConnection conn = new NpgsqlConnection(Conection_string);
            conn.Open();
            string SQL = "SELECT DISTINCT nominal_size FROM tryba where tryba." + kolonka + "='" + comboBox3.Text + "' ORDER by tryba.nominal_size ;";
            NpgsqlDataAdapter da = new NpgsqlDataAdapter(SQL, conn);
            DataSet ds = new DataSet();
            da.Fill(ds);
            BindingSource bs = new BindingSource(ds, ds.Tables[0].TableName);
            comboBox4.DataSource = bs;
            comboBox4.DisplayMember = "avtor";
            conn.Close();
 
            Single[] mas = new Single[comboBox4.Items.Count];
            for (int i = 0; i < comboBox4.Items.Count; i++)
            {
                mas[i] = float.Parse(comboBox4.Items[i].ToString().Replace(".", System.Globalization.CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator));
            }