I have a form with two comboboxes .

combobox1 unitupc

combobox2 production line 

First, the unitupc gets loaded and then for each unitupc chosen the combobox2 gets populated. The issue I am having is that for each unitupc chosen by the user the previous values is stored in the combobox2 and the list keeps getting added, how does one clear the combobox each time the unitupc is chosen and reload it in?

Here is a picture of the form with the issue i am talking about:

![enter image description here](https://i.stack.imgur.com/ZlA2X.png)

**EDIT CODE ADDED**

```
  private void DimensionSelection_Load(object sender, EventArgs e)
        {
            // TODO: This line of code loads data into the 'CorsicanaNetWeightDataSet10.Net_Weight_Master_Data_Report' table. You can move, or remove it, as needed.

            // TODO: This line of code loads data into the 'corsicanaNetWeightDataSet9.ProductionLine' table. You can move, or remove it, as needed.

            prodline = new productweightdataset();
            // TODO: This line of code loads data into the 'corsicanaNetWeightDataSet3.ProductionLine' table. You can move, or remove it, as needed.
            //this.productionLineTableAdapter.Fill(this.corsicanaNetWeightDataSet3.ProductionLine,comboBox3.Text.ToString());
            // TODO: This line of code loads data into the 'corsicanaNetWeightDataSet2.ItemDescription' table. You can move, or remove it, as needed.
            //this.itemDescriptionTableAdapter.Fill(this.corsicanaNetWeightDataSet2.ItemDescription);

            loadprod(); 

            this.reportViewer1.RefreshReport();
            reportViewer1.Visible = false;
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            //if (comboBox1.SelectedIndex > -1)
            //{
            //    button1.Enabled = true;
            //}

            Loadproduction();
            comboBox2.Refresh();
        }

        private void loadprod()
        {
            try
            {
                using (MSSQL.SqlConnection connection = new MSSQL.SqlConnection(constr))
                {
                    connection.Open();
                    using (MSSQL.SqlCommand command = new MSSQL.SqlCommand("SELECT [Unit UPC Base Item] as Unitupc, [Item Description] AS ItemDescription FROM ItemDesc", connection))
                    {
                        {
                            MSSQL.SqlDataAdapter myadapter = new System.Data.SqlClient.SqlDataAdapter();
                            myadapter.SelectCommand = command;
                            myadapter.Fill(prodline, "DataTable1");

                        }

                    }

                    //fill drop down
                    comboBox1.DataSource = prodline.DataTable1;
                    comboBox1.ValueMember = "ItemDescription";
                    comboBox1.DisplayMember = "ItemDescription";
                    comboBox3.DataSource = prodline.DataTable1;
                    comboBox3.ValueMember = "Unitupc";
                    comboBox3.DisplayMember = "Unitupc";
                    if (comboBox1.Items.Count > 0)
                    {

                        comboBox1.SelectedIndex = 0;
                        Loadproduction();
                    }

                }
            }
            catch (Exception) { /*Handle error*/ }

        }

        //private void comboBox3_SelectedIndexChanged(object sender, EventArgs e)
        //{
        //    this.productionLineTableAdapter1.Fill(this.corsicanaNetWeightDataSet9.ProductionLine, comboBox3.Text.ToString());
        //}

        private void Loadproduction()
        {
           if (comboBox1.SelectedValue.ToString().Trim().Length > 0)
            {
                try
                {
                    using (MSSQL.SqlConnection connection = new MSSQL.SqlConnection(constr))
                    {
                        connection.Open();

                        using (MSSQL.SqlCommand command = new MSSQL.SqlCommand("SELECT TOP (100) PERCENT dbo.[Production Lines].[Production Line Description] AS prodline FROM dbo.[Production Lines] LEFT OUTER JOIN dbo.[Net Weight Master Data] ON dbo.[Production Lines].[Production Line] = dbo.[Net Weight Master Data].[Production Line] RIGHT OUTER JOIN dbo.ItemDesc ON dbo.[Net Weight Master Data].[Unit UPC Base Item] = dbo.ItemDesc.[Unit UPC Base Item] WHERE (dbo.ItemDesc.[Unit UPC Base Item] = '0001') GROUP BY dbo.[Production Lines].[Production Line], dbo.[Production Lines].[Production Line Description], dbo.ItemDesc.[Unit UPC Base Item] ORDER BY dbo.ItemDesc.[Unit UPC Base Item]", connection))
                        {
                            MSSQL.SqlParameter myparam = new MSSQL.SqlParameter();
                            myparam.Direction = ParameterDirection.Input;
                            myparam.ParameterName = "@unitupc";
                            myparam.SqlDbType = SqlDbType.NVarChar;
                            myparam.Size = 50;
                            myparam.Value = comboBox3.Text;
                            command.Parameters.Add(myparam);
                            MSSQL.SqlDataAdapter myadapter = new System.Data.SqlClient.SqlDataAdapter();
                            myadapter.SelectCommand = command;
                            myadapter.Fill(prodline, "DataTable2");
                            comboBox2.DataSource = prodline.DataTable2;
                            comboBox2.DisplayMember = "prodline";

                        }
                    }
                }
                catch (Exception) { /*Handle error*/ }
            }
        }

```