C#NET2008 ReLoad Listbox1

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C#3.5

Hullo Good Friends, :laugh:
Please help me. I need your help. ;)
I encounter error message when I trid to refill the Listbox after the user input the data into the parameter textbox. I tried to clear the display in the listbox and it generate this error message:

Items collection cannot be modified when the DataSource property is set.

This is the coding that generate it:
this.listBox1.Items.Clear();

Here are the overall coding:

private void FLoadListBox(string strTagName)
  {
             this.listBox1.Items.Clear();
  if (this.txtInput.text == "Product")
  {
        string strSql = "Select Product from TblSales";
  }

  else
      {
       string strSql = "Select Orders from TblSales";
  }

              sqlconn = new SqlConnection(connstr);
              sqlconn.Open();
              SqlDataAdapter DA = new SqlDataAdapter(strSql, sqlconn);
              System.Data.DataSet DS = new DataSet("DS");
              DS.Clear();
              DS.Tables.Clear();
              DA.Fill(DS, "Display");

  if (this.txtInput.text == "Product")
  {
           this.listBox1.DisplayMember ="Product";
       this.listBox1.DataSource = DS.Tables["Display"];
  }
  else
  {
       this.listBox1.DisplayMember ="Orders";
       this.listBox1.DataSource = DS.Tables["Display"];
  }
  sqlconn.close();
}

Please help me. How to clear the current display on Listbox before refilling it with new dataset