How to resist the drop down value.

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C#

ASP.NET

I have a drop downlist and 2 list box . I want that i am not able to select anything from listbox 1 and 2 until i am not selecting the item from dropdownlist but every time i select anything from dropdownlist it postback and its value is not resisting .

<div>
    <asp:DropDownList ID="ddlAdmin" runat="server" >
    <table width="70%">
    <tr><td colspan="3">
    <asp:ListBox ID="ProductListBox" runat="server" SelectionMode="Multiple" ></td>
    <td><asp:Button ID="BtnMoveLeft" runat="server" Text="Add Permission" OnClick="BtnMoveleft_Click" /></td>
    <td>
    
    <asp:ListBox ID="PermissionListBox" runat="server" SelectionMode="Multiple" ></td>
    </tr>
   </table>
    </div>

protected void Page_Load(object sender, EventArgs e)
    {
        FillProductList();
        FillAdminDropDownList();
    }

    protected void FillAdminDropDownList()
    {
        DataManager myData = new DataManager(DbFunctions.GetConnectionString());
        MySqlConnection myPConn = myData.GetPooledConnection(new MySqlConnection());


        MySqlCommand cmd = new MySqlCommand("SELECT adminuserid,adminusername FROM adminusermast", myPConn);
        DataTable dt = new DataTable();
        myPConn.Open();
        MySqlDataAdapter adptr = new MySqlDataAdapter(cmd);
        adptr.Fill(dt);
        myPConn.Close();
        if (dt.Rows.Count > 0)
        {
            ddlAdmin.Items.Clear();
            ddlAdmin.Items.Add("SELECT");
            foreach (DataRow drow in dt.Rows)
            {
                ListItem lst = new ListItem();
                lst.Value = drow[1].ToString();
                lst.Text = drow[0].ToString();
                ddlAdmin.Items.Add(lst);
            }
        }
    }
    protected void FillProductList()
    {
            DataManager myData = new DataManager(DbFunctions.GetConnectionString());
            MySqlConnection myPConn = myData.GetPooledConnection(new MySqlConnection());
            MySqlCommand com = new MySqlCommand("Select productid,productname from productmast", myPConn);
            MySqlDataReader Reader = null;
            myPConn.Open();
           Reader = com.ExecuteReader();
           while (Reader.Read())
           {

               ListItem newItem = new ListItem();
               newItem.Text = Reader["productname"].ToString();
               newItem.Value = Reader["productid"].ToString();
               ProductListBox.Items.Add(newItem);
           }
           Reader.Close();
       myPConn.Close();
    }

    protected void BtnMoveleft_Click(object sender, EventArgs e)
    {
        for (int i = ProductListBox.Items.Count - 1; i >= 0; i--)
        {
            if (ProductListBox.Items[i].Selected == true)
            {
                PermissionListBox.Items.Add(ProductListBox.Items[i]);
                ListItem li = ProductListBox.Items[i];
                ProductListBox.Items.Remove(li);
            }
        }
    }