DropDownList SelectedIndexChanged event fires twice on few Dropdowns

Could anyone explain me why txtPreppin_SelectedIndexChanged event executes twice after txtPreppin value changed, then change value in second dropdownlist txtScanner


<asp:DropDownList
                    ID="txtPreppin" runat="server" CssClass="padding-input"
                    AutoPostBack="True" onselectedindexchanged="txtPreppin_SelectedIndexChanged">
                    <asp:ListItem Text="-Select-" Value=''>-Select-</asp:ListItem>
                    <asp:ListItem>Acevedo, Noe</asp:ListItem>
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                                   </asp:DropDownList>
                <br />
                <label for="recordtype">
                                 Date:</label><asp:TextBox ID="txtprepDate" runat="server"
                    Enabled="False" ReadOnly="True"></asp:TextBox>
                <asp:CalendarExtender ID="txtprepDate_CalendarExtender" runat="server"
                    Enabled="True" TargetControlID="txtprepDate" Format="MM/dd/yyyy">
                </asp:CalendarExtender>
               
               
               
        </td>
        <td style="width: 306px">
           
        <label for="recordtype">
                                 Scanner:</label><asp:DropDownList
                ID="txtScanner" runat="server" CssClass="padding-input"
                AutoPostBack="True" onselectedindexchanged="txtScanner_SelectedIndexChanged">
                    <asp:ListItem Text="-Select-" Value=''>-Select-</asp:ListItem>
                     <asp:ListItem>Bueno, Nancy</asp:ListItem>
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                               </asp:DropDownList>
       
                <label for="recordtype">
                                 Date:</label>
                                 <br />
                                 <asp:TextBox ID="txtScanDate" runat="server"
                Enabled="False" ReadOnly="True"></asp:TextBox>
                <asp:CalendarExtender ID="txtScanDate_CalendarExtender" runat="server"
                    Enabled="True" TargetControlID="txtScanDate" Format="MM/dd/yyyy">
                </asp:CalendarExtender>
                         





 protected void txtScanner_SelectedIndexChanged(object sender, EventArgs e)
    {

        if (Page.IsPostBack)
        {

           
            if (txtScanner.SelectedItem.Text != null)
            {
                txtScanDate.Enabled = true;
                txtScanDate.ReadOnly = false;
                txtScanDate.Text = DateTime.Today.ToString("MM/dd/yyy");
            }

            else
            {
                txtScanDate.Enabled = false;
                txtScanDate.ReadOnly = true;
            }

        }
    }
    protected void txtPreppin_SelectedIndexChanged(object sender, EventArgs e)
    {


        if (Page.IsPostBack)
        {
           
            if (txtPreppin.SelectedItem.Text != null)
            {
                txtprepDate.Enabled = true;
                txtprepDate.ReadOnly = false;
                txtprepDate.Text = DateTime.Today.ToString("MM/dd/yyy");
            }

            else
            {
                txtprepDate.Enabled = false;
                txtprepDate.ReadOnly = true;
            }
        }
    }
LVL 9
TonyRebaAsked:
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Lalit ChandraCommented:
You disabled the viewstate for dropdowns by setting EnableViewState="false" to them. Take a look to a note on MSDN:

A list control must persist some values between posts to the server for this event to work correctly. Be sure that view state is enabled for the list control.

Also OnSelectedIndexChanged event handler is the same for the each of you dropdowns: OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged".

All this means that after a postback your dropdowns think that the selected value was changed if the selected value is different from the first list item's value. That's why you've got your event handler fire twice after second postback or even thrice after third.

Just enable viewstate for dropdowns to prevent such a behaviour.
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TonyRebaAuthor Commented:
View state is enabled.
Thasts not the problem.
0
TonyRebaAuthor Commented:
View state is enabled.
Thasts not the problem.
0
TonyRebaAuthor Commented:
View State was enabled on the page, but not on the masterpage.....
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TonyRebaAuthor Commented:
Viewstate wasnt enabled on Master Page
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