Dropdownlist change value automatically in AutoPostBack

Question

Hello Programmers

 

I using dropdownlist with autopostback, its working good but if I select a certain value change this value to another value, I have not any update control like panel.

 

what is a problem?

<asp:DropDownList runat="server" Font-Size="Medium" Font-Names="Calibri" CssClass="form-control" Width="250px" Height="30" ID="ddl_device" OnSelectedIndexChanged="ddl_device_SelectedIndexChanged" AutoPostBack="True" >
<asp:ListItem Value="">Select Items</asp:ListItem>
<asp:ListItem Value="0">CPU</asp:ListItem>
<asp:ListItem Value="1">Monitor Extra</asp:ListItem>
<asp:ListItem Value="1">laptop</asp:ListItem>
<asp:ListItem Value="1">Printer</asp:ListItem>
<asp:ListItem Value="1">Fax Machine</asp:ListItem>
<asp:ListItem Value="1">Scanner</asp:ListItem>
</asp:DropDownList>

protected void ddl_device_SelectedIndexChanged(object sender, EventArgs e)
{
if (ddl_device.SelectedValue == "")
{
Response.Write("<script>alert('Please you must be Select Type!');</script>");
}
else if (ddl_device.SelectedValue == "1")
{
TextBox1.Enabled = true;
TextBox1.Text = "";
TextBox2.Enabled = false;
TextBox2.Text = "";
TextBox3.Enabled = false;
TextBox3.Text = "";
}
else if (ddl_device.SelectedValue == "0")
{
TextBox1.Enabled = true;
TextBox1.Text = "";
TextBox2.Enabled = true;
TextBox2.Text = "";
TextBox3.Enabled = true;
TextBox3.Text = "";
}