drop down list Selected index changed event not working - asp.net

Question

On my form,I am having one drop down list.I have filled drop down list on page load event.Even i have check if not is post back.My drop down list's auto post back property is true.But still the event is not firing.I have to show some value in text box on selected index changed of drop down list.
my design code is as below:
enter code here
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<div>
<table class="tblContentcss">
<tr>
<td class="tdContent1css">
<asp:Label runat="server" ID="lblUserRole" Text="Role"></asp:Label>
</td>
<td class="tdContent2css">
:
</td>
<td class="tdContent3css">
<asp:TextBox runat="server" ID="txtUserRole" Text="User" ReadOnly="true"></asp:TextBox>
</td>
</tr>
<tr>
<td class="tdContent1css">
<asp:Label runat="server" ID="lblUserID" Text="User ID"></asp:Label>
</td>
<td class="tdContent2css">
:
</td>
<td class="tdContent3css">
<asp:DropDownList ID="ddlUserID" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlUserID_SelectedIndexChanged">
</asp:DropDownList>
</td>
</tr>
<tr>
<td class="tdContent1css">
<asp:Label runat="server" ID="lblUserName" Text="User Name"></asp:Label>
</td>
<td class="tdContent2css">
:
</td>
<td class="tdContent3css">
<asp:TextBox runat="server" ID="txtUserName" AutoPostBack="true"></asp:TextBox>
</td>
</tr>
<tr>
<td class="tdContent1css">
<asp:Label runat="server" ID="lblUserPassword" Text="Password"></asp:Label>
</td>
<td class="tdContent2css">
:
</td>
<td class="tdContent3css">
<asp:TextBox runat="server" ID="txtPassword" TextMode="Password"></asp:TextBox>
</td>
</tr>
<tr>
<td class="tdContent1css">
<asp:Label runat="server" ID="Label1" Text="Password"></asp:Label>
</td>
<td class="tdContent2css">
:
</td>
<td class="tdContent3css">
<asp:Button runat="server" ID="btnSave" Text="Save" OnClick="btnSave_Click" />
</td>
</tr>
</table>
</div>
</asp:Content>
and in.cs on page_load
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
fillUserID();
}
}
protected void ddlUserID_SelectedIndexChanged(object sender, EventArgs e)
{
fillUserID();
SqlConnection con = dal.GetConnection();
dsUserName = new DataSet();
query = "SELECT CUSTID,(FNAME+' '+MNAME+' '+LNAME) AS USERNAME FROM CUSTOMER where CUSTID=" + Convert.ToInt32(ddlUserID.SelectedValue) + "";
dsUserName = dal.ReturnDataset(query, con);
if (dsUserName.Tables.Count > 0 && dsUserName.Tables[0].Rows.Count > 0)
{
txtUserName.Text = dsUserName.Tables[0].Rows[0]["USERNAME"].ToString();
}
}
what is the problem,i am not getting.
Suggest me any solution

Answer 1

When the autopostback is set True, after the page is rendered, a javascript is added on the onchange event of the dropdown (SELECT in HTML) to trigger the postback,
View the source after the page is rendered and check whether the javascript is getting added to the onchange event of the dropdown.
If you are adding some javascript on the onchange event of the dropdown in the code-behind, that might also stop the page to postback.
I get that from this link: https://www.codeproject.com/Questions/138183/Drop-down-selected-index-change-event-not-working