In this article I will explain with an example, how to use SelectedIndexChanged event of DropDownList placed inside AJAX UpdatePanel in ASP.Net using C# and VB.Net.
The AsyncPostBackTrigger needs to be specified in the AJAX UpdatePanel in order to fire the SelectedIndexChanged event of the DropDownList using Partial PostBack.
The SelectedIndexChanged event will work / fire / trigger only when the AutoPostBack property of the ASP.Net DropDownList is set to True.
 
 

HTML Markup

The HTML Markup consists of following controls:
ScriptManager – For enabling AJAX.
UpdatePanel – For refreshing control.
DropDownList – For selecting Fruits.
The DropDownList specified with OnSelectedIndexChanged event handler and AutoPostBack property set to True.
Then AsyncPostBackTrigger is specified in the AJAX UpdatePanel in order to fire the SelectedIndexChanged event of the DropDownList using Partial PostBack.
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:DropDownList ID="ddlFruits" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged">
            <asp:ListItem Text="Mango" Value="1" />
            <asp:ListItem Text="Apple" Value="2" />
            <asp:ListItem Text="Banana" Value="3" />
            <asp:ListItem Text="Guava" Value="4" />
            <asp:ListItem Text="Orange" Value="5" />
        </asp:DropDownList>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ddlFruits" EventName="SelectedIndexChanged" />
    </Triggers>
</asp:UpdatePanel>
 
 

The SelectedIndexChanged event of ASP.Net DropDownList

When an item is changed in ASP.Net DropDownList, the following OnSelectedIndexChanged event handler is executed.
Inside the event handler, the selected Text and Value of the DropDownList Selected Item is fetched and displayed using JavaScript Alert message box.
C#
protected void OnSelectedIndexChanged (object sender, EventArgs e)
{
    string message = ddlFruits.SelectedItem.Text + " - " + ddlFruits.SelectedItem.Value;
    ScriptManager.RegisterStartupScript((senderas Control), this.GetType(), "alert", "alert('" + message + "');", true);
}
 
VB.Net
Protected Sub OnSelectedIndexChanged (sender As Object, e As EventArgs)
    Dim message As String = ddlFruits.SelectedItem.Text & " - " & ddlFruits.SelectedItem.Value
    ScriptManager.RegisterStartupScript(CType(sender, Control), Me.GetType(), "alert", "alert('" & message & "');", True)
End Sub
 
 

Screenshot

DropDownList SelectedIndexChanged event inside AJAX UpdatePanel in ASP.Net using C# and VB.Net
 
 

Demo

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Downloads

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