In this article I will explain with an example, how to use the SelectedIndexChanged event of ASP.Net DropDownList in C# and VB.Net.
The SelectedIndexChanged event of 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 an ASP.Net DropDownList specified with OnSelectedIndexChanged event handler and AutoPostBack property set to True.
<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" />
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.

protected void OnSelectedIndexChanged(object sender, EventArgs e)
    string message = ddlFruits.SelectedItem.Text + " - " + ddlFruits.SelectedItem.Value;
    ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('" + message + "');", true);
Protected Sub OnSelectedIndexChanged(sender As Object, e As EventArgs)
    Dim message As String = ddlFruits.SelectedItem.Text & " - " & ddlFruits.SelectedItem.Value
    ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('" & message & "');", True)
End Sub
ASP.Net DropDownList SelectedIndexChanged event example in C# and VB.Net