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How to use DropDownList AutoPostBack in asp.net c#

DropDownList AutoPostBack Feature
DropDownList control allows users to select an item from a single selection drop-down list. We can specify DropDownList items by placing ListItem elements between the opening and closing tags of the DropDownList control.

The .NET developers can also populate DropDownList from a data source object such as SqlDataSource, ObjectDataSource, LinqDataSource, ArrayList, Array, etc. The DropDownList DataBind method binds the data source to the DropDownList control.

Users can select an item from DropDownList and manually submit the form using submit button to send their selection to the server. This way we can get user selection by clicking a submit button. But ASP.NET DropDownList is a more user-friendly server control. It has a property name AutoPostBack. AutoPostBack property works with the SelectedIndexChanged event.

AutoPostBack property gets or sets a value that indicates whether a postback to the server automatically occurs when the user changes the list selection. The OnSelectedIndexChanged method raises the SelectedIndexChanged event that allows developers to provide a custom handler for the event.

The SelectedIndexChanged event is raised when the user changes the DropDownList selection. So when if the user changes the DropDownList item selection then the page automatically posts to the web server and the developer can display user selection details on the web page after postback occurs.

The following C# example source code describes more how can you use the DropDownList AutoPostBack feature in ASP.NET to provide a better user experience.
DropDownListAutoPostBack.aspx

<%@ Page Language="C#" %>

<!DOCTYPE html>

<script runat="server">
    protected void DropDownList1_SelectedIndexChanged(object sender, System.EventArgs e)
    {
        Label1.Text = "You Selected: " + DropDownList1.SelectedItem.Text;
    }
</script>

<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>How to use DropDownList AutoPostBack feature</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <h2 style="color:Navy">DropDownList: AutoPostBack</h2>
        <asp:Label 
             ID="Label1"
             runat="server"
             Font-Bold="true"
             ForeColor="Purple"
             Font-Size="Large"
             >
        </asp:Label>
        <br /><br />
        <asp:Label 
             ID="Label2"
             runat="server"
             Font-Bold="true"
             ForeColor="OrangeRed"
             Text="asp.net controls"
             >
        </asp:Label>
        <asp:DropDownList 
             ID="DropDownList1"
             runat="server"
             AutoPostBack="true"
             OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"
             >
             <asp:ListItem>HyperLink</asp:ListItem>
             <asp:ListItem>PasswordRecovery</asp:ListItem>
             <asp:ListItem>PlaceHolder</asp:ListItem>
             <asp:ListItem>LoginName</asp:ListItem>
             <asp:ListItem>Label</asp:ListItem>
        </asp:DropDownList>
    </div>
    </form>
</body>
</html>
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