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## ASP.NET DropDownList SelectedIndex Property The `SelectedIndex` property is a core member of the ASP.NET `DropDownList` Web Server Control. It is used to get or set the zero-based index of the currently selected item in the dropdown list. --- ## Definition and Usage The `SelectedIndex` property serves two primary purposes: * **Get:** Retrieves the index of the currently selected item. The index is zero-based, meaning the first item in the list has an index of `0`, the second has `1`, and so on. If no item is selected, it returns `-1`. * **Set:** Programmatically selects an item in the dropdown list by specifying its index. --- ## Syntax ```syntax ``` Or in your code-behind (C# / VB.NET): ```csharp // C# - Get the selected index int index = myDropDownList.SelectedIndex; // C# - Set the selected index myDropDownList.SelectedIndex = 2; // Selects the third item ``` ```vb ' VB.NET - Get the selected index Dim index As Integer = myDropDownList.SelectedIndex ' VB.NET - Set the selected index myDropDownList.SelectedIndex = 2 ' Selects the third item ``` --- ## Code Example The following example demonstrates how to retrieve the selected item's text when a user clicks a button. ### ASPX Page (Markup and Inline Code) ```html <%@ Page Language="VB" %>

Select a name from the list:

``` --- ## Key Considerations 1. **Zero-Based Indexing:** Always remember that the index starts at `0`. If your `DropDownList` has 5 items, the valid range for `SelectedIndex` is `0` to `4`. 2. **Default Selection:** By default, if you do not explicitly set the `SelectedIndex` or set a `Selected` attribute on a `ListItem`, the `SelectedIndex` defaults to `0` (the first item in the list). 3. **Clearing Selection:** Setting `SelectedIndex` to `-1` clears the selection, meaning no item will be selected. 4. **Out of Range Exception:** If you attempt to set the `SelectedIndex` to a value greater than or equal to the number of items in the list (e.g., setting it to `5` when there are only 3 items), ASP.NET will throw an `ArgumentOutOfRangeException`.