the reference answer GPT ᴼ ᴾ ᴱ ᴺ ᴬ ᴵ < br / > according to the information you provide, I think you might need to check the following:
Did you bind the data source of the DropDownList control in the Page_Load event of the ASP.NET Web Forms page. If so, the Page_Load event reloads and rebinds the data source every time a PostBack occurs on the page(including when the user selects other options in the DropDownList), causing the content of the DropDownList control to refresh.
Solution: Place the data source binding code in a method other than the Page_Load event, and ensure that the method is only called the first time the page is loaded, such as in the Page_Init event.
Did you use the UpdatePanel control or other AJAX techniques? These techniques can update parts of a page without refreshing the entire page, but can cause problems if used incorrectly.
Solution: Ensure that the UpdatePanel control or other AJAX techniques are configured correctly and that the entire page is not refreshed every time the user selects an option in the DropDownList.
Did you manually call the DataBind method of the DropDownList control in your code? If so, each call to the DataBind method rebinds the data source and refreshes the DropDownList control.
Solution: Instead of calling the DataBind method manually, set the DataSource property of the DropDownList and call the DataBind method of the DropDownList when you need to rebind the data source, such as in Page_Load or other events.
Hope the above solution can help you solve the problem. If you are still in doubt, provide more details or code examples to help you better.