if you set in HTML, Selected=”True” for DropDownlist and later try to set it programmatically, ‘ll get the error
A DropDownList cannot have multiple items selected.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.Web.HttpException: A DropDownList cannot have multiple items selected.