On an ASP.NET page with a couple DropDownLists, this problem surfaced.
When the page is called, it tries (unsuccessfully) to select the appropriate item in the DropDown, based on data retrieved from the database. The reported error is “A dropdown list cannot have multiple items selected“, even though I'm only making one selection.
Most of the offerings I found on the Internet said that one or the other of these selection methods must be used.
[YourDropDownName].Items.FindByValue([YourDataFromDataBase]).Selected = true;
ListItem objItem = [YourDropDownName].Items.FindByValue([YourDataFromDataBase]);
[YourDropDownName].SelectedIndex = [YourDropDownName].Items.IndexOf(objItem);
I think both of them are OK to use.
The solution is to call the ClearSelection() method of the DropDownlist immeidately before selecting the value for it. I found it here, at the very bottom of the post. I never had to do that before, but it works!