I said I’d share this information about how to resolve this error since it took me almost a whole day to fix it. The ‘Cannot have multiple items selected in a DropDownList‘ exception will be thrown when the developer attempts to select a ListItem in a DropDownList when an item in that same DropDownList has already been selected earlier in previous code.
When searching on Google and the forums.asp.net site most of the suggested fixes to this problem suggested explicitly deselecting all selected items in a dropdown before selecting another. This can be done via something like ddl.clearselection() or ddl.SelectedIndex = -1. It seemed like a fairly logical approach, however it did not work for me, no matter what variation of deselection I tried.
On further investigation, it seemed the problem stemmed from the fact that I was adding the same ListItem to multiple dropdowns like so:
ListItem areaItem = new ListItem(Areas.Rows[i][0].ToString(),Areas.Rows[i][0].ToString());
ddlArea.Items.Insert(i, areaItem);
ddlArea2.Items.Insert(i, areaItem);
ddlArea3.Items.Insert(i, areaItem);
ddlArea4.Items.Insert(i, areaItem);
ddlArea5.Items.Insert(i, areaItem);
but because a ListItem object is a reference object when I changed the selectedItem (and by doing so set a particular ListItems selected property to true) property of say ddlArea4 I also changed it in all the other dropdowns which contained the ListItem which was now selected. Remember reference objects do not keep a copy of an object they simply store a reference to the original object, thus a change in one dropdown list was leading to a change in all so code like the following which aims to explicitly deselect items before selecting another would not work:
ddlArea4.ClearSelection();
ddlArea4.SelectedIndex = -1;
ddlArea4.Items.FindByValue(itdp.Rows[2][1].ToString()).Selected = true; //the ListItem that this will select is present in multiple dropdowns, if any of them have anything selected an error will be thrown
The solution to the problem is to create new instances of ListItems for each option you need to add to dropdown lists, so instead of the first bit of code above, I tried something like:
ddlArea.Items.Add(new ListItem(Areas.Rows[i][0].ToString(),Areas.Rows[i][0].ToString()));
ddlArea2.Items.Add(new ListItem(Areas.Rows[i][0].ToString(),Areas.Rows[i][0].ToString()));
ddlArea3.Items.Add(new ListItem(Areas.Rows[i][0].ToString(),Areas.Rows[i][0].ToString()));
ddlArea4.Items.Add(new ListItem(Areas.Rows[i][0].ToString(),Areas.Rows[i][0].ToString()));
ddlArea5.Items.Add(new ListItem(Areas.Rows[i][0].ToString(),Areas.Rows[i][0].ToString()));
It meant when I selected a particular ListItem in a particular dropdown only the one ListItem and dropdown was affected. It worked well and it even allowed me to remove the extra deselection logic which in this case it turned out I did not need.
This was quite an uncommon scenario that led to this problem and tracking down the solution was difficult. I think in over 90% of cases the problem with the “Cannot have multiple items selected in a DropDownList” error would come from the developer trying to select more than one item in a DropDownList in a more obvious way such as simply having two or more .Selected = true statements perhaps in different parts of his or her code without realising. Therefore always give something like ClearSelection() or SelectedIndex = -1 a go first before thinking too hard about this problem, it will most likely resolve it.
