System.Web.HttpException: Cannot have multiple items selected in a DropDownList

Question

During page load, index 0 was already selected. Then this code statement selected index 1:

dropDownList.Items.FindByValue(myValue).Selected = true; 
// assume myValue is found at index 1 of dropDownList.Items

On completion of page load, the page reads: "System.Web.HttpException: Cannot have multiple items selected in a DropDownList."

Why did I get the exception? And how can I fix it?

Answer 1

I noticed that both index 0 and index 1 had the properties "Selected" set to true (dropDownList.Items[0].Selected and dropDownList.Items[1].Selected both were true). However, dropDownList.SelectedIndex was still 0, even though index 1 was set most recently.

I tried resolving this by clearing the list selection beforehand.

dropDownList.ClearSelection();
dropDownList.Items.FindByValue(myValue).Selected = true;

But that didn't help. Same exception occurred.

What did help, was setting the selected value another way:

dropDownList.SelectedIndex = dropDownList.Items.IndexOf(dropDownList.Items.FindByValue(myValue));

Now the selection change propogates throughout the list.

So, don't use dropDownList.Items[x].Selected = true/false to change the selected value of a DropDownList. Instead, use dropDownList.SelectedIndex = x;