如何检查C＃中未选择ListBox的选定值？

Question

This code will display the selected value from the listbox. Eg if I sellect Item 1 I will get the following output: You have selected Item 1 .

Label1.Text = "You have selected " + DropDownList1.SelectedValue + "<br />";

But if I don't select anything and click on Submit button, I will get: You have selected

What would I need to make it display "You have not selected anything. Please select at least 1 item."

UPDATE: I am using ASP.NET WebForms.

Answer 1

Update:
The below answer is actually incorrect (left for history). Once you access the SelectedIndex property, when nothing is selected, the list will immediately make the first item selected, and return zero.

So pretty much the only choice that remains is to have some kind of "dummy item" very first in the list, and check for SelectedIndex == 0 .

The above, however, is only true for DropDownList . Other controls derived from ListControl (ie ListBox or RadioButtonList ) will correctly show SelectedIndex == -1 .

Here goes the incorrect answer :
Check the SelectedIndex property. If nothing is selected, it will have the value of -1.

if ( DropDownList1.SelectedIndex < 0 )
{
    Label1.Text = "You have not selected anything";
}
else
{
    Label1.Text = "You have selected " + DropDownList1.SelectedValue;
}

Answer 2

Be carefull!:

Use the SelectedIndex property to programmatically specify or determine the index of the selected item from the DropDownList control. An item is always selected in the DropDownList control . You cannot clear the selection from every item in the list at the same time.

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.dropdownlist.selectedindex.aspx