DropdownList - How To Add Item Property

I have created a dropdownlist which I choose wizard way to fill data in it. and its property like:
DataTextField=Item
DataValueField=ItemCode

I go to properties of this dropdownlist and then I add Items property as the following:
Enabled=true
Selected=false
Text=---choose---
Value=-1

And the code behind is that:
dim strItem as string= ddlItem.selectedItems.text
if ddlItem.selectedItems.text.trim="---choose---" then
Label1. text=" You haven't choosen any items yet!"
exit sub
else
Label1. text=" You have choose"
end if

But when I start debugging this:
strItem always gets "---choose---" text eventhough I choosed other items.