drop down menu - SelectedIndex of DropDown list not setting correctly in C# -

in aspx form, returning page several dropdown boxes, , want set values selected before leaving page.

i returning selectedindex values through url parameters pickup in code:

string oldvalue = request.querystring["oldvalue"].tostring(); 

oldvalue accurately takes on correct value. however, when try set dropdown selectedindex value, continually selectedindex set "-1"

example (temporarily bypassing querystring , using value directly):

ddlsearchcollege.selectedindex = 3; 

i have breakpoint here, , when hover mouse on selectedindex, value shows "-1" instead of 3.