why textchanged and selectedindexchange events don't post correct values in asp.net?

why textchanged and selectedindexchange events don't post correct values
in asp.net?

how to use selected index changed event of drop down list in asp.net??when
I set auto post back ==true, dropdown reload again and select value
changed to first index. i have problems like this problem in text changed
event of textbox that I know only auto post property. but auto post back
not correct solution.