Hi textbox_Leave event is fired everytime AutoCompleteStringCollection is cleared..any ideas how I can prevent the leave event to fire while clearing AutoCompleteStringCollection

You some boolan flag. Like this:

bool bChecking;
        void textBox1_Leave()
            if (!bChecking)
                //put your code in the event in here!

        void YourCurrentMethod()
            //do some code, which you want to do, and which fires Leave event of textBox!
            //just before the code fires the Leave event do:
            bChecking = true;
            //and on the end set the flag back to false, so when the Leave event will be really called, it will be executed as well:
            bChecking = false;
            //and set the focus on textBox: