Why are you trying to cast and convert?
type varname = (type) othervar;
That is a cast.
type varname = othervar as type;
That is using the
operator to check the type, and return null if a cast is not possible.
Sint the cast has a higher precedence, you are doing a cast operation (which will throw and exception if it doesn't work) then doing a check-and-cast-if-possible as operator (which will return the instance as the type or null) on the result ...
Remove the cast, and check the result for null.
But ... since you are getting an exception, you will get a null from the
operator - so what you need to do is use the debugger to find out exactly what FindControl is returning - since it clearly isn't a DropDownList, and the finding out what it is should help you to work out why not.