Is exception handling time-consuming?

I have a hashtable and I may add repeated elements to it. It is pretty time-consuming to get hash code. So I don't want to write like following:

if(!ht.Contains(x))
ht.Add(x);

I want to write like this:

try{
ht.Add(x);
}
catch(ArgumentException e){
}

I am wondering that how fast exception handling is? Will it generate an interrupt?

Thanks

no actually using try .. catch blocks is actually a good idea :)
I mean you should know prior to use it. will that statement cause some exception. if that the case the put it in try ..catch blocks. the execution won't get slow
untill
each statement throws exception.
:D

one more thing to remember that
statement like simple ones which never throws exception should not be the candidates for exceptions.

-Paresh