Is exception handling time-consuming?

[mcwave]

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