Performance issue with generation of random unique numbers

Question

I have a situation where by I need to create tens of thousands of unique numbers. However these numbers must be 9 digits and cannot contain any 0\'s. My current approach is

Answer 1

I think first thing would be to use StringBuilder, instead of concatenation - you'll be pleasantly surprised. Antoher thing - use a more efficient data structure, for example HashSet<> or HashTable.

If you could drop the quite odd requirement not to have zero's - then you could of course use just one random operation, and then format your resulting number the way you want.