how to generate a voucher code in c#?

Whats a promo code?

What are promo codes? Promotional codes are alphanumeric strings that online stores offer to encourage purchases on their website and are typically associated with an overarching promotional marketing strategy. The discount associated with a promo code can apply to individual products or an entire order.

I need to generate a voucher code[ 5 to 10 digit] for one time use only. what is the best way to generate and check if been used?

edited: I would prefer alpha-numeric characters - amazon like gift voucher codes that must be unique.