How to generate 8 bytes unique id from GUID?

Question

I try to use long as unique id within our C# application (not global, and only for one session) for our events. Do you know if the following will generate an unique long id?

Answer 1

No, it won't. A GUID has 128 bit length, a long only 64 bit, you are missing 64 bit of information, allowing for two GUIDs to generate the same long representation. While the chance is pretty slim, it is there.