How to generate a LONG guid?

Question

I would like to generate a long UUID - something like the session key used by gmail. It should be at least 256 chars and no more than 512. It can contain all alpha-numeric chars and a few special chars (the ones below the function keys on the keyboard). Has this been done already or is there a sample out there?

C++ or C#

Update: A GUID is not enough. We already have been seeing collisions and need to remedy this. 512 is the max as of now because it will prevent us from changing stuff that was already shipped.

Update 2: For the guys who are insisting about how unique the GUID is, if someone wants to guess your next session ID, they don't have to compute the combinations for the next 1 trillion years. All they have to do is use constrain the time factor and they will be done in hours.

Answer 1

If your GUIDs are colliding, may I ask how you're generating them?

It is astronomically improbable that GUIDs would collide as they are based on:

• 60 bits - timestamp during generation
• 48 bits - computer identifier
• 14 bits - unique ID
• 6 bits are fixed

You would have to run the GUID generation on the same machine about 50 times in the exact same instant in time in order to have a 50% chance of collision. Note that instant is measured down to nanoseconds.

Update:

As per your comment "putting GUIDs into a hashtable"... the GetHashCode() method is what is causing the collision, not the GUIDs:

public override int GetHashCode()
{
    return ((this._a ^ ((this._b << 0x10) | ((ushort) this._c))) ^ ((this._f << 0x18) | this._k));
}

You can see it returns an int, so if you have more than 2^32 "GUIDs" in the hashtable, you are 100% going to have a collision.