Auto-generate Unique ID within the constructor

Guid

If you do not have any constrain about ID type you can use a GUID:

Dim id As Guid = Guid.NewGuid()

You may even keep it as string:

Dim id As String = Guid.NewGuid().ToString("N")

That should be granted to be unique across different machines (to satisfy your requirement that it has to be suitable for use as a primary key in a database). See also this post.

Timestamp

That was worse case, if you do not have such strict requirement (uniqueness across a network) you may use a timestamp. Of course, in this case, you have to consider more issues:

• Legal time: time goes back and forward twice per year.
• Zones: what if user enter data in London and then he moves to New York?
• Concurrency: you have to assume no one else adds records to your database (you may have collisions if they use a different technique). Also you can't apply this if execution is concurrent (multiple instance of your program running together).
• Timer granularity: system date has a coarse granularity: if you construct many objects in a short period of time then you may have duplicate IDs. Workaround in this post.

Counter

If all these conditions are satisfied:

• Multiple instances of your application won't run in parallel.
• You're working on a single computer (not a network).
• Database is empty every time your application starts.

You may use a Shared counter incremented each time a new object is constructed. If system timer granularity isn't an issue (see paragraph about timestamp)you may use system up time as ID. With limitations about granularity it should work even with multiple instances of the same application running concurrently.

If you use a Shared field you have to deal with synchronization issues. As suggested in this comment you may use a SyncLock. As alternative you may use an Interlocked.Increment operation.

Hash code

If all condistions for a counter are satisfied and this one also:

• Your application is 32 bit.
• Your object is not a ValueType and it doesn't override GetHashCode() method.

You can use hash-code (obtained with GetHashCode()) because (from MSDN):

In other words, two objects for which the ReferenceEquals method returns true have identical hash codes.

Because Object.ReferenceEquals() returns true only if you compare same instance then each instance will have a unique hash code (because in a 32 bit application hash code is object reference itself). Be aware this is an implementation detail and it may change.

Random number

I know this may shock someone but a good random number generator for a 64 bit value has a very low probability of collisions. I repeat very very low probability (see this article for more math details). Just do not use System.Random for this!

According to which seed you use you may be able to generate random numbers in a network scenario too (do not forget - citation needed - that earlier drafts for one local network protocol proposed a 32 bit random number as address, it has been changed because of bad feedback from users but it doesn't mean it can't work).