Getting Duplicate Random Numbers - vb.net

Question

I'm doing basic random number generation using this Shared Function:
Public Shared Function RandomNumber(ByVal MaxNumber As Integer, Optional ByVal MinNumber As Integer = 0) As Integer
'initialize random number generator
Dim r As New Random(Date.Now.Ticks And &HFFFF)
If MinNumber > MaxNumber Then
Dim t As Integer = MinNumber
MinNumber = MaxNumber
MaxNumber = t
End If
Return r.Next(MinNumber, MaxNumber)
End Function
Called like this: dim x as integer = Random(2100000000)
Very simple, and the seed value comes straight from a MS example.
HERE'S THE PROBLEM: I'm getting duplicate numbers on occasion, but always created at times that are usually at least 5 or 10 minutes apart. I can see if I was calling the function multiple times per second or millisecond, because that'd kind of "breaks" the seed. But these are showing up at extended time spans. What else could be causing this?

Answer 1

Duplicate seed issue?
It might be better defining r as static so that it is initialised once when first invoked. Refer to this answer Random integer in VB.NET

Answer 2

The Random constructor takes an Integer as its parameter which is 32-bits. As spencer7593 said, with only 16 bits, you're repeating the sequence every 6.5ms. Try:
Dim r As New Random(Date.Now.Ticks And &HFFFFFFFF)
However, this will do the same thing:
Dim r As New Random()
Better yet, don't create a new Random object each time:
Private Static r As New Random()
Public Shared Function RandomNumber(MaxNumber As Integer, Optional MinNumber As Integer = 0) As Integer
...
Return r.Next(MinNumber, MaxNumber)
End Function

Answer 3

Q: What else could be causing this?
A: It could be happening purely by random. Random numbers are just that: random. At any point in time, whether its seconds or hours away from another point in time, its just as likely for a number to appear as any other number. There is no guarantee that a number won't be repeated.
On the other hand, it looks like your seed value is only on the order of 16 bits. And that's like a total of 65,536 possibilities. There's 10,000 ticks in a millisecond, so ever 6.5 milliseconds you have the possibility of reusing the same seed.
It's not at all clear whether the VB Random is using some other kind of entropy beyond that seed or not. (But gathering entropy for inclusion would slow down the initialization, so it may not be, as a performance consideration.)
According the docs, creating two Random objects using the same seed value results in Random objects that create duplicate sequences of unique numbers.
http://msdn.microsoft.com/en-us/library/ctssatww.aspx
I think that answers the question why it is happening.
I guess the next question is why do you need to instantiate a new Random object? If you need multiple objects, then instantiating several of them, but making sure you are using a different seed value for each one would be one approach.
But before you go there, I recommend you consider using just one Random. Calls to get random numbers can be serviced from an existing Random, rather than creating a new one every time you need a random number.

Answer 4

Try it another way:
Public Function RandomNumber2(ByVal MaxNumber As Integer, Optional ByVal MinNumber As Integer = 0) As Integer
' Initialize the random-number generator.
Randomize()
' Generate random value between MaxNumber and MinNumber.
Return CInt(Int((MaxNumber * Rnd()) + MinNumber))
End Function
See Randomize Function (Visual Basic) for more details. Hope this helps.