Random String in VB - vb.net

Question

I need to generate a lot of random 2 character strings for my application. it's a VB console application. basically what I have tried for random strings is this:
Private Function GenerateRandomString(ByVal intLenghtOfString As Integer) As String
'Create a new StrinBuilder that would hold the random string.
Dim randomString As New StringBuilder
'Create a new instance of the class Random
Dim randomNumber As Random = New Random
'Create a variable to hold the generated charater.
Dim appendedChar As Char
'Create a loop that would iterate from 0 to the specified value of intLenghtOfString
For i As Integer = 0 To intLenghtOfString
'Generate the char and assign it to appendedChar
appendedChar = Convert.ToChar(Convert.ToInt32(26 * randomNumber.NextDouble()) + 65)
'Append appendedChar to randomString
randomString.Append(appendedChar)
Next
'Convert randomString to String and return the result.
Return randomString.ToString()
End Function
AND THIS:
Private Function RandomStringGenerator(ByVal intLen As Integer) As String
Dim r As New Random()
Dim i As Integer
Dim strTemp As String = ""
For i = 0 To intLen
strTemp = strTemp & Chr(Int((26 * r.NextDouble()) + 65))
Next
Return r.Next
End Function
But when run, it displays something like this:
SR
SR
SR
SR
SR
SR
SR
SR
SR
SR
BR
BR
BR
BR
BR
BR
BR
KR
KR
KR
KR
and so on.
What is going on? I thought that I used to, a long time ago, be able to just do random.Next.

Answer 1

I've run into similar issues before with the Random object. The problem is that when you instantiate Random it's default seed value is the number of milliseconds since windows started up. And since you are generating random characters at several a millisecond you end up with the same seed number.
Instead you should create a shared random object instead of instantiating a new one on each call.

Answer 2

In another forum I answered a similar question and came up with this generalized function that could be used for your problem. It includes a metric that can be examined to see if there is a bias in the characters being generated.
Dim charUC As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
Dim charLC As String = charUC.ToLower
Dim charNUM As String = "0123456789"
Dim charSPEC As String = "``!##$%^&*()-_=+[{]}\|;:',<.>/?" & ControlChars.Quote
Dim charCounts As New Dictionary(Of Char, Integer)
Dim PRNG As New Random 'note - defined at class level
Private Function GetRandChars(ByVal numChars As Integer, _
Optional ByVal includeUpperCase As Boolean = False, _
Optional ByVal includeLowerCase As Boolean = False, _
Optional ByVal includeNumbers As Boolean = False, _
Optional ByVal includeSpecial As Boolean = False) As String
If numChars <= 0 Then Throw New ArgumentException 'must specify valid character count
Dim includeSel As New System.Text.StringBuilder 'contains set of characters
If includeUpperCase Then includeSel.Append(charUC) 'UC to set
If includeLowerCase Then includeSel.Append(charLC) 'LC to set
If includeNumbers Then includeSel.Append(charNUM) 'numbers to set
If includeSpecial Then includeSel.Append(charSPEC) 'specials to set
If includeSel.Length = 0 Then Throw New ArgumentException 'must tell function at least one include
Dim rv As New System.Text.StringBuilder 'return value
'generate specified number of characters
For ct As Integer = 1 To numChars
Dim chSel As Char = includeSel(PRNG.Next(includeSel.Length)) 'select random character
rv.Append(chSel)
'do counts
If charCounts.ContainsKey(chSel) Then
charCounts(chSel) += 1
Else
charCounts.Add(chSel, 1)
End If
Next
Return rv.ToString 'return the random string
End Function