VB.NET - Randomize() with a function call in a string.replace method - vb.net

Question

I have a chat system and i want to put a "random string generator".
In my chat i have to write "%random%" and it is replaces with a random string.
I have a problem though, if i type "%random%%random%%random%" for example, it will generate the same string 3 times.
• Here is my function:
Public Function getRandomString(ByVal len As Integer) As String
Randomize()
Dim stringMap as string = "abcdefghijklmnopqrstuwvxyz0123456789"
Dim rndString As String = ""
Dim rnd As New Random()
For i As Integer = 0 To len - 1
Randomize()
rndString &= stringMap.Substring(rnd.Next(0, stringMap.Length), 1)
Next
Return rndString
End Function
• And here is my function call:
Dim msg As String = "Random string: %random%%random%%random%"
msg = msg.Replace("%random%", getRandomString(8))
MsgBox(msg)
The output for example: Random string: 5z15if725z15if725z15if72
I guess this is because it keeps the 1st return value in memory and pastes it, how can i fix that ?
Do i have to make a string.replace function myself ? Thanks

Answer 1

Oh no! You shouldn't call Randomize() here at all! Random is used in combination with the Rnd() function of VB. Creating a new Random object is enough here.
The reason you are getting the same results every time is because you are creating a new Random every time. You should reuse the same object to get different results.
'Create the object once
Private Shared rnd As New Random()
Public Function getRandomString(ByVal len As Integer) As String
Dim stringMap as string = "abcdefghijklmnopqrstuwvxyz0123456789"
Dim rndString As String = ""
For i As Integer = 0 To len - 1
rndString &= stringMap.Substring(rnd.Next(0, stringMap.Length), 1)
Next
Return rndString
End Function
EDIT: I realize that in addition to the above changes, you need to call the getRandomString function for every "%random%". String.Replace only calls the function once and pastes the result everywhere. With Regex, you could do something like this:
msg = new Regex("%random%").Replace(input, Function (match) getRandomString(8))

Answer 2

An easy way to do it is to find the first occurrence of "%random%", replace that, then repeat as necessary.
Written as a console application:
Option Infer On
Module Module1
Dim rand As New Random
Public Function getRandomString(ByVal len As Integer) As String
Dim stringMap As String = "abcdefghijklmnopqrstuwvxyz0123456789"
Dim rndString As String = ""
For i As Integer = 0 To len - 1
rndString &= stringMap.Substring(rand.Next(0, stringMap.Length), 1)
Next
Return rndString
End Function
Function ReplaceRandoms(s As String) As String
Dim stringToReplace = "%random%"
Dim r = s.IndexOf(stringToReplace)
While r >= 0
s = s.Remove(r, stringToReplace.Length).Insert(r, getRandomString(stringToReplace.Length))
r = s.IndexOf(stringToReplace)
End While
Return s
End Function
Sub Main()
Dim msg As String = "Random string: %random%%random%%random%"
msg = ReplaceRandoms(msg)
Console.WriteLine(msg)
Console.ReadLine()
End Sub
End Module