VB.NET Random unique generator - vb.net

Question

I'l trying to generate a unique random number generator with the snippet of code from below, but it's not working. The IF section is suppose to test if it's the first random number generated, if it is, it's suppose to add the first random number to the ArrayList, if it's not the first random number, it's supposed to check if the random number is already in the ArrayList and if it's in the ArrayList it's suppose to MsgBox and generate a new unique random number that is not already in the ArrayList and add it to the ArrayList, but it's not doing any of those. Any help would be greatly appreciated.
Public Class Form1
Dim r As New Random
Dim dLowestVal As Integer = 1
Dim dHighestVal As Integer = 26
Dim dItemAmount As Integer = 1
Dim RollCheck As New HashSet(Of Integer)
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click
End
End Sub
Private Sub btnRollDice_Click(sender As Object, e As EventArgs) Handles btnRollDice.Click
lblRandomNo.Text = r.Next(dLowestVal, dHighestVal)
lblItemAmount.Text = dItemAmount
If dItemAmount = 1 Then
RollCheck.Add(Val(lblRandomNo.Text))
ElseIf (RollCheck.Contains(Val(lblRandomNo.Text))) Then
MsgBox("Already Exists")
lblRandomNo.Text = r.Next(dLowestVal, dHighestVal)
RollCheck.Add(Val(lblRandomNo.Text))
End If
dItemAmount = dItemAmount + 1
Thanks in advance.

Answer 1

You could replace your whole method with this simple one
' This is globally declared at the top of your form
Dim values As New List(Of Integer)
' This is called when you construct your form
' It will store consecutive integers from 1 to 25 (25 elements)
values = Enumerable.Range(1, 25).ToList()
This is the method that extract an integer from your values that is not already used
Private Sub Roll()
' Get an index in the values list
Dim v = r.Next(0, values.Count)
' insert the value at that index to your RollCheck HashSet
RollCheck.Add(values(v))
' Remove the found value from the values list, so the next call
' cannot retrieve it again.
values.Remove(values(v))
End Sub
And you can call it from the previous event handler in this way
Private Sub btnRollDice_Click(sender As Object, e As EventArgs) Handles btnRollDice.Click
if values.Count = 0 Then
MessageBox("No more roll available")
else
Roll()
End Sub
End Sub

Answer 2

The point of the HashSet is that since it doesn't allow duplicates you can just check the return value of Add() to determine whether the number was successfully inserted or if it already exists in the list.
If you want to keep trying until it succeeds all you have to do is wrap it in a loop:
If dHighestVal - dLowestVal >= RollCheck.Count Then
'If the above check passes all unique values are MOST LIKELY already in the list. Exit to avoid infinite loop.
MessageBox.Show("List is full!")
Return 'Do not continue.
End If
Dim Num As Integer = r.Next(dLowestVal, dHighestVal)
'Iterate until a unique number was generated.
While Not RollCheck.Add(Num)
MessageBox.Show("Already exists!")
Num = r.Next(dLowestVal, dHighestVal)
End While
lblRandomNo.Text = Num
An alternative way of writing the loop is: While RollCheck.Add(Num) = False.