Auto Incrementing letters

Try this code
(for sake of simplicity it assumes the input string being uppercase)

Public Function Increment(ByVal s As String) As String
    Dim index As Integer = s.Length - 1
    Dim n As Integer
    While index > 0 And s.Chars(index) = "Z"
        index = index - 1
    End While
 
    Increment = ""
    For n = 0 To index - 1
        Increment = Increment + s.Chars(n)
    Next
 
    If index > 0 Then
        Increment = Increment + Microsoft.VisualBasic.ChrW(Microsoft.VisualBasic.AscW(s.Chars(index)) + 1)
 

        For n = index + 1 To s.Length - 1
            Increment = Increment + "A"
        Next
    Else
        Increment = "A" + s
    End If
 
End Function
 

Sub Main()
    Dim nxt As String
    nxt = Increment("ZZABZZ")
End Sub

Hi
 
You will need to load the the alphabet in an array

Dim letters(25) As String
 
letters(0) = "A"
letters(1) = "B"
letters(2) = "C"
letters(3) = "D"
letters(4) = "E"
etc.

Then You will need to grab the Text box's contents.

Dim letterGrab As String
 
letterGrab = tbLetter.Text

Now get the letter's index

Dim found As Boolean = False
Dim FoundIndex As Integer = 0
For i As Integer = 0 To 25
 
    If letters(i) = letterGrab Then
        found = True
        FoundIndex = i
        Exit For
    End If
Next

Finally get the new Letter and put it in the text box

If found = True Then
    FoundIndex = FoundIndex + 1
    tbLetter.Text = letters(FoundIndex)
        
Else
'What to do if end of array reached
End If

 
If you need case insensitive comparison, change code block 3 to this:

Dim found As Boolean = False
Dim FoundIndex As Integer = 0
For i As Integer = 0 To 25
Dim CaseDiff As Integer  = String.Compare(letters(i), letterGrab, True)
 
    If CaseDiff = 0 Then
        found = True
        FoundIndex = i
        Exit For
    End If
Next

 
Hope This Helps
Jacques