Code Scanner Version 1

Hi this is a small scanner to scan some source code, I made this after I wanted to try and write a small interpreter for my self.
At the moment it supports the scanning of String,Symbols and Numbers.

Anyway this is the beginning of it. This is just the scanner. Hope you may find it sufull commends suguestions welcome.

vbnet Code:

Option Explicit On
Imports System.Text
 
Public Class Form1
 
    Private Tokens As List(Of Token)
    Private idx As Integer = 0
 
    Private Enum TokenTypes
        StrText = 0
        Symbols = 1
        Digits = 2
        EOC = 3
    End Enum
 
    Private Structure Token
        Dim Token As String
        Dim Type As TokenTypes
    End Structure
 
    Private Sub AddToken(ByVal Data As String, ByVal Type As TokenTypes)
        Dim tok As Token
        'Add token and type.
        tok.Type = Type
        tok.Token = Data
        Tokens.Add(tok)
    End Sub
 
    Private Sub InitScanner()
        'Init defaults.
        idx = 0
        Tokens = New List(Of Token)
    End Sub
 
    Private Sub Scan(ByVal Source As String)
 
        'While less than length loop.
        While idx <= (Source.Length - 1)
 
            Dim ch As Char = Source(idx)
 
            'Check for end of code.
            If (idx >= Source.Length - 1) Then
                AddToken("EOC", TokenTypes.EOC)
            End If
 
            'Skip white space
            If Char.IsWhiteSpace(ch) Then
                idx += 1
                ch = ""
            ElseIf Char.IsLetter(ch) Then
                Dim sb As New StringBuilder()
                'Get letters
 
                While Char.IsLetter(ch)
                    ch = Source(idx)
                    If Char.IsLetter(ch) Then
                        sb.Append(ch)
                    End If
                    'Inc counter
                    idx += 1
                End While
 
                'Add token and type
                AddToken(sb.ToString(), TokenTypes.StrText)
 
            ElseIf Char.IsSymbol(ch) Then
                Dim sb As New StringBuilder()
                'Get symbols
                While Char.IsSymbol(ch)
                    ch = Source(idx)
                    If Char.IsSymbol(ch) Then
                        sb.Append(ch)
                    End If
                    'INC counter
                    idx += 1
                End While
 
                'Add token and type
                AddToken(sb.ToString(), TokenTypes.Symbols)
 
            ElseIf Char.IsNumber(ch) Then
                Dim sb As New StringBuilder()
                'get digits
                While Char.IsNumber(ch)
                    ch = Source(idx)
                    If Char.IsNumber(ch) Then
                        sb.Append(ch)
                    End If
                    'INC counter
                    idx += 1
                End While
 
                'Add token and type
                AddToken(sb.ToString(), TokenTypes.Digits)
            Else
                Throw New ArgumentException("Bad Token Found.")
            End If
            'idx += 1
        End While
 
    End Sub
 
    Private Sub cmdShow_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdShow.Click
        Dim tok As New Token
        Dim s As String
 
        'Example code to scan
        s = "Var X" & vbCrLf
        s = s & "X = Hello World" & vbCrLf
        s = s & "B = 123" & vbCrLf
        s = s & "Print B" & vbCrLf
 
        'Init scanner defaults
        InitScanner()
        'Add source
        Scan(s)
 
        For Each tok In Tokens
            'Show token and type
            MessageBox.Show("Token = " & tok.Token & " Type = " & tok.Type.ToString(),
                            "Scanner Demo", MessageBoxButtons.OK, MessageBoxIcon.Information)
        Next tok
    End Sub
End Class