VB6

.cls files are used for VB "classes". These are COM classes. .bas files are used for global routines.
The name of the .cls file is important: the progid for that class is made up: projectname.classname.XX
Class initialize/terminate
```
      Private Sub Class_Initialize()
        'body
      End Sub

      Private Sub Class_Terminate()
        'body
      End Sub
     
```
Class_initialize() corresponds to the FinalInitialize() in COM/ATL. It is called once, when the first reference to this component is created.
Class_terminate() corresponds to the FinalTerminate() in COM/ATL. It is called once, when the reference count goes to 0.
'Options Explicit' causes the compiler to check for missing declarations etc.
all data declarations are private by default. 'Public' causes the data to be visible in the component's interface
Const xx = 9 creates a manifest constant. These can be public too.
Variant types are VARIANT in C++. Variant's can hold any other data type.
App.LogEvent "msg" causes a message to be written to the NT event log
Set objXX = CreateObject("a.progid.goes.here") is late binding (run-time) of an object. The 'Set' is required. The IDE's intellisense does not work.
```
        dim XX as AReferenceGoesHere
        set XX = new AReferenceGoesHere

      
```
The 'Set' is required. Intellisense works.
Set XX = Nothing causes the object to be released. In COM/ATL the FinalRelease()

If-then-else

      if  abc = 21 then
         abc = 22
      ElseIf abc = 22 Then
         abc = 23
      end if

Loop structures

      dim i
      for i = 1 to 10
         'loop body
      next i

      Do While Not aValue
        ' loop body
      loop

      For Each Child In Children
         'loop body
      Next

Misc. Flow of control

        Exit Sub
        Exit Function

Exceptions Typical error handling:

      Sub xx
        On Error GoTo ErrHandler

        'sub body here
        Exit Sub

       ErrHandler:
          'error handler code here
          errNbr = Err.Number        'holds the error id
          errDescr = Err.Description 'a string explaining the error
     End Sub

This causes any errors to set the Err global variable and continue on.

      On Error Resume Next

This causes error handling to revert to the system default (i.e. put up a msg box.

      On Error GoTo 0

String manipulations:

         UCase$("abc")    ' returns "ABC"
         Right$("ABC", 2) ' returns "BC"
         Left$("ABC", 1)  ' returns "A"
         Len$("ABC")      ' returns 3
         Trim("  ABC  ")  ' returns "ABC"
         Mid("ABC", 1, 1) ' returns "B"

Conversions

         CInt("123")  ' returns an integer 123
         CStr(123)    ' returns a string "123"
         CSng("1.23") ' returns a single (C++ => float)

Properties:

       Public Property Let CustomerID(RHS As String)
         mLocalVrbl = RHS
       End Property

       Public Property Get CustomerID() As Variant
         CustomerID = mLocalVrbl
       End Property

Parameters:
```
      private sub XX(ByVal theIndex As String, ByRef theValue As String)
      
```
All parameters are passed by reference. The ByVal is default and is not required.
ByVal indicates that the reference is is a constant, e.g. it is an [in] parameter. The value contained in theIndex is not constant, it can be changed. However Strings are immutable and so the caller sees no changes. If theIndex was an object, it can be changed by XX.
ByRef indicates that it is an [out] parameter. The reference can be changed. If theValue points to a new string, the caller sees the change.

File System interaction

        Dim fs As Scripting.FileSystemObject
        Set fs = New Scripting.FileSystemObject
        If Not fs.FolderExists(sBuildPath) Then
           fs.CreateFolder (sBuildPath & "\")
        endif

        Dim aFlatFile As Scripting.TextStream
        Set aFlatFile = fs.CreateTextFile(sFileName, True)
        aFlatFile.WriteLine ("abc" & Chr(9) & "def")
        aFlatFile.Close

The Message Box debugging methd
```
       MsgBox mAvariable
      
```