VB6 Legacy and COM Interop Patterns for maintaining VB6 code and strategies for migrating to VB.NET with COM interop. VB6 to VB.NET Migration Key Differences ' VB6 - Variant types Dim data As Variant data = 123 data = "Hello" ' VB.NET - Strong typing with Option Strict On Option Strict On Dim data As Object ' Still avoid when possible Dim number As Integer = 123 Dim text As String = "Hello" ' VB6 - Default properties Text1 . Text = "Hello" Text1 = "Hello" ' Uses default Text property ' VB.NET - Explicit properties required TextBox1 . Text = "Hello" ' Must be explicit ' VB6 - ByRef default Sub ProcessData ( data As String ) ' ByRef by default ' VB.NET - ByVal default Sub ProcessData ( data As String ) ' ByVal by default Sub ProcessData ( ByRef data As String ) ' Explicit ByRef ' VB6 - On Error On Error Resume Next On Error GoTo ErrorHandler ' VB.NET - Try-Catch Try ' Code Catch ex As Exception ' Handle error End Try Common Migration Issues ' VB6 - Fixed-length strings Dim name As String * 50 ' VB.NET - Use regular string and PadRight Dim name As String = "John" . PadRight ( 50 ) ' VB6 - Currency type Dim amount As Currency ' VB.NET - Use Decimal Dim amount As Decimal ' VB6 - Control arrays Dim TextBox ( 5 ) As TextBox ' VB.NET - Use collection Dim textBoxes As New List ( Of TextBox ) ( ) ' VB6 - Let/Set keywords Let x = 5 Set obj = New MyClass ' VB.NET - Assignment without keywords Dim x As Integer = 5 Dim obj As New MyClass ( ) COM Interop from VB.NET Early Binding (Type Library Reference) ' Add COM reference in project ' Tools -> Add Reference -> COM -> Excel Object Library Imports Excel = Microsoft . Office . Interop . Excel Public Sub ExportToExcel ( data As DataTable ) Dim excelApp As Excel . Application = Nothing Dim workbook As Excel . Workbook = Nothing Dim worksheet As Excel . Worksheet = Nothing Try excelApp = New Excel . Application ( ) workbook = excelApp . Workbooks . Add ( ) worksheet = CType ( workbook . Worksheets ( 1 ) , Excel . Worksheet ) ' Write headers For col = 0 To data . Columns . Count - 1 worksheet . Cells ( 1 , col + 1 ) = data . Columns ( col ) . ColumnName Next ' Write data For row = 0 To data . Rows . Count - 1 For col = 0 To data . Columns . Count - 1 worksheet . Cells ( row + 2 , col + 1 ) = data . Rows ( row ) ( col ) Next Next excelApp . Visible = True Catch ex As Exception MessageBox . Show ( $"Excel export failed: {ex.Message}" ) Finally ' Release COM objects If worksheet IsNot Nothing Then System . Runtime . InteropServices . Marshal . ReleaseComObject ( worksheet ) End If If workbook IsNot Nothing Then System . Runtime . InteropServices . Marshal . ReleaseComObject ( workbook ) End If If excelApp IsNot Nothing Then System . Runtime . InteropServices . Marshal . ReleaseComObject ( excelApp ) End If End Try End Sub Late Binding (No Type Library) Imports System . Reflection Public Sub CreateExcelLateBound ( ) Dim excelType As Type = Type . GetTypeFromProgID ( "Excel.Application" ) If excelType Is Nothing Then MessageBox . Show ( "Excel not installed" ) Return End If Dim excelApp As Object = Nothing Try ' Create COM object excelApp = Activator . CreateInstance ( excelType ) ' Call methods via reflection excelType . InvokeMember ( "Visible" , BindingFlags . SetProperty , Nothing , excelApp , New Object ( ) { True } ) Dim workbooks As Object = excelType . InvokeMember ( "Workbooks" , BindingFlags . GetProperty , Nothing , excelApp , Nothing ) ' Add workbook workbooks . GetType ( ) . InvokeMember ( "Add" , BindingFlags . InvokeMethod , Nothing , workbooks , Nothing ) Finally If excelApp IsNot Nothing Then System . Runtime . InteropServices . Marshal . ReleaseComObject ( excelApp ) End If End Try End Sub COM Object Release Pattern ' ✅ Good: Proper COM object cleanup Public Sub UseComObject ( ) Dim excelApp As Excel . Application = Nothing Dim workbook As Excel . Workbook = Nothing Try excelApp = New Excel . Application ( ) workbook = excelApp . Workbooks . Add ( ) ' Use objects Finally ' Release in reverse order of creation If workbook IsNot Nothing Then workbook . Close ( False ) Marshal . ReleaseComObject ( workbook ) workbook = Nothing End If If excelApp IsNot Nothing Then excelApp . Quit ( ) Marshal . ReleaseComObject ( excelApp ) excelApp = Nothing End If GC . Collect ( ) GC . WaitForPendingFinalizers ( ) End Try End Sub ' ❌ Bad: Not releasing COM objects (memory leak) Dim excelApp = New Excel . Application ( ) excelApp . Workbooks . Add ( ) ' No cleanup - Excel process remains in memory! Creating COM-Visible .NET Components COM-Visible Class Imports System . Runtime . InteropServices < ComVisible ( True )

< Guid ( "12345678-1234-1234-1234-123456789012" )

< ClassInterface ( ClassInterfaceType . None )

< ProgId ( "MyCompany.Calculator" )

Public Class Calculator Implements ICalculator Public Function Add ( a As Integer , b As Integer ) As Integer Implements ICalculator . Add Return a + b End Function Public Function Subtract ( a As Integer , b As Integer ) As Integer Implements ICalculator . Subtract Return a - b End Function End Class ' Interface for COM < ComVisible ( True )

< Guid ( "87654321-4321-4321-4321-210987654321" )

< InterfaceType ( ComInterfaceType . InterfaceIsDual )

Public Interface ICalculator Function Add ( a As Integer , b As Integer ) As Integer Function Subtract ( a As Integer , b As Integer ) As Integer End Interface Register for COM

Register assembly for COM

regasm MyAssembly.dll /tlb /codebase

Unregister

regasm MyAssembly.dll /u

Generate type library

tlbexp MyAssembly.dll Legacy VB6 Patterns to Modernize Error Handling ' VB6 style (avoid in new code) Public Function GetCustomer ( id As Integer ) As Customer On Error GoTo ErrorHandler ' Code here Exit Function ErrorHandler : MsgBox "Error: " & Err . Description Resume Next End Function ' VB.NET modern style Public Function GetCustomer ( id As Integer ) As Customer Try ' Code here Catch ex As ArgumentException MessageBox . Show ( $"Invalid argument: {ex.Message}" ) Return Nothing Catch ex As Exception MessageBox . Show ( $"Error: {ex.Message}" ) Throw End Try End Function File I/O ' VB6 style (avoid) Dim fileNum As Integer = FreeFile ( ) FileOpen ( fileNum , "data.txt" , OpenMode . Output ) PrintLine ( fileNum , "Hello World" ) FileClose ( fileNum ) ' VB.NET modern style Using writer = New StreamWriter ( "data.txt" ) writer . WriteLine ( "Hello World" ) End Using ' Or async Await File . WriteAllTextAsync ( "data.txt" , "Hello World" ) Collections ' VB6 Collection (avoid in new code) Dim customers As New Collection ( ) customers . Add ( customer , "key1" ) Dim item = customers ( "key1" ) ' VB.NET generic collections Dim customers = New Dictionary ( Of String , Customer ) ( ) customers . Add ( "key1" , customer ) Dim item = customers ( "key1" ) ' Or List(Of T) Dim customerList = New List ( Of Customer ) ( ) customerList . Add ( customer ) Migration Strategy Incremental Migration ' 1. Start with COM interop wrapper ' Wrap VB6 COM component in VB.NET Public Class VB6Wrapper Private vb6Component As Object Public Sub New ( ) vb6Component = CreateObject ( "VB6Project.Component" ) End Sub Public Function ProcessData ( data As String ) As String Return vb6Component . ProcessData ( data ) . ToString ( ) End Function End Class ' 2. Gradually replace with native VB.NET Public Class ModernComponent Public Function ProcessData ( data As String ) As String ' New VB.NET implementation Return data . ToUpper ( ) End Function End Class Best Practices ✅ DO ' Use modern VB.NET features in new code Option Strict On Option Explicit On ' Release COM objects explicitly Marshal . ReleaseComObject ( comObject ) GC . Collect ( ) ' Use Try-Catch instead of On Error Try ' Code Catch ex As Exception ' Handle End Try ' Use generics instead of Collections Dim items = New List ( Of Customer ) ( ) ' Use async for I/O Await File . WriteAllTextAsync ( "file.txt" , content ) ❌ DON'T ' Don't use VB6 compatibility module Imports Microsoft . VisualBasic . Compatibility . VB6 ' Legacy only! ' Don't use Option Strict Off Option Strict Off ' Avoid! ' Don't use On Error in new code On Error Resume Next ' VB6 style ' Don't forget to release COM objects Dim excelApp = New Excel . Application ( ) ' ... (no cleanup - memory leak!) ' Don't use late binding when early binding available Dim obj = CreateObject ( "Excel.Application" ) ' Use typed reference instead