VB6 to .NET 10: What's Actually Different Under the Hood

The gap between VB6 and .NET 10 is more than a version upgrade - it's a generational leap in computing architecture. While your VB6 apps may still run (mostly) on Windows 11, understanding the major differences under the hood reveals why migration isn't just about modernization. It's about getting capabilities that are architecturally impossible in VB6. And I say all of this as a die-hard former VB6 product manager (yes, I'm that old), it's amazing what you can do with 2025 tech vs 1995 tech...

Memory Management: From Manual Chaos to Managed Elegance

VB6: Reference Counting and Memory Leaks

VB6 uses COM reference counting for memory management. Every object maintains a counter, incremented when referenced and decremented when released. When the counter hits zero, the object is destroyed.

 ' VB6 - Manual cleanup required
Dim objExcel As Object
Set objExcel = CreateObject("Excel.Application")
' ... use the object
Set objExcel = Nothing  ' Manual cleanup - forget this and leak memory

The problem? Circular references create immortal objects:

 ' VB6 - Memory leak from circular reference
Class Parent
    Public Child As ChildClass
End Class

Class ChildClass
    Public Parent As Parent  ' Circular reference = memory leak
End Class

.NET 10: Generational Garbage Collection

.NET uses a sophisticated generational garbage collector that automatically handles circular references and optimizes based on object lifetime patterns.

 // .NET 10 - Automatic memory management
public class Parent
{
    public Child Child { get; set; }
}

public class Child
{
    public Parent Parent { get; set; }  // No memory leak - GC handles it
}

// Objects cleaned up automatically when no longer reachable
var excel = new ExcelApplication();
// No manual cleanup needed - GC handles it

.NET 10's GC divides objects into three generations:

• Gen 0: Short-lived objects (collected frequently)
• Gen 1: Medium-lived objects (buffer between Gen 0 and Gen 2)
• Gen 2: Long-lived objects (collected rarely)

This generational approach means .NET 10 apps can handle millions of object allocations per second with minimal performance impact - something that would bring VB6 to its knees.

Threading: From Single-Threaded Apartment to True Parallelism

VB6: The Single-Threaded Bottleneck

VB6 was designed for single-threaded apartment (STA) COM components. While you could create "multi-threaded" applications using ActiveX EXEs, true parallel processing was nearly impossible.

 ' VB6 - Pseudo-multithreading with Timer control
Private Sub Timer1_Timer()
    ' This still runs on the main thread
    ' True parallel processing is not possible
    ProcessData
End Sub

.NET 10: Modern Async/Await and Parallel Processing

.NET 10 provides multiple threading models with async/await patterns that make concurrent programming almost trivial:

 // .NET 10 - True parallel processing
public async Task ProcessMillionRecordsAsync()
{
    var tasks = records.Select(async record => 
    {
        await ProcessRecordAsync(record);
    });
    
    await Task.WhenAll(tasks);  // Process all records in parallel
}

// Or use Parallel LINQ for CPU-bound operations
var results = records.AsParallel()
    .WithDegreeOfParallelism(Environment.ProcessorCount)
    .Select(r => ProcessRecord(r))
    .ToList();

.NET 10 also introduces improvements to the ThreadPool and better work-stealing algorithms, meaning your migrated application can automatically scale across all available CPU cores.

Type System: From Variants to Generics

VB6: The Variant Tax

VB6's Variant type provided flexibility at a massive performance cost:

 ' VB6 - Runtime type checking with Variants
Dim myData As Variant
myData = 123
myData = "Now I'm a string"
myData = CreateObject("Some.Object")

' Every operation requires runtime type checking
If IsNumeric(myData) Then
    myData = myData + 1  ' Runtime overhead for type coercion
End If

.NET 10: Strong Typing with Generics

.NET's generic type system provides compile-time type safety without sacrificing flexibility:

 // .NET 10 - Compile-time type safety with zero runtime overhead
public class Repository<T> where T : class
{
    private readonly List<T> _items = new();
    
    public void Add(T item) => _items.Add(item);
    public T Get(int id) => _items[id];  // Type-safe at compile time
}

// Use with any type - no boxing/unboxing overhead
var customerRepo = new Repository<Customer>();
var orderRepo = new Repository<Order>();

Performance: Interpreted P-Code vs JIT-Compiled Machine Code

VB6: P-Code Interpretation

VB6 compiles to P-Code (pseudo-code) by default, which is interpreted at runtime:

 ' VB6 - This loop runs as interpreted P-Code
For i = 1 To 1000000
    total = total + i
Next i

Even when compiled to native code, VB6's optimizer is primitive compared to modern standards.

.NET 10: Tiered JIT Compilation

.NET 10 uses tiered compilation with the RyuJIT compiler:

1. Tier 0: Quick compilation for fast startup
2. Tier 1: Optimized recompilation for hot paths

 // .NET 10 - This loop gets progressively optimized
for (int i = 0; i < 1_000_000; i++)
{
    total += i;  // RyuJIT may vectorize this using SIMD instructions
}

.NET 10's JIT can:

• Vectorize loops using SIMD instructions
• Inline methods across assembly boundaries
• Eliminate bounds checking when proven safe
• Perform escape analysis to stack-allocate objects

Performance improvements can be 10-100x for computational workloads.

Error Handling: From GOTO Hell to Structured Exceptions

VB6: The On Error Goto Maze

VB6's error handling is notorious for creating unmaintainable code:

 ' VB6 - Error handling nightmare
Private Function ProcessData() As Boolean
    On Error GoTo ErrorHandler
    
    ' Some code
    Open "file.txt" For Input As #1
    
    On Error GoTo FileError  ' Changed error handler mid-function
    ' More code
    
    On Error GoTo 0  ' Disabled error handling
    ' Dangerous code
    
    On Error Resume Next  ' Ignore all errors
    ' Even more dangerous code
    
    Exit Function
    
ErrorHandler:
    MsgBox "Error: " & Err.Description
    Resume Next
    
FileError:
    MsgBox "File Error"
    ProcessData = False
End Function

.NET 10: Structured Exception Handling

.NET provides structured, hierarchical exception handling:

 // .NET 10 - Clear, structured error handling
public async Task<bool> ProcessDataAsync()
{
    try
    {
        await using var file = File.OpenRead("file.txt");
        await ProcessFileAsync(file);
        return true;
    }
    catch (FileNotFoundException ex)
    {
        _logger.LogError(ex, "File not found");
        return false;
    }
    catch (IOException ex) when (ex.HResult == -2147024864)
    {
        _logger.LogError(ex, "File locked");
        await Task.Delay(1000);
        return await ProcessDataAsync();  // Retry
    }
    finally
    {
        // Cleanup code always runs
        await CleanupAsync();
    }
}

Runtime Features: From Windows-Only to Cross-Platform

VB6: Windows API Imprisonment

VB6 is forever tied to Windows:

 /' VB6 - Windows API calls
Private Declare Function GetWindowsDirectory Lib "kernel32" _
    Alias "GetWindowsDirectoryA" (ByVal lpBuffer As String, _
    ByVal nSize As Long) As Long

.NET 10: Cross-Platform Freedom

.NET 10 runs on Windows, Linux, macOS, and ARM processors:

 // .NET 10 - Platform-agnostic code
var path = Environment.GetFolderPath(Environment.SpecialFolder.System);
// Works on Windows, Linux, macOS

// Platform-specific code when needed
if (OperatingSystem.IsWindows())
{
    // Windows-specific implementation
}
else if (OperatingSystem.IsLinux())
{
    // Linux-specific implementation
}

Modern Language Features VB6 Can't Touch

.NET 10 introduces language features that fundamentally change how you write code:

Pattern Matching

 var result = shape switch
{
    Circle { Radius: > 10 } => "Large circle",
    Rectangle { Width: var w, Height: var h } when w == h => "Square",
    Triangle t when t.IsEquilateral => "Equilateral triangle",
    _ => "Other shape"
};

Records and Immutability

 public record Customer(string Name, string Email);
var customer = new Customer("John", "john@example.com");
var updated = customer with { Email = "newemail@example.com" };

Nullable Reference Types

 string? nullable = null;  // Explicitly nullable
string nonNullable = "Can't be null";  // Compiler enforces non-null

Span<T> for Zero-Allocation Processing

 ReadOnlySpan<char> span = "Hello World".AsSpan(0, 5);
// Process string without allocations

The Performance Reality Check

Here's what our customers typically see after migration:

• Memory usage: 40-60% reduction
• CPU utilization: 50-70% reduction for same workload
• Throughput: 5-20x improvement for data processing
• Startup time: 30-50% faster with .NET 10's tiered compilation
• Scalability: Linear scaling up to available CPU cores (vs VB6's single-thread limit)

Why This Matters for Your Migration

Understanding these architectural differences is important for migration planning:

1. Don't just translate code - reimagine it using modern patterns
2. Use async/await instead of recreating VB6's synchronous patterns
3. Use generics instead of variants for type-safe, performant code
4. Embrace the GC instead of manual memory management patterns
5. Design for parallelism from the start

The jump from VB6 to .NET 10 isn't incremental - it's transformational. Your migrated app won't just run on modern infrastructure; it will be able to take advantage of architectural improvements that deliver order-of-magnitude improvements in performance, reliability and maintainability.

At GAPVelocity AI our hybrid AI migration tools understand these differences. We do more than convert syntax - we transform your application so that you can use all of .NET 10's architectural advantages - ensuring your migrated code is truly modern - not just VB6 in C# clothing.

If you're stuck on how to move your VB6 apps to .NET 10, contact us and we'll show you how to quickly and securely transform your legacy code into modern, high-performance .NET 10 apps.