One customers brought an issue to our attention. He was using a migrated VB.NET component in a heavy multi-threaded environment (the component is called by ASP.NET pages) and found that occasionally the program crashed with an unexplainable error message. A quick look at the stack trace confirmed the initial suspect: the error was caused by two threads that initiates a call to the same method at the same time. We have fixed the problem in our library in a few minutes (the fix will appear in version 1.10.03, due in a few days), but we realized that the problem isn't confined to our library. To explain why, a short digression is in order.

VB6 components use Single-Threaded Apartment (STA) model: each thread lives in an “apartment” of its own. In plain English, this means that each thread works with a different set of global variables. For example, assume that the ActiveX DLL project contains a BAS module with the following variable declaration:

    ' ...(in the UserInfo.bas file)
    Public UserName As String

Thanks to STA threading model, each client – running in a different thread – can write this variable and read it back, without worrying about another client (in another thread) overwriting the variable with a different value. As you see, STA makes multi-threading programming virtually as easy as single-thread (i.e. traditional) programming. Of course,when writing multi-threaded apps you have to take care of many other potential concurrency issues – for example, two threads working on the same temporary file or registry key – but at least you don’t need to worry about global variables.

By contrast, components written in VB.NET – and all .NET languages, for that matter – use free-threading, which means that all threads can access all variables at the same time. Therefore, the VB.NET component exposes the same UserName field to all threads, which means that any thread can overwrite the value stored there by another thread.

To simplify programming in free-threaded contests, the .NET Framework supports the ThreadStatic attribute. You can apply this attribute only to static class fields – either fields defined in modules or fields defined in classes and marked with the Shared kewword:

    <ThreadStatic()> _
    Public UserName As String

All fields marked with the ThreadStatic attribute are stored in the Thread Local Storage (TLS) area. The separation among threads is automatic, because each thread owns a different TLS.

In theory, when migrating a VB6 DLL meant to be used by free-threaded clients, you should scrutinize each and every variable in BAS modules and decide whether to tag them with the ThreadStatic attribute. To make this task as easy as possible, as well as to avoid any manual edit of the resulting VB.NET code, starting with version 1.10.03 VB Migration Partner supports the ThreadSafe pragma, which can be applied at the project-, file-, method- and variable-level scope. In most cases, therefore, all you need to do is using a single pragma that affects all the variables in the current project:

    '## project:ThreadSafe True
    Public UserName As String