Answer

In VB.NET, the following mechanisms are available for multithreading:

• The Thread class (System.Threading.Thread) — manual creation and running of threads. Requires detailed management, suitable for simple scenarios.
• The BackgroundWorker class — convenient for asynchronous operations in Windows Forms applications (UI remains responsive).
• The Task Parallel Library (TPL) namespace, including Task classes and the Async/Await keyword. A modern and recommended approach.

Synchronization:

• Monitors (SyncLock), mutexes, semaphores — to protect critical sections and access shared resources.

Example:

' Starting a parallel task
Imports System.Threading.Tasks

Sub StartJob()
    Task.Run(Sub()
        ' Long operation
        Console.WriteLine("Working in a separate thread")
    End Sub)
End Sub

' Using SyncLock for synchronization
Dim locker As New Object()

Sub SafeIncrement()
    SyncLock locker
        ' Code here runs atomically
    End SyncLock
End Sub

Trick question

Can you freely access Windows Forms controls from any thread? Why/Why not?

Answer:

No, access to WinForms controls is only possible from the thread in which they were created (usually the main UI thread). Violating this leads to unpredictable errors or crashes. To update the UI from another thread, the Invoke or BeginInvoke methods are used.

Example:

If TextBox1.InvokeRequired Then
    TextBox1.Invoke(Sub()
        TextBox1.Text = "Data from thread"
    End Sub)
Else
    TextBox1.Text = "Data from thread"
End If

Examples of real errors due to lack of knowledge of the subtleties of the topic

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Story

In a server application, no synchronization mechanisms were used when working with shared lists. The result — periodic InvalidOperationException exceptions and "data corruption".

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Story

In a Windows application, controls were being updated from a background thread via BackgroundWorker. The application sometimes "crashed" without explicit messages — reason: direct UI changes outside of thread-safe access.

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Story

The programmer started several threads, each of which modified a global variable without SyncLock. The result — data races, incorrect calculation outcomes, elusive bugs, and frequent reporting failures.