[VB6] - Multithreading in VB6 part 1

The most recent solution.

Hello everyone! Many people wonder multithreaded programs written in VB6. Write multithreaded programs in VB6 quite real, I have many examples that I also published in my blog, but there are restrictions, one way or another can be circumvented. I consider this question in this post will not, and will consider more correct (in terms of programming in VB6) method of of multithreading - using objects. In this method, there are no restrictions, unlike threading Standart EXE, and has all the advantages of OOP. Also, I hasten to note that the IDE is not intended for debugging multithreaded programs, so to debug such programs in the IDE will not work. For debugging I use another debugger. You can also debug streams separately, and then collect the EXE.
Using multiple threads, we have the ability to call methods asynchronously while maintaining synchronicity; ie we can call methods as well as in a separate thread, and in his. For example methods require large computational load should cause asynchronously and receive, at the end of the notice in the form of events. Such methods (properties) that work fast, you can call synchronously.
One of the problems create a thread on VB6 in Standart EXE, is the inability to use WinAPI calls functions through Declare. Unlike the functions declared in a type library and entering the import, Declared-function after each call to set the properties of the object variable Err.LastDllError. This is done by calling the function __vbaSetSystemError of MSVBVM. Object Err, is thread-dependent, and the reference to it is in the thread local storage (TLS). For each thread must create its own object Err, otherwise the function call __vbaSetSystemError, runtime inquiry link from the storage, and we have it is not there (or rather there is 0) and will read the wrong address, as a consequence of crash.
To prevent this behavior, you can declare a function in tlb, then the function will not be called __vbaSetSystemError. You can also initialize the Err object, create an object instance of the DLL in the new thread, then the runtime initializes the object itself. But to create a new object, you must first initialize the thread to work with COM, it needs to call CoInitialize (Ex), but we can not call functions. It is possible to declare a tlb (it only one), then all is fair; it can also be called from assembler code or in any other way. I always go to another. Why do I LastDllError? I can just simply call GetLastError himself when I need to. So I just find the address of the function __vbaSetSystemError and write the first instruction output from the procedure (ret). This is certainly not so nice, but reliably and quickly. You can have only one function CoInitialize, and then restore __vbaSetSystemError.
Now we can call Declared-function in a new thread, which gives us endless possibilities. After creating the object (CreateObject), we can call its methods, properties, events receive from him, etc., but just a link between streams can not be passed because errors can occur because of concurrent access to data, etc. To send a link exists between threads marshaling. We will use the universal marshaller, because we ActiveX DLL has a type library. The principle of work, I will not describe in detail, it has a lot of articles online. The general sense is that instead of a direct call to the object, the RPC request to another computer / process / thread. For processing queries need to use the message loop, and once it happened, then the communication between threads is done through the posts.
To test, I wrote a simple ActiveX DLL that lets you download a file from a network that has several methods and generates events.

Code:

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' Класс MultithreadDownloader - класс загрузчика
' © Кривоус Анатолий Анатольевич (The trick), 2014
 
Option Explicit
 
Public Enum ErrorCodes
    OK
    NOT_INITIALIZE
    ERROR_CREATING_DST_FILE
End Enum
 
Private Declare Function InternetCloseHandle Lib "wininet" (ByRef hInternet As Long) As Boolean
Private Declare Function InternetOpen Lib "wininet" Alias "InternetOpenW" (ByVal lpszAgent As Long, ByVal dwAccessType As Long, ByVal lpszProxy As Long, ByVal lpszProxyBypass As Long, ByVal dwFlags As Long) As Long
Private Declare Function InternetOpenUrl Lib "wininet" Alias "InternetOpenUrlW" (ByVal hInternet As Long, ByVal lpszUrl As Long, ByVal lpszHeaders As Long, ByVal dwHeadersLength As Long, ByVal dwFlags As Long, ByRef dwContext As Long) As Long
Private Declare Function InternetReadFile Lib "wininet" (ByVal hFile As Long, lpBuffer As Any, ByVal dwNumberOfBytesToRead As Long, ByRef lpdwNumberOfBytesRead As Long) As Integer
Private Declare Function HttpQueryInfo Lib "wininet" Alias "HttpQueryInfoW" (ByVal hRequest As Long, ByVal dwInfoLevel As Long, lpBuffer As Any, ByRef lpdwBufferLength As Long, ByRef lpdwIndex As Long) As Long
Private Declare Function CreateFile Lib "kernel32" Alias "CreateFileW" (ByVal lpFileName As Long, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, lpSecurityAttributes As Any, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) As Long
Private Declare Function WriteFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToWrite As Long, lpNumberOfBytesWritten As Long, lpOverlapped As Any) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
 
Private Const INTERNET_OPEN_TYPE_PRECONFIG  As Long = 0
Private Const INTERNET_FLAG_RELOAD          As Long = &H80000000
Private Const HTTP_QUERY_CONTENT_LENGTH    As Long = 5
Private Const HTTP_QUERY_FLAG_NUMBER        As Long = &H20000000
Private Const CREATE_ALWAYS                As Long = 2
Private Const FILE_ATTRIBUTE_NORMAL        As Long = &H80
Private Const INVALID_HANDLE_VALUE          As Long = -1
Private Const GENERIC_WRITE                As Long = &H40000000
 
Public Event Complete()
Public Event Error(ByVal Code As Long)
Public Event Progress(ByVal Size As Currency, ByVal TotalSize As Currency, cancel As Boolean)
 
Private mBufferSize As Long
Private mError      As ErrorCodes
 
Dim hInternet  As Long
 
Public Property Get ErrorCode() As ErrorCodes
    ErrorCode = mError
End Property
 
Public Property Get BufferSize() As Long
    BufferSize = mBufferSize
End Property
Public Property Let BufferSize(ByVal Value As Long)
    If Value > &H1000000 Or Value < &H400 Then Err.Raise vbObjectError, "MultithreadDownloader", "Wrong buffer size": Exit Property
    mBufferSize = Value
End Property
 
Public Sub Download(URL As String, Filename As String)
    Dim hFile  As Long
    Dim hDst    As Long
    Dim fSize  As Currency
    Dim total  As Long
    Dim prgSize As Currency
    Dim cancel  As Boolean
    Dim buf()  As Byte
   
    If hInternet = 0 Then mError = NOT_INITIALIZE: RaiseEvent Error(mError): Exit Sub
    hFile = InternetOpenUrl(hInternet, StrPtr(URL), 0, 0, INTERNET_FLAG_RELOAD, 0)
   
    If hFile = 0 Then mError = Err.LastDllError: RaiseEvent Error(mError): Exit Sub
   
    If HttpQueryInfo(hFile, HTTP_QUERY_CONTENT_LENGTH Or HTTP_QUERY_FLAG_NUMBER, fSize, 8, 0) Then
        hDst = CreateFile(StrPtr(Filename), GENERIC_WRITE, 0, ByVal 0&, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0)
        If hDst = INVALID_HANDLE_VALUE Then mError = ERROR_CREATING_DST_FILE: RaiseEvent Error(mError): Exit Sub
        ReDim buf(mBufferSize - 1)
        Do
            If InternetReadFile(hFile, buf(0), mBufferSize, total) = 0 Then
                mError = Err.LastDllError
                RaiseEvent Error(mError)
                InternetCloseHandle hFile
                Exit Sub
            End If
            WriteFile hDst, buf(0), total, 0, ByVal 0&
            prgSize = prgSize + CCur(total) / 10000@
            RaiseEvent Progress(prgSize, fSize, cancel)
        Loop While (total = mBufferSize) And Not cancel
        CloseHandle hDst
        RaiseEvent Complete
    Else
        mError = Err.LastDllError
        RaiseEvent Error(mError)
    End If
    InternetCloseHandle hFile
    mError = OK
End Sub
 
Private Sub Class_Initialize()
    ' Инициализация WinInet
    hInternet = InternetOpen(StrPtr(App.ProductName), INTERNET_OPEN_TYPE_PRECONFIG, 0, 0, 0)
    mBufferSize = &H10000
End Sub
 
Private Sub Class_Terminate()
    ' Деинициализация
    If hInternet Then InternetCloseHandle hInternet
End Sub

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The code basically simple, if you read the description of the API functions. When calling the method "Download", starts will download from time to time (depending on the size of the buffer) event is generated Progress. If an error occurs, an event "Error", and at the end of the "Complete". "BufferSize" - sets the size of the buffer, which is generated when filling event. Demo code and contains bugs.*
Class I named "MultithreadDownloader", and the library "MTDownloader", respectively ProgID of the object - "MTDownloader.MultithreadDownloader". After compiling obtain a description of the interfaces through OleView, PEExplorer etc. In my example, CLSID = {20FAEF52-0D1D-444B-BBAE-21240219905B}, IID = {DF3BDB52-3380-4B78-B691-4138300DD304}. I also put a check "RemoteServerFiles" to get the output type library for our DLL, and will connect it instead of DLL for guaranteed start of the application.

Client application code:
Form:

Code:

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' frmDownloader.frm - форма загрузчика
' © Кривоус Анатолий Анатольевич (The trick), 2014
 
Option Explicit
 
' Объявляем объектную переменную с подпиской на события
Dim WithEvents Downloader As MTDownloader.MultithreadDownloader
 
Dim param  As ThreadData  ' Данные потока
Dim tid    As Long        ' ИД потока
Dim hThread As Long        ' Описатель потока
Dim mCancel As Boolean      ' Если отмена закачки
Dim mActive As Boolean      ' Если активна закачка
 
' // Отмена
Private Sub cmdCancel_Click()
    mCancel = True
End Sub
 
' // Скачать файл
Private Sub cmdDownload_Click()
    Dim ptr As Long
    ' Проверяем, идет ли уже вызов
    If WaitForSingleObject(param.hEvent, 0) = WAIT_OBJECT_0 Then
        ' Упаковываем параметры
        ptr = MT_DOWNLOAD_packParam(txtURL.Text, txtPath.Text)
        If ptr Then
            mCancel = False
            mActive = True
            ' Очистка прогрессбара
            picProgress.Cls
            ' Отправляем запрос на асинхронный вызов метода в другом потоке
            PostThreadMessage tid, WM_MT_DOWNLOAD, 0, ptr
        Else
            MsgBox "Не удалось упаковать параметры", vbCritical
        End If
    Else
        MsgBox "Скачивание еще идет", vbInformation
    End If
End Sub
 
' // Окончание загрузки
Private Sub Downloader_Complete()
    mActive = False
    MsgBox "Загрузка завершена"
End Sub
 
' // Ошибка загрузки
Private Sub Downloader_Error(ByVal Code As Long)
    mActive = False
    MsgBox "Ошибка"
End Sub
 
' // Прогресс
Private Sub Downloader_Progress(ByVal Size As Currency, ByVal TotalSize As Currency, cancel As Boolean)
    Dim sVal    As String
    Dim wTxt    As Single
   
    cancel = mCancel
    picProgress.Cls
    picProgress.Line (0, 0)-(Size / TotalSize, 1), vbRed, BF
   
    sVal = Format(Size / TotalSize, "##0%")
    wTxt = picProgress.TextWidth(sVal)
    picProgress.CurrentX = (1 - wTxt) / 2
    picProgress.CurrentY = 0
    picProgress.Print sVal
    picProgress.Refresh
   
End Sub
 
Private Sub Form_Initialize()
    InitCommonControlsEx 3435973.8623@
End Sub
 
Private Sub Form_Load()
    Dim iid As UUID
    Dim obj As MTDownloader.MultithreadDownloader
    ' Удаляем вылет Declared функций
    RemoveLastDllError
    ' Создаем синхронизирующий объект
    param.hEvent = CreateEvent(ByVal 0&, 1, 0, 0)
    ' Создаем поток
    hThread = CreateThread(ByVal 0&, 0, AddressOf ThreadProc, ByVal VarPtr(param), 0, tid)
    If hThread = 0 Then
        MsgBox "Не удалось создать поток", vbCritical
        End
    End If
    ' Ждем инициализацию объекта
    WaitForSingleObject param.hEvent, INFINITE
    ' Если успешно
    If param.IStream Then
        ' Преобразуем интерфейс в бинарную форму
        IIDFromString StrPtr(IID_MultithreadDownloader), iid
        ' Получаем отмаршаленный указатель на объект
        CoGetInterfaceAndReleaseStream param.IStream, iid, obj
        Set Downloader = obj
    Else
        MsgBox "Не удалось создать объект", vbCritical
        End
    End If
    ' Проверяем корректность инициализации объекта
    If Downloader.ErrorCode = NOT_INITIALIZE Then
        MsgBox "Объект не инициализирован", vbCritical
        End
    End If
    ' Задаем размер буфера в 100 кб
    Downloader.BufferSize = &H10000
End Sub
 
Private Sub Form_Unload(cancel As Integer)
    If mActive Then
        MsgBox "Идет загрузка"
        cancel = True
        Exit Sub
    End If
    If tid Then
        ' Освобождаем объект
        Set Downloader = Nothing
        ' Отправляем запрос на завершение потока
        PostThreadMessage tid, WM_QUIT, 0, 0
        ' Ждем завершение, т.к. поток ссылается на param
        WaitForSingleObject hThread, INFINITE
        ' Закрываем описатели
        CloseHandle hThread
        CloseHandle param.hEvent
    End If
End Sub

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Standart module:

Code:

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' modMain.bas - главный модуль загрузчика
' © Кривоус Анатолий Анатольевич (The trick), 2014
 
Option Explicit
 
Public Type POINTAPI
    x  As Long
    y  As Long
End Type
Public Type msg
    hwnd    As Long
    message As Long
    wParam  As Long
    lParam  As Long
    time    As Long
    pt      As POINTAPI
End Type
Public Type UUID
    Data1          As Long
    Data2          As Integer
    Data3          As Integer
    Data4(0 To 7)  As Byte
End Type
 
Public Declare Function GetMessage Lib "user32" Alias "GetMessageW" (lpMsg As msg, ByVal hwnd As Long, ByVal wMsgFilterMin As Long, ByVal wMsgFilterMax As Long) As Long
Public Declare Function TranslateMessage Lib "user32" (lpMsg As msg) As Long
Public Declare Function DispatchMessage Lib "user32" Alias "DispatchMessageW" (lpMsg As msg) As Long
Public Declare Function PostThreadMessage Lib "user32" Alias "PostThreadMessageW" (ByVal idThread As Long, ByVal msg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
Public Declare Function GetMem1 Lib "msvbvm60" (Src As Any, Dst As Any) As Long
Public Declare Function GetMem4 Lib "msvbvm60" (Src As Any, Dst As Any) As Long
Public Declare Function VirtualProtect Lib "kernel32" (ByVal lpAddress As Long, ByVal dwSize As Long, ByVal flNewProtect As Long, lpflOldProtect As Long) As Long
Public Declare Function GetModuleHandle Lib "kernel32" Alias "GetModuleHandleW" (ByVal lpModuleName As Long) As Long
Public Declare Function GetProcAddress Lib "kernel32" (ByVal hModule As Long, ByVal lpProcName As String) As Long
Public Declare Function CreateThread Lib "kernel32" (lpThreadAttributes As Any, ByVal dwStackSize As Long, ByVal lpStartAddress As Long, lpParameter As Any, ByVal dwCreationFlags As Long, lpThreadId As Long) As Long
Public Declare Function CoInitialize Lib "ole32" (pvReserved As Any) As Long
Public Declare Function CoUninitialize Lib "ole32" () As Long
Public Declare Function CLSIDFromString Lib "ole32" (ByVal lpsz As Long, pclsid As UUID) As Long
Public Declare Function IIDFromString Lib "ole32" (ByVal lpsz As Long, lpiid As UUID) As Long
Public Declare Function CoCreateInstance Lib "ole32" (rclsid As UUID, ByVal pUnkOuter As Long, ByVal dwClsContext As Long, riid As UUID, ppv As Any) As Long
Public Declare Function CoMarshalInterThreadInterfaceInStream Lib "ole32.dll" (riid As UUID, ByVal pUnk As IUnknown, ppStm As Long) As Long
Public Declare Function CoGetInterfaceAndReleaseStream Lib "ole32.dll" (ByVal pStm As Long, riid As UUID, pUnk As Any) As Long
Public Declare Function WaitForSingleObject Lib "kernel32" (ByVal hHandle As Long, ByVal dwMilliseconds As Long) As Long
Public Declare Function SetEvent Lib "kernel32" (ByVal hEvent As Long) As Long
Public Declare Function ResetEvent Lib "kernel32" (ByVal hEvent As Long) As Long
Public Declare Function CreateEvent Lib "kernel32" Alias "CreateEventW" (lpEventAttributes As Any, ByVal bManualReset As Long, ByVal bInitialState As Long, ByVal lpName As Long) As Long
Public Declare Function lstrcpy Lib "kernel32" Alias "lstrcpyW" (lpString1 As Any, lpString2 As Any) As Long
Public Declare Function lstrlen Lib "kernel32" Alias "lstrlenW" (lpString As Any) As Long
Public Declare Function HeapAlloc Lib "kernel32" (ByVal hHeap As Long, ByVal dwFlags As Long, ByVal dwBytes As Long) As Long
Public Declare Function HeapFree Lib "kernel32" (ByVal hHeap As Long, ByVal dwFlags As Long, lpMem As Any) As Long
Public Declare Function GetProcessHeap Lib "kernel32" () As Long
Public Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
Public Declare Function InitCommonControlsEx Lib "comctl32" (icc As Any) As Long
 
Public Const CLSCTX_INPROC_SERVER          As Long = 1
Public Const PAGE_EXECUTE_READWRITE        As Long = &H40&
Public Const CLSID_MultithreadDownloader    As String = "{20FAEF52-0D1D-444B-BBAE-21240219905B}"
Public Const IID_MultithreadDownloader      As String = "{DF3BDB52-3380-4B78-B691-4138300DD304}"
Public Const WM_APP                        As Long = &H8000&
Public Const WM_QUIT                        As Long = &H12
Public Const WM_MT_DOWNLOAD                As Long = WM_APP    ' Сообщение потоку чтобы вызвать метод
Public Const INFINITE                      As Long = -1&
Public Const WAIT_OBJECT_0                  As Long = 0
 
Public Type ThreadData
    hEvent  As Long    ' Объект синхронизации
    IStream As Long    ' Объект потока, получающий ссылку на отмаршалленый объект MultithreadDownloader
End Type
 
' // Удаляем вылеты Declare функций, жертвуя Err.LastDllError
' // Если использовать tlb, то не нужна.
Public Sub RemoveLastDllError()
    Dim hMod    As Long
    Dim lpProc  As Long
    ' Получаем адрес функции __vbaSetSystemError
    hMod = GetModuleHandle(StrPtr("msvbvm60"))
    lpProc = GetProcAddress(hMod, "__vbaSetSystemError")
    ' Делаем ret
    VirtualProtect lpProc, 1, PAGE_EXECUTE_READWRITE, 0
    GetMem1 &HC3, ByVal lpProc
End Sub
 
' // Функция потока
Public Function ThreadProc(value As ThreadData) As Long
    Dim clsid  As UUID
    Dim iid    As UUID
    Dim obj    As MTDownloader.MultithreadDownloader
 
    ' Инициализируем COM
    CoInitialize ByVal 0&
    ' Инициализируем CLSID и IID для создания и управления объектом
    IIDFromString StrPtr(IID_MultithreadDownloader), iid
    CLSIDFromString StrPtr(CLSID_MultithreadDownloader), clsid
    ' Создаем объект MTDownloader.MultithreadDownloader
    If CoCreateInstance(clsid, 0, CLSCTX_INPROC_SERVER, iid, obj) = 0 Then
        ' Маршаллинг для отлова событий в другом потоке
        CoMarshalInterThreadInterfaceInStream iid, obj, value.IStream
        ' Объект инициализирован
        SetEvent value.hEvent
    Else
        ' Объект не инициализирован
        SetEvent value.hEvent
        ' Деинициализация
        CoUninitialize
        ' Выход
        Exit Function
    End If
   
    Dim msg As msg
    Dim ret As Long
    Dim URL As String
    Dim fle As String
   
    ' Цикл обработки сообщений в новом потоке
    Do
        ret = GetMessage(msg, 0, 0, 0)
        If ret = -1 Or ret = 0 Then Exit Do
        ' Проверяем сообщения
        Select Case msg.message
        Case WM_MT_DOWNLOAD
            ' Получаем запакованные параметры, они лежат последовательно
            ret = lstrlen(ByVal msg.lParam)
            URL = Space(ret)
            lstrcpy ByVal StrPtr(URL), ByVal msg.lParam
            ret = lstrlen(ByVal msg.lParam + (ret + 1) * 2)
            fle = Space(ret)
            lstrcpy ByVal StrPtr(fle), ByVal msg.lParam + LenB(URL) + 2
            ' Сбрасываем событие, чтобы нельзя было вызвать метод еще раз пока не отработает предыдущий вызов
            ResetEvent value.hEvent
            ' Вызываем метод
            obj.Download URL, fle
            ' Устанавливаем событие - объект свободен
            SetEvent value.hEvent
            ' Очищаем параметры
            HeapFree GetProcessHeap(), 0, ByVal msg.lParam
        Case Else
            TranslateMessage msg
            DispatchMessage msg
        End Select
    Loop
   
    ' Удаляем объекты
    Set obj = Nothing
    ' Деинициализация
    CoUninitialize
   
End Function
 
' // Упаковка параметров последовательно
Public Function MT_DOWNLOAD_packParam(URL As String, fileName As String) As Long
    MT_DOWNLOAD_packParam = HeapAlloc(GetProcessHeap(), 0, LenB(URL) + LenB(fileName) + 4)
    If MT_DOWNLOAD_packParam Then
        lstrcpy ByVal MT_DOWNLOAD_packParam, ByVal StrPtr(URL)
        lstrcpy ByVal MT_DOWNLOAD_packParam + LenB(URL) + 2, ByVal StrPtr(fileName)
    End If
End Function

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We examine in detail the code. When loading forms (Form_Load), we patch the runtime error to exclude the use Declared functions in an uninitialized flow (RemoveLastDllError). The principle I described above. If we create an object in another thread, we need to somehow check in the main thread, whether to create the object. For this, I use a simple synchronization objects - an event with manual reset. Initialize it in a reset state. Then create a stream function ThreadProc, as a parameter to pass the structure of sync events and links to the stream object (Stream), which need to be marshaled. This object returns a reference to marshal a pointer to the object. If successful, forward trigger events (WaitForSingleObject). On this main thread suspends execution until we establish event hEvent. In a new thread first initialize COM (CoInitialize), translate the CLSID and IID in binary form, create an object (CoCreateInstance). Here, if you do not need error handling, you can use:

Code:

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CreateObject("MTDownloader.MultithreadDownloader")

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In this code to create an object I used CoCreateInstance, as before the creation of the first object we can not include error handling (the reason described above), after the creation of the first object can be further create objects through "CreateObject". If the error handling is not necessary, you can immediately use CreateObject. If successful, marshals for this call the "CoMarshalInterThreadInterfaceInStream", which writes the Stream (stream) information to create a proxy object in another thread. Set the event, thereby indicate the main flow of the initialization was successful. On failure, and set the event and perform deinitialization "COM" in the stream output (flow completed). A sign of a successful initialization becomes a pointer to the IStream. Further, in this stream to enter the standard loop. Because we have established event, the main flow and wakes up, we check whether successfully passed the initialization. If "IStream" contains a pointer, then all is well, otherwise an error. Then get a pointer to a proxy object from the stream by calling "CoGetInterfaceAndReleaseStream", thus also release the object "Stream". Assign our object variable, subscribe to the event, a pointer to a proxy object. All these manipulations we can now access the object in another thread and receive events from it. Check whether correctly initialized object itself (hInternet <> 0), and set the buffer size to 64 KB, the information will be updated when the next batch of uploaded data in 64 KB. The initialization is completed. To it was impossible to perform several queries on the download, we will synchronize requests to create an event. Otherwise, if just a few clicks on the button Download, the data will be downloaded sequentially, if mistakenly press 2 times, then the file will be downloaded 2 times and overwritten, the error will not. When pressed we check the status of the event, if it is established that download in the moment. To transfer data to another stream, perform transportation (marshalling) parameters in the other thread (MT_DOWNLOAD_packParam). To do this, allocate memory on the heap and copy the data (in this case the URL and FileName) into it, and give a link to the newly created thread. I decided to save the easiest way - 2 unicode-strings series with a trailing null terminals. A reference to the parameters in turn flow through PostThreadMessage, as the number of the message using the first free identifier WM_APP, which I called WM_MT_DOWNLOAD. In another thread in the cycle, when receiving a message WM_MT_DOWNLOAD, take out the parameters of the heap and invokes the Download, pre-reset event hEvent. All. While a method we can not call him again, and thanks marshaled we receive notification from the object as the events in the main stream. Event handlers elementary and self-explanatory. The only thing that I want to add that the file size I selected Currency, as 64-bit integers no, but it's almost Currency same, only divided by 10000₁₀.
In addition to the asynchronous calls have also remains the possibility of a synchronous call, that is, in the form code can legitimately write "Downloader.Download URL, FileName". You can compare the advantages and disadvantages of synchronous and asynchronous calls.* Example does not require registration of ActiveX DLL, enough to put it in the same folder thanks to the manifesto. As a result, we have a multi-threaded application that runs on any machine without prompting the admin rights.
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Multithreading on VB6 real and feasibility. In this article, I described a method of creating an object in a separate thread and subscribe to its events. If we do not need to have a connection with different object streams, the code is repeatedly shortened (retracted cycle, marshaling, etc.); You can even create an object in inline assembly - that will limit the debug this code in the IDE. But all these methods I describe it some other time. Good luck to everyone!

Attachment 123705

I had previously asked the question of how to tell if threads use different CPU cores when available. To answer that question, I modified TrickMTDownloader to service 2 separate threads. I then started a download of a 180 MB jpeg, and quickly started a simple page file, with the following results.
Start 1 31833.39
Start 2 31833.82
End 2 31833.85
End 1 31833.9
Download 1 is a 180 KB file - elapsed time 510 ms
Download 2 is a 1 KB file - elapsed time 30 ms
I am assuming that this means the second download runs on a different CPU core.

I then compiled the program and ran the test again. The results were pretty much the same, indicating that network speed was the limiting factor. I have only included the test program in the attached download. To run this program you will need the registered ActiveX DLL program from the trick download.

I used arrays for most of the download variables, but I had difficulty with the Downloader object. I left most of the translated comments in place.

J.A. Coutts