[multithreading] Multi-threading in VBA

Does anybody here know how to get VBA to run multiple threads? I am using Excel.

I am adding this answer since programmers coming to VBA from more modern languages and searching Stack Overflow for multithreading in VBA might be unaware of a couple of native VBA approaches which sometimes help to compensate for VBA's lack of true multithreading.

If the motivation of multithreading is to have a more responsive UI that doesn't hang when long-running code is executing, VBA does have a couple of low-tech solutions that often work in practice:

1) Userforms can be made to display modelessly - which allows the user to interact with Excel while the form is open. This can be specified at runtime by setting the Userform's ShowModal property to false or can be done dynamically as the from loads by putting the line

UserForm1.Show vbModeless

in the user form's initialize event.

2) The DoEvents statement. This causes VBA to cede control to the OS to execute any events in the events queue - including events generated by Excel. A typical use-case is updating a chart while code is executing. Without DoEvents the chart won't be repainted until after the macro is run, but with Doevents you can create animated charts. A variation of this idea is the common trick of creating a progress meter. In a loop which is to execute 10,000,000 times (and controlled by the loop index i ) you can have a section of code like:

If i Mod 10000 = 0 Then
    UpdateProgressBar(i) 'code to update progress bar display
    DoEvents
End If

None of this is multithreading -- but it might be an adequate kludge in some cases.

I was looking for something similar and the official answer is no. However, I was able to find an interesting concept by Daniel at ExcelHero.com.

Basically, you need to create worker vbscripts to execute the various things you want and have it report back to excel. For what I am doing, retrieving HTML data from various website, it works great!

Take a look:

http://www.excelhero.com/blog/2010/05/multi-threaded-vba.html

'speed up thread
     dim lpThreadId as long
     dim test as long
     dim ptrt as long
'initparams
     ptrt=varptr(lpThreadId)
     Add = CODEPTR(thread)
'opensocket(191.9.202.255) change depending on configuration
     numSock = Sock.Connect("191.9.202.255", 1958)    
'port recieving
     numSock1=sock.open(5963)
'create thread
     hThread= CreateThread (byval 0&,byval 16384, Add , byval 0&, ByVal 1958, ptrt )
     edit3.text=str$(hThread)


' use 
Declare Function CreateThread Lib "kernel32" Alias "CreateThread" (lpThreadAttributes As long, ByVal dwStackSize As Long, lpStartAddress As Long, lpParameter As long, ByVal dwCreationFlags As Long, lpThreadId As Long) As Long

Sub MultiProcessing_Principle()
    Dim k As Long, j As Long
    k = Environ("NUMBER_OF_PROCESSORS")
    For j = 1 To k
        Shellm "msaccess", "C:\Autoexec.mdb"
    Next
    DoCmd.Quit
End Sub

Private Sub Shellm(a As String, b As String) ' Shell modificirani
    Const sn As String = """"
    Const r As String = """ """
    Shell sn & a & r & b & sn, vbMinimizedNoFocus
End Sub

I know the question specifies Excel, but since the same question for Access got marked as duplicate, so I will post my answer here. The principle is simple: open a new Access application, then open a form with a timer inside that application, send the function/sub you want to execute to that form, execute the task if the timer hits, and quit the application once execution has finished. This allows the VBA to work with tables and queries from your database. Note: it will throw errors if you've exclusively locked the database.

This is all VBA (as opposed to other answers)

The function that runs a sub/function asynchronously

Public Sub RunFunctionAsync(FunctionName As String)
    Dim A As Access.Application
    Set A = New Access.Application
    A.OpenCurrentDatabase Application.CurrentProject.FullName
    A.DoCmd.OpenForm "MultithreadingEngine"
    With A.Forms("MultiThreadingEngine")
        .TimerInterval = 10
        .AddToTaskCollection (FunctionName)
    End With
End Sub

The module of the form required to achieve this

(form name = MultiThreadingEngine, doesn't have any controls or properties set)

Public TaskCollection As Collection

Public Sub AddToTaskCollection(str As String)
    If TaskCollection Is Nothing Then
        Set TaskCollection = New Collection
    End If
    TaskCollection.Add str
End Sub
Private Sub Form_Timer()
    If Not TaskCollection Is Nothing Then
        If TaskCollection.Count <> 0 Then
            Dim CollectionItem As Variant
            For Each CollectionItem In TaskCollection
                Run CollectionItem
            Next CollectionItem
        End If
    End If
    Application.Quit
End Sub

Implementing support for parameters should be easy enough, returning values is difficult, however.

As said before, VBA does not support Multithreading.

But you don't need to use C# or vbScript to start other VBA worker threads.

I use VBA to create VBA worker threads.

First copy the makro workbook for every thread you want to start.

Then you can start new Excel Instances (running in another Thread) simply by creating an instance of Excel.Application (to avoid errors i have to set the new application to visible).

To actually run some task in another thread i can then start a makro in the other application with parameters form the master workbook.

To return to the master workbook thread without waiting i simply use Application.OnTime in the worker thread (where i need it).

As semaphore i simply use a collection that is shared with all threads. For callbacks pass the master workbook to the worker thread. There the runMakroInOtherInstance Function can be reused to start a callback.

'Create new thread and return reference to workbook of worker thread
Public Function openNewInstance(ByVal fileName As String, Optional ByVal openVisible As Boolean = True) As Workbook
    Dim newApp As New Excel.Application
    ThisWorkbook.SaveCopyAs ThisWorkbook.Path & "\" & fileName
    If openVisible Then newApp.Visible = True
    Set openNewInstance = newApp.Workbooks.Open(ThisWorkbook.Path & "\" & fileName, False, False) 
End Function

'Start macro in other instance and wait for return (OnTime used in target macro)
Public Sub runMakroInOtherInstance(ByRef otherWkb As Workbook, ByVal strMakro As String, ParamArray var() As Variant)
    Dim makroName As String
    makroName = "'" & otherWkb.Name & "'!" & strMakro
    Select Case UBound(var)
        Case -1:
            otherWkb.Application.Run makroName
        Case 0:
            otherWkb.Application.Run makroName, var(0)
        Case 1:
            otherWkb.Application.Run makroName, var(0), var(1)
        Case 2:
            otherWkb.Application.Run makroName, var(0), var(1), var(2)
        Case 3:
            otherWkb.Application.Run makroName, var(0), var(1), var(2), var(3)
        Case 4:
            otherWkb.Application.Run makroName, var(0), var(1), var(2), var(3), var(4)
        Case 5:
            otherWkb.Application.Run makroName, var(0), var(1), var(2), var(3), var(4), var(5)
    End Select
End Sub

Public Sub SYNCH_OR_WAIT()
    On Error Resume Next
    While masterBlocked.Count > 0
        DoEvents
    Wend
    masterBlocked.Add "BLOCKED", ThisWorkbook.FullName
End Sub

Public Sub SYNCH_RELEASE()
    On Error Resume Next
    masterBlocked.Remove ThisWorkbook.FullName
End Sub

Sub runTaskParallel()
    ...
    Dim controllerWkb As Workbook
    Set controllerWkb = openNewInstance("controller.xlsm")

    runMakroInOtherInstance controllerWkb, "CONTROLLER_LIST_FILES", ThisWorkbook, rootFold, masterBlocked
    ...
End Sub

As you probably learned VBA does not natively support multithreading but. There are 3 methods to achieve multithreading:

1. COM/dlls - e.g. C# and the Parallel class to run in separate threads
2. Using VBscript worker threads - run your VBA code in separate VBscript threads
3. Using VBA worker threads executed e.g. via VBscript - copy the Excel workbook and run your macro in parallel.

I compared all thread approaches here: http://analystcave.com/excel-multithreading-vba-vs-vbscript-vs-c-net/

Considering approach #3 I also made a VBA Multithreading Tool that allows you to easily add multithreading to VBA: http://analystcave.com/excel-vba-multithreading-tool/

See the examples below:

Multithreading a For Loop

Sub RunForVBA(workbookName As String, seqFrom As Long, seqTo As Long)
    For i = seqFrom To seqTo
        x = seqFrom / seqTo
    Next i
End Sub

Sub RunForVBAMultiThread()
    Dim parallelClass As Parallel 

    Set parallelClass = New Parallel 

    parallelClass.SetThreads 4 

    Call parallelClass.ParallelFor("RunForVBA", 1, 1000) 
End Sub

Run an Excel macro asynchronously

Sub RunAsyncVBA(workbookName As String, seqFrom As Long, seqTo As Long)
    For i = seqFrom To seqTo
        x = seqFrom / seqTo
    Next i
End Sub

Sub RunForVBAAndWait()
    Dim parallelClass As Parallel

    Set parallelClass  = New Parallel

    Call parallelClass.ParallelAsyncInvoke("RunAsyncVBA", ActiveWorkbook.Name, 1, 1000) 
    'Do other operations here
    '....

    parallelClass.AsyncThreadJoin 
End Sub