How to make a very simple asynchronous method call

Question

I just have a simple vb.net website that need to call a Sub that performs a very long task that works with syncing up some directories in the filesystem (details not important).

When I call the method, it eventually times out on the website waiting for the sub routine to complete. However, even though the website times out, the routine eventually completes it's task and all the directories end up as they should.

I want to just prevent the timeout so I'd like to just call the Sub asynchronously. I do not need (or even want) and callback/confirmation that it ran successfully.

So, how can I call my method asynchronously inside a website using VB.net?

If you need to some code:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Call DoAsyncWork()
End Sub

Protected Sub DoAsyncWork()
        Dim ID As String = ParentAccountID
        Dim ParentDirectory As String = ConfigurationManager.AppSettings("AcctDataDirectory")
        Dim account As New Account()
        Dim accts As IEnumerable(Of Account) = account.GetAccounts(ID)

        For Each f As String In My.Computer.FileSystem.GetFiles(ParentDirectory)
            If f.EndsWith(".txt") Then
                Dim LastSlashIndex As Integer = f.LastIndexOf("\")
                Dim newFilePath As String = f.Insert(LastSlashIndex, "\Templates")
                My.Computer.FileSystem.CopyFile(f, newFilePath)
            End If
        Next

        For Each acct As Account In accts
            If acct.ID <> ID Then
                Dim ChildDirectory As String = ConfigurationManager.AppSettings("AcctDataDirectory") & acct.ID
                If My.Computer.FileSystem.DirectoryExists(ChildDirectory) = False Then
                    IO.Directory.CreateDirectory(ChildDirectory)
                End If
                My.Computer.FileSystem.DeleteDirectory(ChildDirectory, FileIO.DeleteDirectoryOption.DeleteAllContents)
                My.Computer.FileSystem.CopyDirectory(ParentDirectory, ChildDirectory, True)
            Else
            End If
        Next
End Sub

Answer 1

You could do this with a simple thread:

Add :

 Imports System.Threading

And wherever you want it to run :

 Dim t As New Thread(New ThreadStart(AddressOf DoAsyncWork))
 t.Priority = Threading.ThreadPriority.Normal
 t.Start()

The call to t.Start() returns immediately and the new thread runs DoAsyncWork in the background until it completes. You would have to make sure that everything in that call was thread-safe but at first glance it generally seems to be so already.

Answer 2

I wouldn't recommend using the Thread class unless you need a lot more control over the thread, as creating and tearing down threads is expensive. Instead, I would recommend using a ThreadPool thread. See this for a good read.

You can execute your method on a ThreadPool thread like this:

System.Threading.ThreadPool.QueueUserWorkItem(AddressOf DoAsyncWork)

You'll also need to change your method signature to...

Protected Sub DoAsyncWork(state As Object) 'even if you don't use the state object

Finally, also be aware that unhandled exceptions in other threads will kill IIS. See this article (old but still relevant; not sure about the solutions though since I don't reaslly use ASP.NET).

Answer 3

I also was looking for information on Asynchronous programming in VB. In addition to this thread, I also found the following: beginning with Visual Studio 2012 and .Net Framework 4.5, VB was given two new keywords to make a method asynchronous right in the declaration, without using Thread or Threadpool. The new keywords are "Async" and "Await". You may refer to the following links if you wish:

http://msdn.microsoft.com/library/hh191443%28vs.110%29.aspx

https://msdn.microsoft.com/en-us/library/hh191564%28v=vs.110%29.aspx