Async/Await为何出现多线程？如何限定仅在主线程运行？

原本以为使用Async/Await的程序仅会占用1个线程：编译器遇到await关键字时，会在同一线程处理其他任务，直至等待的操作完成，仅等待的目标操作可能在其他线程执行，但程序本身不会创建新线程。但实际VB代码中出现主线程和工作线程卡在同一位置，疑惑是否是Task.WhenAll和Task.WhenAny导致了多线程，想知道如何让所有逻辑仅在主线程运行。

日志记录代码 #

Public Sub LogEvents(ByVal whatToLog As String, Optional ByVal canRandom As Boolean = True)
    Static logNumber As Integer
    Dim timeStamp As String

    timeStamp = CStr(Microsoft.VisualBasic.Now)

    whatToLog = timeStamp & "  " & whatToLog & Microsoft.VisualBasic.vbNewLine

    Try
        Debug.Print(whatToLog)
        System.IO.File.AppendAllText("log.txt", whatToLog, defaultEncoding)
    ' ... 省略后续代码
End Sub

任务创建代码 #

For Each account In uniqueAccounts().Values
    Dim newtask = account.getMarketInfoAsync().ContinueWith(Sub() account.LogFinishTask("Geting Market Info", starttime))
    LogEvents("marketinfo account " + account.Exchange + " is being done by task " + newtask.Id.ToString + " " + newtask.ToString)
    tasklist.Add(newtask)
    'newtask.ContinueWith(Sub() LogEvents(account.ToString))
Next

后续调用代码 #

LogEvents("Really Start Getting Market Detail of All")
Try
    Await jsonHelper.whenAllWithTimeout(tasklist.ToArray, 500000)
Catch ex As Exception
    Dim b = 1
End Try

whenAllWithTimeout函数代码 #

Public Shared Async Function whenAllWithTimeout(taskar As Task(), timeout As Integer) As Task
    Dim timeoutTask = Task.Delay(timeout)
    Dim maintask = Task.WhenAll(taskar)
    Await Task.WhenAny({timeoutTask, maintask})
    If maintask.IsCompleted Then
        Dim b = 1

        For Each tsk In taskar
            LogEvents("Not Time Out. Status of task " + tsk.Id.ToString + " is " + tsk.IsCompleted.ToString)
        Next
    End If

    If timeoutTask.IsCompleted Then
        Dim b = 1
        For Each tsk In taskar
            LogEvents("status of task " + tsk.Id.ToString + " is " + tsk.IsCompleted.ToString)
        Next
    End If
End Function

为什么会出现多线程？ #

1. ContinueWith的默认调度逻辑：ContinueWith默认会把后续任务放到线程池线程执行，不会自动回到原上下文（比如主线程），这是工作线程出现的核心原因。Async/Await本身会在await恢复时回到原同步上下文，但ContinueWith不遵循这个规则。
2. Task.WhenAll/WhenAny不创建线程：这两个方法只是等待任务完成，不会主动生成新线程，但如果传入的任务本身在多线程执行（比如getMarketInfoAsync内部用了线程池，或者ContinueWith导致的），自然会出现多线程运行的情况。
3. LogEvents的竞态风险：LogEvents里的Static logNumber是线程不安全的，加上多线程同时调用文件写入操作，可能导致线程阻塞或冲突。

如何让所有逻辑在主线程运行？ #

1. 用Await替换ContinueWith：这是最简洁可靠的方式，Async/Await会自动处理上下文切换，确保后续代码回到主线程：

   ' 串行执行的写法
   For Each account In uniqueAccounts().Values
       Await account.getMarketInfoAsync()
       account.LogFinishTask("Geting Market Info", starttime)
       LogEvents($"marketinfo account {account.Exchange} completed")
   Next
   
   ' 如果需要并行执行所有任务
   Dim taskList As New List(Of Task)()
   For Each account In uniqueAccounts().Values
       ' 先收集所有异步任务
       taskList.Add(account.getMarketInfoAsync())
   Next
   ' 等待所有任务完成，恢复时回到主线程
   Await Task.WhenAll(taskList)
   ' 统一执行后续逻辑
   For Each account In uniqueAccounts().Values
       account.LogFinishTask("Geting Market Info", starttime)
   Next
   

2. 给ContinueWith指定主线程调度器：如果一定要用ContinueWith，需要显式指定调度器，强制后续任务在主线程执行：

   Dim newtask = account.getMarketInfoAsync().ContinueWith(
       Sub() account.LogFinishTask("Geting Market Info", starttime),
       TaskScheduler.FromCurrentSynchronizationContext()
   )
   

3. 修复LogEvents的线程安全问题：即使在单线程运行，也建议优化方法的安全性，避免后续引入多线程时出问题：

   Private Shared logLock As New Object()
   Public Sub LogEvents(ByVal whatToLog As String, Optional ByVal canRandom As Boolean = True)
       Static logNumber As Integer
       Dim timeStamp As String = CStr(Microsoft.VisualBasic.Now)
       whatToLog = timeStamp & "  " & whatToLog & Microsoft.VisualBasic.vbNewLine
   
       SyncLock logLock
           Debug.Print(whatToLog)
           System.IO.File.AppendAllText("log.txt", whatToLog, defaultEncoding)
       End SyncLock
       ' ... 省略后续代码
   End Sub
   

4. 检查getMarketInfoAsync内部实现：如果这个方法内部用了Task.Run包裹同步操作，会主动切换到线程池线程。要完全在主线程运行，需要确保它使用真正的异步IO API（比如HttpClient的异步方法），而不是模拟异步。

内容的提问来源于stack exchange，提问作者user4951