多个异步函数，首先返回正确的结果

Question

我在网上找不到我想要的东西，也找不到正确的条款。就是这样。

我有一些异步函数：

Public Async Function Func1(Arg as T) As Task(Of Reply) 
Public Async Function Func2(Arg as T) As Task(Of Reply)
Public Async Function Func3(Arg as T) As Task(Of Reply)
.
.

它们都接受相同的参数并返回相同的类型“回复”。 在函数中，Reply.Info 分配有一个有效对象或 Nothing。

Public Class Reply
    Public Property Info As Object = Nothing

    Public ReadOnly Property Available As Boolean
        Get
            Return Info IsNot Nothing
        End Get
    End Property
End Class

我想在不需要等待所有异步函数完成的情况下执行以下操作。它们可以被终止，只要第一个完成的满足返回标准 - First Non-Nothing Reply.Info。

Public Async Function RunFunc(Arg as T) as Task(Of Reply)
    Dim Reply1 As Reply = Await Func2(Arg)
    Dim Reply2 As Reply = Await Func3(Arg)
    Dim Reply3 As Reply = Await Func4(Arg)

'Return the first one where Reply.Available = True
End Function

如果有人能对此提供一些见解，那就太好了。

解决方案

基于 Richard Zhang 和 this 的解决方案

Dim TaskList As New List(Of Task(Of Reply)) From {
    Func1(Search),
    Func2(Search),
    Func3(Search)
}

Dim CompletedTask As Task(Of Reply)
Do
    CompletedTask = Await Task.WhenAny(TaskList.ToArray)
    TaskList.Remove(CompletedTask)
Loop While CompletedTask.IsFaulted AndAlso TaskList.Count > 0
TaskList.Clear()

If Not CompletedTask.IsFaulted Then
    Dim CompletedReply As Reply = Await CompletedTask
    If CompletedReply.Available AndAlso CompletedReply.SomeOtherProperty Then
        Return Reply
    Else
        Return 'Handles non-expected but non-error return
    End If
Else
    Return 'Handles all error situations
End If

虽然并不完美，但性能确实得到了显着提升。

Answer 1

我们可以使用Task.WhenAny来满足我们的需求

Private Async Sub InitTask(Of T)(ByVal Arg As T)
    Dim tasks = New List(Of Task(Of Reply))()
    tasks.Add(Func1(Arg))
    tasks.Add(Func2(Arg))
    tasks.Add(Func3(Arg))
    Dim completeTask = Await Task.WhenAny(tasks.ToArray())
    Dim result = Await completeTask
End Sub

考虑到你的实际返回值可能为空，可以在相关函数中判断是否为空并抛出异常。 Task 会过滤掉抛出异常的任务，只取最终结果。

Private Async Function Func1(Of T)(ByVal Arg As T) As Task(Of Reply)
    ' get reply
    If reply.Available Then
        Return reply
    Else
        Throw New ArgumentException("not invalid data")
    End If
End Function