C# 中异步委托的 VB.NET 等价物是什么？

Question

我正在尝试将以下扩展方法 (source) 从 C# 转换为 VB:

public static Task ForEachAsync<T>(this IEnumerable<T> source,
                                   int dop, Func<T, Task> body)
{
    return Task.WhenAll(
        from partition in Partitioner.Create(source).GetPartitions(dop)
        select Task.Run(async delegate {
            using (partition)
                while (partition.MoveNext())
                    await body(partition.Current);
        }));
}

delegate 的常规等价物是 Sub()，AFAIK，但我没想到它能在这种情况下工作，因为 Async 关键字（而且它没有).所以，我尝试使用 Function() 代替：

<System.Runtime.CompilerServices.Extension>
Public Function ForEachAsync(Of T)(source As IEnumerable(Of T),
                                   dop As Integer, body As Func(Of T, Task)) As Task
    Return Task.WhenAll(
        From partition In Partitioner.Create(source).GetPartitions(dop)
        Select Task.Run(Async Function() 'As Task '<-- see below.
                            Using partition
                                Do While partition.MoveNext()
                                    Await body(partition.Current)
                                Loop
                            End Using
                        End Function))
End Function

但这仍然无法编译并显示以下错误：

在WhenAll:

Overload resolution failed because no accessible 'WhenAll' can be called with these arguments:
    'Public Shared Overloads Function WhenAll(Of TResult)(tasks As IEnumerable(Of Task(Of TResult))) As Task(Of TResult())': Type parameter 'TResult' cannot be inferred.
    'Public Shared Overloads Function WhenAll(Of TResult)(ParamArray tasks As Task(Of TResult)()) As Task(Of TResult())': Type parameter 'TResult' cannot be inferred.

在Await body(partition.Current):

'Await' may only be used in a query expression within the first collection expression of the initial 'From' clause or within the collection expression of a 'Join' clause.
[警告] Async Function()：^{（如果我添加 As Task，它就会消失）}

Function '<anonymous method>' doesn't return a value on all code paths. A null reference exception could occur at run time when the result is used.

我做错了什么？在 VB 中执行此操作的正确方法是什么？

Answer 1

不是问题的真正答案，但是，如果它是真正的异步代码，则不需要对执行进行分区：

<System.Runtime.CompilerServices.Extension>
Public Function ForEachAsync(Of T)(
    source As IEnumerable(Of T),
    body As Func(Of T, Task)) As Task
    Return Task.WhenAll(        
        From item In source
        Select body(item))
End Function

Answer 2

在 C# 中，异步 Lambda 可以用 delegate type or using the invocation operator () followed by the => 标记表示为 lambda 运算符以调用匿名方法：

Task.Run(async ()=> { } );
Task.Run(async delegate { } );

在 VB.Net 中，匿名方法可以通过使用 Sub() 或 Function() 的 Lambda 表达式调用，内联和 Sub / End Sub, Function() / End Function 块:

Task.Run(Async Sub() [operation on captured variables])
Task.Run(Sub()
             [operation on captured variables]
         End Sub))

Task.Run(Async Function() [operation on captured variables])
Task.Run(Function()
             Return [operation on captured variables]
         End Function))

VB.Net 的 LINQ to SQL 不允许在 Select 子句中等待，因为：

Await may only be used in a query expression within the first collection expression of the initial From clause or within the collection expression of a Join clause

Stephen Toub 的 Async/Await FAQ 中引用了它。

Select Task.Run(Async Function() ... ) 尝试 return 一个 IEnumerable(Of TResult) 而不是 IEnumerable(Of Task).

Language-Integrated Query (LINQ) (Visual Basic) 中的更多内容。

相反，LINQ to Objects - 在没有其他中间提供者的情况下使用 IEnumerable/IEnumerable<T> 集合 - 确实允许 Select 方法上的 async/await 模式：

<Extension>
Public Function ForEachAsync(Of T)(source As IEnumerable(Of T), dop As Integer, body As Func(Of T, Task)) As Task
    Return Task.WhenAll(
        Partitioner.Create(source).GetPartitions(dop).
        Select(Function(p) (
                   Task.Run(Async Function()
                                Using p
                                    While p.MoveNext()
                                        Await body(p.Current)
                                    End While
                                End Using
                            End Function))))
End Function

LINQ to SQL 的 C# 版本允许它。
为什么，因为同样的规则也应该适用于 C# 实现？

The .NET Language Strategy:

C#:

We will keep growing C# to meet the evolving needs of developers and remain a state of the art programming language. We will innovate aggressively, while being very careful to stay within the spirit of the language.

VB.Net:

We will keep a focus on the cross-language tooling experience, recognizing that many VB developers also use C#. We will focus innovation on the core scenarios and domains where VB is popular.

因此，2010 年断言的 VB and C# Coevolution 发生了变化：C# 和 VB.Net 功能更新已分离。因此，鉴于新的语言策略，VB.Net 和 C# 不再 显示大致相等的采用率 。

C# 中异步委托的 VB.NET 等价物是什么？

What's the VB.NET equivalent of async delegate in C#?

c#

vb.net

lambda

c#-to-vb.net

async-await