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[英]'System.InvalidOperationException' when LINQ to SQL using WCF on WP8
[英]System.InvalidOperationException when using GetAwaiter().GetResult() with ServiceBusReceiver.PeekMessagesAsync
我們使用GetAwaiter().GetResult()
因為 PowerShell 的Cmdlet.ProcessRecord()
不支持異步/等待。
class Program
{
static async Task Main(string[] args)
{
var topicPath = "some-topic";
var subscriptionName = "some-subscription";
var connectionString = "some-connection-string";
var subscriptionPath = EntityNameHelper.FormatSubscriptionPath(
topicPath,
subscriptionName
);
var serviceBusClient = new ServiceBusClient(connectionString);
var receiver = serviceBusClient.CreateReceiver(queueName: subscriptionPath);
// This one works. :-)
await foreach (var item in GetMessages(receiver, maxMessagesPerFetch: 5))
{
Console.WriteLine("async/await: " + item);
}
// This one explodes.
var enumerator = GetMessages(receiver, maxMessagesPerFetch: 5).GetAsyncEnumerator();
while (enumerator.MoveNextAsync().GetAwaiter().GetResult())
{
// Unhandled exception. System.InvalidOperationException: Operation is not valid due to the current state of the object.
// at NonSync.IAsyncEnumerable.Program.GetMessages(ServiceBusReceiver receiver, Int32 maxMessagesPerFetch)+System.Threading.Tasks.Sources.IValueTaskSource<System.Boolean>.GetResult()
// at NonSync.IAsyncEnumerable.Program.Main(String[] args) in C:\dev\mediavalet\MediaValet.Learning\entropy\NonSynchronousDotNet\NonSync.IAsyncEnumerable\Program.cs:line 42
// at NonSync.IAsyncEnumerable.Program.<Main>(String[] args)
Console.WriteLine("GetAwaiter().GetResult(): " + enumerator.Current);
}
}
public static async IAsyncEnumerable<string> GetMessages(
ServiceBusReceiver receiver,
int maxMessagesPerFetch
)
{
yield return "Foo";
var messages = await receiver.PeekMessagesAsync(maxMessagesPerFetch);
yield return "Bar";
}
}
這里發生了什么? 我們如何在不更改GetMessages
情況下修復它?
根據ValueTask<TResult>
結構的文檔:
永遠不應在
ValueTask<TResult>
實例上執行以下操作:• 多次等待實例。
• 多次調用AsTask
。
• 在操作尚未完成或多次使用時使用.Result
或.GetAwaiter().GetResult()
。
• 使用這些技術中的一種以上來使用實例。如果您執行上述任何操作,則結果未定義。
您可以做的是使用AsTask
方法將ValueTask<bool>
轉換為Task<bool>
:
while (enumerator.MoveNextAsync().AsTask().GetAwaiter().GetResult())
該答案通過代碼示例補充了 Theodor 的答案。 我們的具體問題是我們在ValueTask
完成之前調用GetResult()
。 文檔指定這是不允許的:
一個 ValueTask 實例只能被等待一次,並且消費者在實例完成之前可能不會讀取 Result。 如果這些限制不可接受,請通過調用 AsTask 將 ValueTask 轉換為 Task。 (強調補充)。
using System;
using System.Collections.Generic;
using System.Threading.Tasks;
var enumerator = GetAsyncEnumerable().GetAsyncEnumerator();
while (true)
{
var moveNext = enumerator.MoveNextAsync();
var moveNextAwaiter = moveNext.GetAwaiter();
Console.WriteLine("ValueTask.IsCompleted: {0}", moveNext.IsCompleted);
try
{
if (moveNextAwaiter.GetResult())
{
Console.WriteLine("IAsyncEnumerator.Current: {0}", enumerator.Current);
continue;
}
Console.WriteLine("Done! We passed the end of the collection.");
break;
}
catch (InvalidOperationException)
{
Console.WriteLine("Boom! GetResult() before the ValueTask completed.");
continue;
}
}
async IAsyncEnumerable<int> GetAsyncEnumerable()
{
yield return 1;
await Task.Delay(1000);
yield return 2; // <---- We never access this, because GetResult() explodes.
yield return 3;
}
Output:
ValueTask.IsCompleted: True
IAsyncEnumerator.Current: 1
ValueTask.IsCompleted: False
Boom! GetResult() before the ValueTask completed.
ValueTask.IsCompleted: True
IAsyncEnumerator.Current: 3
ValueTask.IsCompleted: True
Done! We passed the end of the collection.
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