c# - 将 GetAwaiter().GetResult() 与 ServiceBusReceiver.PeekMessagesAsync 一起使用时出现 System.InvalidOperationException

Question

上下文

我们正在使用 GetAwaiter().GetResult()，因为 PowerShell 的 Cmdlet.ProcessRecord() 不支持异步/等待。

代码示例

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";
    }
}

问题

这是怎么回事？我们如何在不更改 GetMessages 的情况下修复它？

Answer 1

根据 ValueTask<TResult> 的文档结构:

The following operations should never be performed on a ValueTask<TResult> instance:

• Awaiting the instance multiple times.
• Calling AsTask multiple times.
• Using .Result or .GetAwaiter().GetResult() when the operation hasn't yet completed, or using them multiple times.
• Using more than one of these techniques to consume the instance.

If you do any of the above, the results are undefined.

您可以做的是转换 ValueTask<bool>到 Task<bool> , 通过使用 AsTask 方法:

while (enumerator.MoveNextAsync().AsTask().GetAwaiter().GetResult())