為什么在C#中異步IO阻塞？[復制]

我創建了一個針對本地檔案資料庫的 WPF 應用程式，以供娛樂/練習。這個想法是物體的檔案是一個 .json 檔案，它存在于磁盤上，檔案夾充當集合。在這個實作中，我有一堆 .json 檔案，它們提供有關視頻的資料以創建一種 IMDB 克隆。

我有這堂課：

public class VideoRepository : IVideoRepository
{
    public async IAsyncEnumerable<Video> EnumerateEntities()
    {
        foreach (var file in new DirectoryInfo(Constants.JsonDatabaseVideoCollectionPath).GetFiles())
        {
            var json = await File.ReadAllTextAsync(file.FullName); // This blocks
            var document = JsonConvert.DeserializeObject<VideoDocument>(json); // Newtonsoft
            var domainObject = VideoMapper.Map(document); // A mapper to go from the document type to the domain type
            yield return domainObject;
        }
        // Uncommenting the below lines and commenting out the above foreach loop doesn't lock up the UI.
        //await Task.Delay(5000);
        //yield return new Video();
    }

    // Rest of class.
}

在呼叫堆疊上，通過 API 層和 UI 層，我在 ViewModel 中有一個 ICommand：

QueryCommand = new RelayCommand(async (query) => await SendQuery((string)query));

private async Task SendQuery(string query)
    {
        QueryStatus = "Querying...";
        QueryResult.Clear();

        await foreach (var video in _videoEndpoints.QueryOnTags(query))
            QueryResult.Add(_mapperService.Map(video));

        QueryStatus = $"{QueryResult.Count()} videos found.";
    }

目標是在處理查詢時向用戶顯示訊息“正在查詢...”。但是，該訊息永遠不會顯示，并且 UI 會鎖定，直到查詢完成，此時會顯示結果訊息。

In VideoRepository, if I comment out the foreach loop and uncomment the two lines below it, the UI doesn't lock up and the 'Querying...' message gets shown for 5 seconds.

Why does that happen? Is there a way to do IO without locking up the UI/blocking?

Fortunately, if this were behind a web API and hit a real database, I probably wouldn't see this issue. I'd still like the UI to not lock up with this implementation though.

EDIT: Dupe of Why File.ReadAllLinesAsync() blocks the UI thread?

Turns out Microsoft didn't make their async method very async. Changing the IO line fixes everything:

//var json = await File.ReadAllTextAsync(file.FullName); // Bad
var json = await Task.Run(() => File.ReadAllText(file.FullName)); // Good

uj5u.com熱心網友回復：

您的目標可能是 .NET 6 之前的 .NET 版本。在這些舊版本中，檔案系統 API 沒有有效實作，甚至不是真正的異步.NET 。.NET 6 中的情況有所改進，但同步檔案系統 API 的性能仍然比異步對應的 API 更高。您的問題可以簡單地通過切換來解決：

var json = await File.ReadAllTextAsync(file.FullName);

對此：

var json = await Task.Run(() => File.ReadAllText(file.FullName));

如果你想變得花哨，你也可以通過使用這樣的自定義 LINQ 運算子來解決 UI 層的問題：

public static async IAsyncEnumerable<T> OnThreadPool<T>(
    this IAsyncEnumerable<T> source,
    [EnumeratorCancellation] CancellationToken cancellationToken = default)
{
    var enumerator = await Task.Run(() => source
        .GetAsyncEnumerator(cancellationToken)).ConfigureAwait(false);
    try
    {
        while (true)
        {
            var (moved, current) = await Task.Run(async () =>
            {
                if (await enumerator.MoveNextAsync())
                    return (true, enumerator.Current);
                else
                    return (false, default);
            }).ConfigureAwait(false);
            if (!moved) break;
            yield return current;
        }
    }
    finally
    {
        await Task.Run(async () => await enumerator
            .DisposeAsync()).ConfigureAwait(false);
    }
}

此運算子卸載到ThreadPool與列舉相關的所有操作IAsyncEnumerable<T>。它可以這樣使用：

await foreach (var video in _videoEndpoints.QueryOnTags(query).OnThreadPool())
    QueryResult.Add(_mapperService.Map(video));

標籤：c# wpf asynchronous io iasyncenumerable

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