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如何处理ContinueWith和WhenAll异常?

result task exception-handling asynchronous c#

-1

M.kazem Akhgary · 技术社区 · 6 年前

我正在尝试异步加载多个文件,并在加载完每个文件后通知UI,

_loadCancellationTokenSource = new CancellationTokenSource();

TaskScheduler scheduler = TaskScheduler.FromCurrentSynchronizationContext();
var files = await picker.PickMultipleFilesAsync();
LoadedFiles.Clear();

loads = await Task.WhenAll(files.Select(file =>
{
    var load = LoadAsync(file);
    load.ContinueWith(t =>
    {
        if (t.IsCompleted) LoadedFiles.Add(file.Path);
        if (t.IsFaulted) NotifyUser(t.Exception.Message, NotifyType.ErrorMessage);
        if (t.IsCanceled) NotifyUser("operation was canceled.", NotifyType.ErrorMessage);
    }, scheduler);
    return load;
}));

private Task<Foo> LoadAsync(StorageFile file)
{
    // exception may be thrown inside load
    return Load(file, _loadCancellationTokenSource.Token);
}

问题是当抛出异常时,它不会被处理。我知道为什么,因为 ContinueWith 创建新任务,但我返回旧任务。

这是因为这是一项无效的任务。但是我不知道如何正确返回结果。我不确定它是否安全使用 t.Result 因为它可能会阻塞UI线程?

a task was cancelled exception

load = (await Task.WhenAll(files.Select(file =>
{
    var load = LoadAsync(file);
    load.ContinueWith(t =>
    {
        if (t.IsFaulted) NotifyUser(t.Exception.Message, NotifyType.ErrorMessage);
        if (t.IsCanceled) NotifyUser("operation was canceled.", NotifyType.ErrorMessage);
    }, _loadCancellationTokenSource.Token, TaskContinuationOptions.NotOnRanToCompletion, scheduler);
    return load.ContinueWith(t =>
    {
        LoadedFiles.Add(file.Path);
        return (file, t.Result);
    }, _loadCancellationTokenSource.Token, TaskContinuationOptions.OnlyOnRanToCompletion, scheduler); ;
})));

1 回复 | 直到 6 年前

M.kazem Akhgary 6 年前

多亏了@Jimi,我才能够通过查看任务状态来解决这个问题。

loads = (await Task.WhenAll(files.Select(file =>
{
    return LoadAsync(file).ContinueWith(t =>
    {
        if (t.IsFaulted) NotifyUser(t.Exception.Message, NotifyType.ErrorMessage);
        if (t.IsCanceled) NotifyUser("operation was canceled.", NotifyType.ErrorMessage);

        if (t.Status == TaskStatus.RanToCompletion)
        {
            LoadedFiles.Add(file.Path);
            return t.Result;
        }

        return null;
    }, scheduler);
}))).Where(x => x != null).ToArray();

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