以下方法无法编译。替代方案？

public static async Task<IEnumerable<object[]>> GetRecordsAsync(
    this Transaction transaction,
    string commandText,
    params SqlParameter[] parameters)
{
    // Get a SqlDataReader
    var reader = await transaction.GetReaderAsync(commandText, parameters);
    var fieldCount = -1;
    // Begin iterating through records asynchronously
    while (await reader.ReadAsync()) // Note we don't loop until .ReadAsync returns a boolean
    {
        // Grab all the field values out
        if (fieldCount < 0)
            fieldCount = reader.FieldCount;
        var fields = new object[fieldCount];
        reader.GetValues(fields);
        // Yield return the field values from this record
        yield return fields;
    }
}

错误信息:

The body of 'TransactionExtensions.GetRecordsAsync(Transaction, string, params SqlParameter[])' cannot be an iterator block because 'Task>' is not an iterator interface type

我看不到适应的方法this answer一个听起来相似(但不同)的问题，因为我不知道先验循环会进行多少次。

编辑:固定格式

最佳答案

根据@SLaks 对问题的评论，这是使用 Reactive Extensions 的一般替代方法:

/// <summary>
/// Turns the given asynchronous functions into an IObservable
/// </summary>
static IObservable<T> ToObservable<T>(
    Func<Task<bool>> shouldLoopAsync,
    Func<Task<T>> getAsync)
{
    return Observable.Create<T>(
        observer => Task.Run(async () =>
            {
                while (await shouldLoopAsync())
                {
                    var value = await getAsync();
                    observer.OnNext(value);
                }
                observer.OnCompleted();
            }
        )
    );
}

示例用法，专为解决问题的具体情况而设计:

/// <summary>
/// Asynchronously processes each record of the given reader using the given handler
/// </summary>
static async Task ProcessResultsAsync(this SqlDataReader reader, Action<object[]> fieldsHandler)
{
    // Set up async functions for the reader
    var shouldLoopAsync = (Func<Task<bool>>)reader.ReadAsync;
    var getAsync = new Func<SqlDataReader, Func<Task<object[]>>>(_reader =>
    {
        var fieldCount = -1;
        return () => Task.Run(() =>
        {
            Interlocked.CompareExchange(ref fieldCount, _reader.FieldCount, -1);
            var fields = new object[fieldCount];
            _reader.GetValues(fields);
            return fields;
        });
    })(reader);

    // Turn the async functions into an IObservable
    var observable = ToObservable(shouldLoopAsync, getAsync);

    // Process the fields as they become available
    var finished = new ManualResetEventSlim(); // This will be our signal for when the observable completes
    using (observable.Subscribe(
        onNext: fieldsHandler, // Invoke the handler for each set of fields
        onCompleted: finished.Set // Set the gate when the observable completes
    )) // Don't forget best practice of disposing IDisposables
        // Asynchronously wait for the gate to be set
        await Task.Run((Action)finished.Wait);
}

(请注意，getAsync 可以在上面的代码块中进行简化，但我喜欢它关于正在创建的闭包的明确性)

...最后:

// Get a SqlDataReader
var reader = await transaction.GetReaderAsync(commandText, parameters);
// Do something with the records
await reader.ProcessResultsAsync(fields => { /* Code here to process each record */ });

关于c# - 带有 yield 返回的异步任务<IEnumerable>？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42882078/