如果您需要并行运行多个异步I/O任务，但需要确保同时不超过X个I/O进程正在运行；且前后的I/O处理任务不应该有这样的限制。
这里有一个场景-假设有1000个任务；每个任务都接受一个文本字符串作为输入参数；转换该文本（预 I/O 处理），然后将该转换后的文本写入文件。目标是使预处理逻辑利用 CPU / 核心的 100％，并使最多有 10 个 I/O 进程并行运行（最多同时打开10个文件进行写操作）。
您能提供一个使用 C# / .NET 4.5 实现此功能的示例代码吗？
http://blogs.msdn.com/b/csharpfaq/archive/2012/01/23/using-async-for-file-access-alan-berman.aspx
我认为使用TPL Dataflow是个不错的主意：你可以创建带有无限并行性的预处理和后处理块，以及带有有限并行性的文件写入块，并将它们链接在一起。类似这样：
var unboundedParallelismOptions =
new ExecutionDataflowBlockOptions
{
MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded
};
var preProcessBlock = new TransformBlock<string, string>(
s => PreProcess(s), unboundedParallelismOptions);
var writeToFileBlock = new TransformBlock<string, string>(
async s =>
{
await WriteToFile(s);
return s;
},
new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 10 });
var postProcessBlock = new ActionBlock<string>(
s => PostProcess(s), unboundedParallelismOptions);
var propagateCompletionOptions =
new DataflowLinkOptions { PropagateCompletion = true };
preProcessBlock.LinkTo(writeToFileBlock, propagateCompletionOptions);
writeToFileBlock.LinkTo(postProcessBlock, propagateCompletionOptions);
// use something like await preProcessBlock.SendAsync("text") here
preProcessBlock.Complete();
await postProcessBlock.Completion;
WriteToFile()可能如下所示：
private static async Task WriteToFile(string s)
{
using (var writer = new StreamWriter(GetFileName()))
await writer.WriteAsync(s);
}
PreProcess 和 PostProcess 方法是什么？ - shashwat听起来你可能想考虑使用Djikstra信号量来控制任务启动的访问。
然而，这听起来像是一个典型的队列/固定消费者数量的问题，可能更适合采用这种方式进行结构化。
我会创建一个扩展方法，使得可以设置最大并行度。SemaphoreSlim 在这里将是救星。
/// <summary>
/// Concurrently Executes async actions for each item of <see cref="IEnumerable<typeparamref name="T"/>
/// </summary>
/// <typeparam name="T">Type of IEnumerable</typeparam>
/// <param name="enumerable">instance of <see cref="IEnumerable<typeparamref name="T"/>"/></param>
/// <param name="action">an async <see cref="Action" /> to execute</param>
/// <param name="maxDegreeOfParallelism">Optional, An integer that represents the maximum degree of parallelism,
/// Must be grater than 0</param>
/// <returns>A Task representing an async operation</returns>
/// <exception cref="ArgumentOutOfRangeException">If the maxActionsToRunInParallel is less than 1</exception>
public static async Task ForEachAsyncConcurrent<T>(
this IEnumerable<T> enumerable,
Func<T, Task> action,
int? maxDegreeOfParallelism = null)
{
if (maxDegreeOfParallelism.HasValue)
{
using (var semaphoreSlim = new SemaphoreSlim(
maxDegreeOfParallelism.Value, maxDegreeOfParallelism.Value))
{
var tasksWithThrottler = new List<Task>();
foreach (var item in enumerable)
{
// Increment the number of currently running tasks and wait if they are more than limit.
await semaphoreSlim.WaitAsync();
tasksWithThrottler.Add(Task.Run(async () =>
{
await action(item).ContinueWith(res =>
{
// action is completed, so decrement the number of currently running tasks
semaphoreSlim.Release();
});
}));
}
// Wait for all tasks to complete.
await Task.WhenAll(tasksWithThrottler.ToArray());
}
}
else
{
await Task.WhenAll(enumerable.Select(item => action(item)));
}
}
示例用法：
await enumerable.ForEachAsyncConcurrent(
async item =>
{
await SomeAsyncMethod(item);
},
5);