【C#】62. 异步读写文件的几种方法： Task.Factory.FromAsync，WriteAsync

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一、这里主要说明2种异步写入文件的方法：

1）异步编程模型API转为Task——使用Task.Factory.FromAsync方法

2）对于StreamWriter使用WriteAsync方法

请记得对stream对象使用FileOptions.Asynchronous选项！

首先来看一下准备函数：

CreateFileContent是用来随机生成要写入的内容（字符串形式）；

SumFileContent

static string CreateFileContent(){var sb = new StringBuilder();for (int i = 0; i < 100000; i++){sb.AppendFormat("{0}", new Random(i).Next(0, 99999));sb.AppendLine();}return sb.ToString();}async static Task<long> SumFileContent(string fileName){using (var stream = new FileStream(fileName, FileMode.Open, FileAccess.Read,FileShare.None, BUFFER_SIZE, FileOptions.Asynchronous))using (var sr = new StreamReader(stream)){long sum = 0;while (sr.Peek() > -1){string line = await sr.ReadLineAsync();sum += long.Parse(line);}return sum;}}static Task SimulateAsynchronousDelete(string fileName){return Task.Run(() => File.Delete(fileName));}

1）Task.Factory.FromAsync方法

using (var stream = new FileStream("test2.txt", FileMode.Create, FileAccess.ReadWrite, FileShare.None, BUFFER_SIZE, FileOptions.Asynchronous)){Console.WriteLine("2. Uses I/O Threads: {0}", stream.IsAsync);byte[] buffer = Encoding.UTF8.GetBytes(CreateFileContent());var writeTask = Task.Factory.FromAsync(stream.BeginWrite, stream.EndWrite, buffer, 0, buffer.Length, null);           await writeTask;}

2）StreamWriter.WriteAsync方法

using (var stream = File.Create("test3.txt", BUFFER_SIZE, FileOptions.Asynchronous))using (var sw = new StreamWriter(stream)){Console.WriteLine("3. Uses I/O Threads: {0}", stream.IsAsync);await sw.WriteAsync(CreateFileContent());}

二、异步从文件中读取内容（四个Task来数字数，WhenAll后再相加）

Console.WriteLine("Starting parsing files in parallel");Task<long>[] readTasks = new Task<long>[4];for (int i = 0; i < 4; i++){readTasks[i] = SumFileContent(string.Format("test{0}.txt", i + 1));}long[] sums = await Task.WhenAll(readTasks);Console.WriteLine("Sum in all files: {0}", sums.Sum());

三、模拟异步删除（因为API中没有异步删除）

Console.WriteLine("Deleting files...");Task[] deleteTasks = new Task[4];for (int i = 0; i < 4; i++){string fileName = string.Format("test{0}.txt", i + 1);deleteTasks[i] = SimulateAsynchronousDelete(fileName);}await Task.WhenAll(deleteTasks);Console.WriteLine("Deleting complete.");

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