【C#】62. 非同步讀寫檔案的幾種方法： Task.Factory.FromAsync，WriteAsync

一、這裡主要說明2種非同步寫入檔案的方法：

1）非同步編程模型API轉為Task——使用Task.Factory.FromAsync方法

2）對於StreamWriter使用WriteAsync方法

請記得對stream對象使用FileOptions.Asynchronous選項。

首先來看一下準備函數：

CreateFileContent是用來隨機產生要寫入的內容（字串形式）；

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、非同步從檔案中讀取內容（四個Task來數字數，WhenAll後再相加）

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、類比非同步刪除（因為API中沒有非同步刪除）

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.");