【C#】62. 異步讀寫文件的幾種方法： Task.Factory.FromAsync，WriteAsync

一、這裏主要說明2種異步寫入文件的方法：

1）異步編程模型API轉爲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是用來隨機生成要寫入的內容（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、異步從文件中讀取內容（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、模擬異步刪除（因爲API中沒有異步刪除）

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.");