在C#/.NET中，将文本内容写入文件最简单的方法是调用 File.WriteAllText() 方法，但这个方法没有异步的实现，要想用异步，只能改用有些复杂的 FileStream.WriteAsync() 方法。
使用 FileStream.WriteAsync() 有2个需要注意的地方，1是要设置bufferSize，2是要将useAsync这个构造函数参数设置为true，示例调用代码如下：
public async Task CommitAsync()
{
var bits = Encoding.UTF8.GetBytes("{\"text\": \"test\"}");
using (var fs = new FileStream(
path: @"C:\temp\test.json",
mode: FileMode.Create,
access: FileAccess.Write,
share: FileShare.None,
bufferSize: ,
useAsync: true))
{
await fs.WriteAsync(bits, , bits.Length);
}
}
看这个方法的帮助文档中对useAsync参数的说明：
// useAsync:
// Specifies whether to use asynchronous I/O or synchronous I/O. However, note
// that the underlying operating system might not support asynchronous I/O,
// so when specifying true, the handle might be opened synchronously depending
// on the platform. When opened asynchronously, the System.IO.FileStream.BeginRead(System.Byte[],System.Int32,System.Int32,System.AsyncCallback,System.Object)
// and System.IO.FileStream.BeginWrite(System.Byte[],System.Int32,System.Int32,System.AsyncCallback,System.Object)
// methods perform better on large reads or writes, but they might be much slower
// for small reads or writes. If the application is designed to take advantage
// of asynchronous I/O, set the useAsync parameter to true. Using asynchronous
// I/O correctly can speed up applications by as much as a factor of 10, but
// using it without redesigning the application for asynchronous I/O can decrease
// performance by as much as a factor of 10.
从中可以得知，只有设置useAsync为true，才真正使用上了异步IO。