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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: 4096, useAsync: true)) { await fs.WriteAsync(bits, 0, 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。
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原文地址：http://www.cnblogs.com/dudu/p/async_file_write_text.html
