C#異步將文本內容寫入文件

在C#/.NET中，將文本內容寫入文件最簡單的方法是調用 File.WriteAllText() 方法，但這個方法沒有異步的實現，要想用異步，只能改用有些復雜的 FileStream.WriteAsync() 方法。

使用 FileStream.WriteAsync() 有2個需要注意的地方，1是要設置bufferSize，2是要將useAsync這個構造函數參數設置為true，示例調用代碼如下：

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);
    }
}

看這個方法的幫助文檔中對useAsync參數的說明：

//   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.

從中可以得知，只有設置useAsync為true，才真正使用上了異步IO。