asp.net ashx一般处理程序实现async await异步操作

目前项目存在页面展示大量图片，效率不高，考虑优化性能，改为ashx+异步下载的方式，废话不说直接贴code：

using System;
using System.Web;
using System.IO;
using System.Web.SessionState;
using System.Threading.Tasks;

public class DownLoadS3FileAsync : System.Web.HttpTaskAsyncHandler
{

    public void ProcessRequest(HttpContext context)
    {
        throw new NotImplementedException();
    }

    public override async Task ProcessRequestAsync(HttpContext context)
    {

        SystemError.CreateErrorLog("开始执行异步啦");
        await Task.Run(() =>
        {
            //调用下载方法
            Download(context);
        });
        SystemError.CreateErrorLog("调用异步完成啦");
    }

    public void Download(HttpContext context)
    {
        SystemError.CreateErrorLog("开始下载啦");
        //文件路径
        string key = context.Request.QueryString["key"];
        byte[] bFile = StorageService.Current.Download(key);

        //输出
        context.Response.Clear();
        context.Response.AddHeader("Content-Disposition", "attachment; filename=" + fileName);
        context.Response.AddHeader("Content-Length", bFile.Length.ToString());
        context.Response.ContentType = contentType;
        context.Response.OutputStream.Write(bFile, , bFile.Length);
        context.Response.Flush();
        context.Response.End();

        SystemError.CreateErrorLog("下载结束了！");
    }
}