Http.IMiddleware 不通过 TaskScheduler 就不要创建任务

Question

(.Net 3.1, Visual Studio 2019)

在 DevExpress 代码示例中：https://github.com/DevExpress-Examples/blazor-server-dxdatagrid-export/blob/19.2.2%2B/CS/DxDataGridExportingWithReports/Helpers/ExportMiddleware.cs，以下 http 中间件代码得到了

的警告

Do not create tasks without passing a TaskScheduler ...

重写代码以启动新任务的正确方法是什么？

public class ExportMiddleware : IMiddleware
{
    ......

    public Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
       ......

        _ = await new TaskFactory().StartNew(() => // warning: Do not create tasks without passing a TaskScheduler
        {
            report.CreateDocument();
            using (MemoryStream fs = new MemoryStream())
            {
                if (format == pdf)
                    report.ExportToPdf(fs);
                else if (format == xlsx)
                    report.ExportToXlsx(fs);
                else if (format == docx)
                    report.ExportToDocx(fs);
                context.Response.Clear();
                context.Response.Headers.Append("Content-Type", "application/" + format);
                context.Response.Headers.Append("Content-Transfer-Encoding", "binary");
                context.Response.Headers.Append("Content-Disposition", "attachment; filename=ExportedDocument." + format);
                context.Response.Body.WriteAsync(fs.ToArray(), 0, fs.ToArray().Length);
                return context.Response.CompleteAsync();
            }
        });

Answer 1

从技术上讲，要回答实际问题，代码应使用 Task.Run 而不是 StartNew:

_ = Task.Run(() =>

但是，这是一个非常糟糕的主意。这不仅是在做 fire-and-forget，它还会在将来的某个随机时间使用 context.Response。它将开始写入响应流，然后在写入完成之前完成流。完全坏了。

我相信更合适的解决方案是完全删除factory/start/run并在必要时使用await：

public async Task InvokeAsync(HttpContext context, RequestDelegate next)
{
  ......
  report.CreateDocument();
  using (MemoryStream fs = new MemoryStream())
  {
    if (format == pdf)
      report.ExportToPdf(fs);
    else if (format == xlsx)
      report.ExportToXlsx(fs);
    else if (format == docx)
      report.ExportToDocx(fs);
    context.Response.Clear();
    context.Response.Headers.Append("Content-Type", "application/" + format);
    context.Response.Headers.Append("Content-Transfer-Encoding", "binary");
    context.Response.Headers.Append("Content-Disposition", "attachment; filename=ExportedDocument." + format);
    await context.Response.Body.WriteAsync(fs.ToArray(), 0, fs.ToArray().Length);
    await context.Response.CompleteAsync();
  }
}

Http.IMiddleware 不通过 TaskScheduler 就不要创建任务

Http.IMiddleware Do not create tasks without passing a TaskScheduler

c#

asp.net

asp.net-core

devexpress

task-parallel-library