asp.net core 3.1 中Synchronous operations are disallowed. Call FlushAsync or set AllowSynchronousIO to true instead

　　　　　在进行　Asp.NetCore.MVC  文件上传时，后台无法正常读取文件流保存，出现：Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.

　　　　

 　　查找资料，发现需要添加允许条件，才可以； 感谢：https://www.cnblogs.com/lonelyxmas/p/12060869.html

　　有三种解决方式：第一种：在处理文件的Action 中添加：　　

var syncIOFeature = HttpContext.Features.Get<IHttpBodyControlFeature>();
if (syncIOFeature != null)
{
    syncIOFeature.AllowSynchronousIO = true;
}

　　第二种：或者在Startup.cs 中注册

 public void ConfigureServices(IServiceCollection services)
        {
            // If using Kestrel:
            services.Configure<KestrelServerOptions>(options =>
            {
                options.AllowSynchronousIO = true;
            });

            // If using IIS:
            services.Configure<IISServerOptions>(options =>
            {
                options.AllowSynchronousIO = true;
            });
}

　　第三种:(不太理解。。。)

            Request.EnableBuffering();
            using (var reader = new StreamReader(Request.Body, encoding: Encoding.UTF8))
            {
                var body = reader.ReadToEndAsync();
                // Do some processing with body…
                // Reset the request body stream position so the next middleware can read it
                Request.Body.Position = 0;
            }