asp.net core 3.1 中Synchronous operations are disallowed. Call FlushAsync or set AllowSynchronousIO t

　　　　　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();
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(options =>
            {
                options.AllowSynchronousIO = true;
            });

            // If using IIS:
            services.Configure(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;
            }