public static IHostBuilder CreateHostBuilder(string[] args) =>
 Host.CreateDefaultBuilder(args)
 .ConfigureWebHostDefaults(webBuilder =>
 {
  webBuilder.ConfigureKestrel((context, options) =>
  {
  //设置应用服务器Kestrel请求体最大为50MB
  options.Limits.MaxRequestBodySize = 52428800;
  });
  webBuilder.UseStartup<Startup>();
});

services.Configure<FormOptions>(options =>
 {
  options.MultipartBodyLengthLimit = long.MaxValue;
 });

public override int Read(byte[] buffer, int offset, int count)
 {
  
  var bufferedData = _innerStream.BufferedData;
　　　　　　int read;
　　　　 read = _innerStream.Read(buffer, offset, Math.Min(count, bufferedData.Count));
  return UpdatePosition(read);
}

private int UpdatePosition(int read)
 {
  _position += read;
  if (_observedLength < _position)
  {
  _observedLength = _position;
  if (LengthLimit.HasValue && _observedLength > LengthLimit.GetValueOrDefault())
  {
   throw new InvalidDataException($"Multipart body length limit {LengthLimit.GetValueOrDefault()} exceeded.");
  }
  }
  return read;
}

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.WebUtilities;
using Microsoft.Net.Http.Headers;
using System.IO;
using System.Threading.Tasks;
 
namespace BigFilesUpload.Controllers
{
 [Route("api/[controller]")]
 public class FileController : Controller
 {
 private readonly string _targetFilePath = "C:\\files\\TempDir";
 
 /// <summary>
 /// 流式文件上传
 /// </summary>
 /// <returns></returns>
 [HttpPost("UploadingStream")]
 public async Task<IActionResult> UploadingStream()
 {
 
  //获取boundary
  var boundary = HeaderUtilities.RemoveQuotes(MediaTypeHeaderValue.Parse(Request.ContentType).Boundary).Value;
  //得到reader
  var reader = new MultipartReader(boundary, HttpContext.Request.Body);
  //{ BodyLengthLimit = 2000 };//
  var section = await reader.ReadNextSectionAsync();
 
  //读取section
  while (section != null)
  {
  var hasContentDispositionHeader = ContentDispositionHeaderValue.TryParse(section.ContentDisposition, out var contentDisposition);
  if (hasContentDispositionHeader)
  {
   var trustedFileNameForFileStorage = Path.GetRandomFileName();
   await WriteFileAsync(section.Body, Path.Combine(_targetFilePath, trustedFileNameForFileStorage));
  }
  section = await reader.ReadNextSectionAsync();
  }
  return Created(nameof(FileController), null);
 }
 
 /// <summary>
 /// 缓存式文件上传
 /// </summary>
 /// <param name=""></param>
 /// <returns></returns>
 [HttpPost("UploadingFormFile")]
 public async Task<IActionResult> UploadingFormFile(IFormFile file)
 {
  using (var stream = file.OpenReadStream())
  {
  var trustedFileNameForFileStorage = Path.GetRandomFileName();
  await WriteFileAsync(stream, Path.Combine(_targetFilePath, trustedFileNameForFileStorage));
  }
  return Created(nameof(FileController), null);
 }
 
 
 /// <summary>
 /// 写文件导到磁盘
 /// </summary>
 /// <param name="stream">流</param>
 /// <param name="path">文件保存路径</param>
 /// <returns></returns>
 public static async Task<int> WriteFileAsync(System.IO.Stream stream, string path)
 {
  const int FILE_WRITE_SIZE = 84975;//写出缓冲区大小
  int writeCount = 0;
  using (FileStream fileStream = new FileStream(path, FileMode.Create, FileAccess.Write, FileShare.Write, FILE_WRITE_SIZE, true))
  {
  byte[] byteArr = new byte[FILE_WRITE_SIZE];
  int readCount = 0;
  while ((readCount = await stream.ReadAsync(byteArr, 0, byteArr.Length)) > 0)
  {
   await fileStream.WriteAsync(byteArr, 0, readCount);
   writeCount += readCount;
  }
  }
  return writeCount;
 }
 
 }
}