在Asp.net核心Razor页面中错误上传图像

我是Asp.net核心的新手，我想上传图片。我一直在关注此视频（https://www.youtube.com/watch?v=Fx-EQ1hT8R0）。在运行应用程序时，当我选择图像并单击提交时，该程序会自动关闭。我不知道错误发生在哪里。

这是HTML页面：

<form method="post" enctype="multipart/form-data">
    <p>
        <input type="file" name="uploadfiles" />
        <input type="submit" value="Upload" class="btn btn-primary" />
    </p>
</form>

这是IndexModel页面：

public class IndexModel : PageModel
    {
        private readonly Razorpages_FileUploadContext _context;
        private readonly IWebHostEnvironment _environment;

        public IndexModel(Razorpages_FileUploadContext context, IWebHostEnvironment environment)
        {
            _context = context;
            _environment = environment;
        }

        public IList<ImageSave> ImageSave { get; set; }

        public async Task OnGetAsync()
        {
            ImageSave = await _context.ImageSave.ToListAsync();
        }

        public async Task<IActionResult> OnPostAsync(IFormFile uploadfiles, ImageSave img)
        {
            if (!ModelState.IsValid)
            {
                return Page();
            }
            string imgext = Path.GetExtension(uploadfiles.FileName);
            if (imgext == ".jpg" || imgext == ".png" || imgext == ".gif")
            {
                var imgsave = Path.Combine(_environment.WebRootPath, "Images", uploadfiles.FileName);
                var stream = new FileStream(imgsave, FileMode.Create);
                await uploadfiles.CopyToAsync(stream);
                stream.Close();
                img.Imgname = uploadfiles.FileName;
                img.Imgpath = imgsave;
                await _context.ImageSave.AddAsync(img);
                await _context.SaveChangesAsync();
            }
            return RedirectToPage("./Index");
        }
    }

我上传图像并提交，程序关闭，cmd中的错误消息：D：\ c_sharp \ LearnRazorPages \ Razorpages_FileUpload \ Razorpages_FileUpload \ bin \ Debug \ netcoreapp3.1 \ Razorpages_FileUpload.exe（进程12992）退出，代码为-1。要在调试停止时自动关闭控制台，请启用工具->选项->调试->在调试停止时自动关闭控制台。

请问你能帮帮我吗！