我用www.example.com的核心网络应用程序创建了一个文档上传网站asp.net，我遇到了一个小错误，但我不知道如何修复它。
在我的网站上，你首先创建一个“文件”，如下所示：

然后它会出现在列表中，如下所示：

当您按下upload attachment时，它会传递上一个表中的id，以确保它上传到正确的文件。

上传页面后面的代码如下，错误是如果在选择文件之前按upload，它会刷新页面显示错误，然后之前传递的ID已经丢失，所以myInv最终为null。

using FarmersPortal.Data;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using File = FarmersPortal.Data.File;

namespace FarmersPortal.Pages.Admin
{
    [Authorize(Roles ="Admin")]
    public class UploadModel : PageModel
    {
        private readonly filedbContext _context;

        public UploadModel(filedbContext context)
        {

            _context = context;
        }
        public int? myID { get; set; }

        [BindProperty]
        public IFormFile file { get; set; }

        [BindProperty]
        public int? ID { get; set; }
        public void OnGet(int? id)
        {
            myID = id;
        }

        [BindProperty]
        public File File { get; set; }
        public async Task<IActionResult> OnPostAsync()
        {
            if (file != null)
            {
                if (file.Length > 0 && file.Length < 300000)
                {
                    var myInv = _context.Files.FirstOrDefault(x => x.Id == ID);
                    var date = DateTime.Today;

                    using (var target = new MemoryStream())
                    {
                        file.CopyTo(target);
                        myInv.UploadDate = date;
                        myInv.Attachment = target.ToArray();
                    }
                    if (myInv == null)
                    {
                        return NotFound();
                    }
                    else
                    {
                        File = myInv;
                    }
                    _context.Files.Update(myInv);
                    await _context.SaveChangesAsync();
                }

            }

            if (File.FileType == "Purchase Order")
            {
                return RedirectToPage("./PurchaseOrders");
            }
            else if (File.FileType == "Remittance")
            {
                return RedirectToPage("./Remittance");
            }
            else if (File.FileType == "Haulage Self Bill")
            {
                return RedirectToPage("./HaulageSelfBill");
            }
            else if (File.FileType == "Growers Return")
            {
                return RedirectToPage("./GrowersReturn");
            }
            return Page();
        }
    }
}

我不知道该怎么办，有什么想法吗？

@page
@model FarmersPortal.Pages.Admin.UploadModel
@{
}

<h1 style="color:white">Upload File</h1>

<style>
    body {
        background-image: url("http://10.48.1.215/PORTAL/hero-range-1.jpg");
        height: 100%;
        background-position: center;
        background-repeat: no-repeat;
        background-size: cover;
    }
</style>

<hr />
<div class="row">
    <div class="col-md-4">
        <form method="post" enctype="multipart/form-data">
            <div class="form-group">
                <div class="col-md-10">
                    <p style="color:white">Upload file</p>
                    <input type="hidden" asp-for="@Model.ID" value="@Model.myID" />
                    <input asp-for="file" class="form-control" accept=".pdf" type="file" />
                    <span asp-validation-for="file" class="text-white"></span>
                </div>
            </div>
            <div class="form-group">
                <div class="col-md-10">
                    <input class="btn btn-success" type="submit" value="Upload" />
                </div>
            </div>
        </form>
    </div>
</div>

<div>
    <a asp-page="Index">Back to List</a>
</div>

上面是前端代码。