以下是一个使用ASP.NET Core 2.1的简单示例，演示如何在同一页上上传文件并显示上传的文件。

首先，在你的ASP.NET Core 2.1项目中，创建一个名为"UploadFile"的控制器：

using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using System.IO;
using System.Threading.Tasks;

namespace YourNamespace.Controllers
{
    public class UploadFileController : Controller
    {
        private readonly IWebHostEnvironment _environment;

        public UploadFileController(IWebHostEnvironment environment)
        {
            _environment = environment;
        }

        [HttpGet]
        public IActionResult Index()
        {
            return View();
        }

        [HttpPost]
        public async Task Index(IFormFile file)
        {
            if (file != null && file.Length > 0)
            {
                var filePath = Path.Combine(_environment.WebRootPath, "uploads", file.FileName);

                using (var stream = new FileStream(filePath, FileMode.Create))
                {
                    await file.CopyToAsync(stream);
                }

                ViewBag.Message = "文件上传成功！";
            }
            else
            {
                ViewBag.Message = "请选择一个文件。";
            }

            return View();
        }
    }
}

下一步是创建一个名为"Index.cshtml"的视图，用于显示文件上传表单和上传后的消息：

@{
    ViewData["Title"] = "文件上传";
}

文件上传

@if (ViewBag.Message != null)
{
    
@ViewBag.Message
}


    

        选择文件：
        
    
    上传

最后，在"Startup.cs"文件中，将路由配置为使用"UploadFile"控制器的"Index"动作：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace YourNamespace
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");
            });
        }
    }
}

完成上述步骤后，你的ASP.NET Core 2.1应用程序就能够在同一页上上传文件并显示上传的文件了。