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Below is a complete, production‐style example demonstrating how to handle file uploads (and editing) in an ASP.NET Core MVC application. In this example, a document (or file) is stored with its file path in the database (here simulated by an in‑memory list) and the physical file is saved in wwwroot/uploads/. The edit page is designed so that if validation fails, the existing file isn’t lost (using a hidden field to preserve the file path). We also include a custom validation attribute for allowed file types, a dedicated file service for file operations, a view model that supports both new file upload and retaining the existing file, and an MVC controller with GET/POST actions.

Below you’ll find full code examples for each part:

---

1. The Model

This model represents our document and stores the file path.

csharp
// Models/Document.cs
namespace MyMvcApp.Models
{
    public class Document
    {
        public int Id { get; set; }
        public string Title { get; set; }
        // FilePath stores a relative path to the file saved in wwwroot/uploads/
        public string FilePath { get; set; }
    }
}

---

2. The ViewModel and Custom Validation

The view model is used by the Edit view. It includes an optional new file upload (an IFormFile) and a hidden property for the existing file path so that if validation fails, the file is not lost. We also add a custom validation attribute to enforce allowed file extensions.

csharp
// ViewModels/DocumentEditViewModel.cs
using Microsoft.AspNetCore.Http;
using System.ComponentModel.DataAnnotations;
using System.IO;
using System.Linq;

namespace MyMvcApp.ViewModels
{
    public class DocumentEditViewModel
    {
        public int Id { get; set; }

        [Required(ErrorMessage = "Title is required")]
        public string Title { get; set; }

        // Optionally upload a new file (if provided, it will replace the existing file)
        [AllowedExtensions(new string[] { ".jpg", ".png", ".pdf", ".docx" })]
        public IFormFile File { get; set; }

        // Hidden field to keep the current file path in case no new file is uploaded.
        public string ExistingFilePath { get; set; }
    }

    // Custom validation attribute to check allowed file extensions.
    public class AllowedExtensionsAttribute : ValidationAttribute
    {
        private readonly string[] _extensions;
        public AllowedExtensionsAttribute(string[] extensions)
        {
            _extensions = extensions;
        }
        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            var file = value as IFormFile;
            if (file != null)
            {
                var extension = Path.GetExtension(file.FileName);
                if (!_extensions.Contains(extension.ToLower()))
                {
                    return new ValidationResult($"The file extension {extension} is not allowed.");
                }
            }
            return ValidationResult.Success;
        }
    }
}

---

3. The File Service

This service encapsulates file operations (saving and deleting) and uses the IWebHostEnvironment to determine the wwwroot path. In a real project, you’d register this service via dependency injection in your Startup.cs or Program.cs.

csharp
// Services/IFileService.cs
using Microsoft.AspNetCore.Http;

namespace MyMvcApp.Services
{
    public interface IFileService
    {
        // Saves the file and returns the relative path (e.g., "uploads/unique_filename.ext")
        string SaveFile(IFormFile file);

        // Deletes the file based on the relative file path.
        bool DeleteFile(string filePath);
    }
}

csharp
// Services/FileService.cs
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using System;
using System.IO;

namespace MyMvcApp.Services
{
    public class FileService : IFileService
    {
        private readonly IWebHostEnvironment _env;
        public FileService(IWebHostEnvironment env)
        {
            _env = env;
        }

        public string SaveFile(IFormFile file)
        {
(_env.WebRootPath, "uploads");
            if (!Directory.Exists(uploadsFolder))
                Directory.CreateDirectory(uploadsFolder);

            // Generate a unique file name to prevent collisions.
            var uniqueFileName = Guid.NewGuid().ToString() + "_" + Path.GetFileName(file.FileName);
            var filePath = Path.Combine(uploadsFolder, uniqueFileName);

            using (var fileStream = new FileStream(filePath, FileMode.Create))
            {
                file.CopyTo(fileStream);
            }

            // Return a relative path to be stored in the database.
            return "uploads/" + uniqueFileName;
        }

        public bool DeleteFile(string filePath)
        {
            if (string.IsNullOrEmpty(filePath))
                return false;

            var fullPath = Path.Combine(_env.WebRootPath, filePath);
            if (File.Exists(fullPath))
            {
                File.Delete(fullPath);
                return true;
            }
            return false;
        }
    }
}

---

4. The Controller

The controller demonstrates how to load the existing document into a view model for editing and then handle the postback. If a new file is uploaded, it replaces the old one; otherwise, the original file path is preserved. Note that in a real application you would use a database context instead of an in‑memory list.

csharp
// Controllers/DocumentController.cs
using Microsoft.AspNetCore.Mvc;
using MyMvcApp.Models;
using MyMvcApp.ViewModels;
using MyMvcApp.Services;
using System.Collections.Generic;
using System.Linq;

namespace MyMvcApp.Controllers
{
    public class DocumentController : Controller
    {
        private readonly IFileService _fileService;

        // In‑memory "database" for demonstration purposes.
        private static List<Document> _documents = new List<Document>
        {
            new Document { Id = 1, Title = "Sample Document", FilePath = "uploads/sample.pdf" }
        };

        public DocumentController(IFileService fileService)
        {
            _fileService = fileService;
        }

        // GET: /Document/Edit/1
        public IActionResult Edit(int id)
        {
            var document = _documents.FirstOrDefault(d => d.Id == id);
            if (document == null)
                return NotFound();

            var viewModel = new DocumentEditViewModel
            {
                Id = document.Id,
                Title = document.Title,
                ExistingFilePath = document.FilePath
            };

            return View(viewModel);
        }

        // POST: /Document/Edit
        [HttpPost]
        [ValidateAntiForgeryToken]
        public IActionResult Edit(DocumentEditViewModel viewModel)
        {
            if (ModelState.IsValid)
            {
                var document = _documents.FirstOrDefault(d => d.Id == viewModel.Id);
                if (document == null)
                    return NotFound();

                document.Title = viewModel.Title;

                // If a new file is uploaded, delete the old file and save the new one.
                if (viewModel.File != null)
                {
                    if (!string.IsNullOrEmpty(document.FilePath))
                    {
                        _fileService.DeleteFile(document.FilePath);
                    }
                    var newFilePath = _fileService.SaveFile(viewModel.File);
                    document.FilePath = newFilePath;
                }
                else
                {
                    // No new file uploaded; preserve the existing file.
                    document.FilePath = viewModel.ExistingFilePath;
                }

                // In a real application, update the database here.
                return RedirectToAction("Index");
            }

            // If validation fails, the view will redisplay the form
            // with the hidden field preserving the current file path.
            return View(viewModel);
        }

        // Simple Index action to list documents.
        public IActionResult Index()
        {
            return View(_documents);
        }
    }
}

---

5. The Views

Edit View (Razor)

The edit view uses a form with enctype="multipart/form-data". It shows the current file (if available) and includes a hidden field to keep the existing file path. If model validation fails, the user won’t be forced to re‑upload the file because the file path is preserved.

html
@* Views/Document/Edit.cshtml *@
@model MyMvcApp.ViewModels.DocumentEditViewModel
@{
    ViewData["Title"] = "Edit Document";
}
<h2>Edit Document</h2>

<form asp-action="Edit" enctype="multipart/form-data" method="post">
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
    <input type="hidden" asp-for="Id" />
    <input type="hidden" asp-for="ExistingFilePath" />

    <div class="form-group">
        <label asp-for="Title"></label>
        <input class="form-control" asp-for="Title" />
        <"></span>
    </div>

    <div class="form-group">
        <label>Current File:</label>
        @if (!string.IsNullOrEmpty(Model.ExistingFilePath))
        {
            <a href="~/@Model.ExistingFilePath" target="_blank">View File</a>
        }
        else
        {
            <p>No file uploaded.</p>
        }
    </div>

    <div class="form-group">
        <label asp-for="File">Upload New File (optional):</label>
        <input type="file" asp-for="File" class="form-control-file" />
        <span asp-validation-for="File" class="text-danger"></span>
    </div>

    <button type="submit" class="btn btn-primary">Save</button>
</form>

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

Index View

This view lists all documents with a link to edit each one.

html
@* Views/Document/Index.cshtml *@
@model IEnumerable<MyMvcApp.Models.Document>
@{
    ViewData["Title"] = "Documents";
}
<h2>Documents</h2>

<table class="table">
    <thead>
        <tr>
            <th>Id</th>
            <th>Title</th>
            <th>File</th>
            <th>Actions</th>
        </tr>
    </thead>
    <tbody>
    @foreach (var item in Model)
    {
        <tr>
            <td>@item.Id</td>
            <td>@item.Title</td>
            <td>
                @if (!string.IsNullOrEmpty(item.FilePath))
                {
                    <a href="~/@item.FilePath" target="_blank">View File</a>
                }
                else
                {
                    <span>No file</span>
                }
            </td>
            <td>
                <a asp-action="Edit" asp-route-id="@item.Id">Edit</a>
            </td>
        </tr>
    }
    </tbody>
</table>

---

6. Registration in Startup/Program

Finally, don’t forget to register the FileService with dependency injection. In your Startup.cs (or Program.cs in .NET 6+), add:

csharp
// In Startup.cs -> ConfigureServices or in Program.cs (for .NET 6+)
services.AddScoped<IFileService, FileService>();

---

Summary

This extensive example covers:

• Models: Representing your document with a file path.
• ViewModels & Validation: Handling file uploads via IFormFile and retaining the existing file path through a hidden field.
• Service Layer: A dedicated file service to abstract file saving and deletion.
• Controller: Managing GET/POST logic to edit a document while handling file operations.
• Views: A Razor view for editing that ensures if validation fails, the existing file remains referenced.

This approach is aligned with industry standards for clean separation of concerns, dependency injection, and robust form handling in ASP.NET Core MVC.