System.IO.IOException: "Unable to remove the file to be replaced" using File.Replace in Azure App Service

Question

We have code deployed to an Azure App Service that uses System.IO.File.Replace() to replace a file in the temporary folder of the web app:

File.Replace("c:\local\temp\source.pdf", "c:\local\temp\destination.pdf", "c:\local\temp\original.pdf", false)

This was working for several years until a couple weeks ago. It started throwing an Exception:

System.IO.IOException: Unable to remove the file to be replaced. at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath) at System.IO.File.InternalReplace(String sourceFileName, String destinationFileName, String destinationBackupFileName, Boolean ignoreMetadataErrors)

I tried connecting in kudu console to the web app and running the same command in powershell:

[System.IO.File]::Replace("c:\local\temp\source.pdf", "c:\local\temp\destination.pdf", "c:\local\temp\original.pdf", $false)

and I got back the same error message: "Unable to remove the file to be replaced."

I tried the same operation on a different app service on the same app service plan and got the same error, but on a different app service plan, the operation worked fine. I tried scaling the app service plan to different sizes but that didn't resolve it.

I opened a Microsoft support ticket but they have not been able to resolve yet.

I am wondering if anyone else has experienced the same issue or has a resolution?

Answer 1

The File.Replace method in C# is designed to atomically replace one file with another. However, it comes with certain conditions and limitations, and its behavior can be platform-specific.

• Here Below is my repro, I've created .net MVC application added a controller.

FileController:

using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using System;
using System.IO;

namespace WebApplication1.Controllers
{
    public class FileController : Controller
    {
        private readonly IWebHostEnvironment _hostingEnvironment;

        public FileController(IWebHostEnvironment hostingEnvironment)
        {
            _hostingEnvironment = hostingEnvironment;
        }

        public IActionResult Index()
        {
            return View();
        }

        [HttpPost]
        public IActionResult ReplaceFile()
        {
            // Get the physical path of the App_Data folder
            string appDataFolderPath = Path.Combine(_hostingEnvironment.ContentRootPath, "App_Data");

            // Specify the file names
            string sourceFileName = "source.pdf";
            string destinationFileName = "destination.pdf";

            // Build the full file paths
            string sourceFilePath = Path.Combine(appDataFolderPath, sourceFileName);
            string destinationFilePath = Path.Combine(appDataFolderPath, destinationFileName);

            try
            {
                if (System.IO.File.Exists(sourceFilePath))
                {
                    System.IO.File.Replace(sourceFilePath, destinationFilePath, null, false);
                    ViewBag.Message = "File replaced successfully!";
                }
                else
                {
                    ViewBag.Message = "Source file does not exist.";
                }
            }
            catch (Exception ex)
            {
                ViewBag.Message = $"Error: {ex.Message}";
            }

            return View("Index");
        }
    }
}

Application Folder Structure:

• Pdf files which you want to replace must be in the App_Data folder, If not it shows Source file does not exist.

Then I deployed it to my App service in azure portal. I can see my application deployed successfully and clicked on browse.

Iam able to see my source.pdf file replacing with destination.pdf.

Answer 2

The answer I got from Microsoft support was that they need to install a hotfix to the app service to fix this issue.