Skip to main content

Downloading files using ASP.NET MVC

For a customer project in ASP.NET MVC 2, we added some functionality to download files. This can easily be done in ASP.NET MVC with the FileStreamResult class. Our original code looked something like this:

   1:  private ActionResult CreateFileStreamResult(string filepath, string fileResultName, string contentType)
   2:  {
   3:              FileStreamResult result = new FileStreamResult(new FileStream(filepath, FileMode.Open), contentType);
   4:              result.FileDownloadName = fileResultName;
   5:              return result;
   6:   }

Not long after deployment in our test enviroment, we discovered that it didn’t work. Instead we got back the following exception:

System.UnauthorizedAccessException: Access to the path is denied 

We double-checked the application pool, but the user account we used had the necessary read privileges on the target folder. After opening the FileStream class in Reflector, I noticed the cause of our problem. If you are using the constructor with FilePath and FileMode alone. System.IO.FileStream implementation of this constructor is

   1:  public FileStream(string path, FileMode mode)
   2:   : this(path, mode, (mode == FileMode.Append) ? FileAccess.Write : FileAccess.ReadWrite, FileShare.Read, 0×1000, FileOptions.None, Path.GetFileName(path), false)
   3:  {
   4:  }

The default constructor asks for ReadWrite access! That’s why a System.UnauthorizedAccessException is thrown in our test environment. The user account under which our application runs has no write access.

As our application needs only Read access to the folder,  we change the code and use the following FileStream constructor:

   1:  using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))

It would probably have been better if  Read access was the default in System.IO.FileStream rather than ReadWrite.

Labels:

Popular posts from this blog

Podman– Command execution failed with exit code 125

After updating WSL on one of the developer machines, Podman failed to work. When we took a look through Podman Desktop, we noticed that Podman had stopped running and returned the following error message: Error: Command execution failed with exit code 125 Here are the steps we tried to fix the issue: We started by running podman info to get some extra details on what could be wrong: >podman info OS: windows/amd64 provider: wsl version: 5.3.1 Cannot connect to Podman. Please verify your connection to the Linux system using `podman system connection list`, or try `podman machine init` and `podman machine start` to manage a new Linux VM Error: unable to connect to Podman socket: failed to connect: dial tcp 127.0.0.1:2655: connectex: No connection could be made because the target machine actively refused it. That makes sense as the podman VM was not running. Let’s check the VM: >podman machine list NAME         ...
Read more

Cache stampede: when our cache turned against us

While investigating some performance issues, we ran into an ASP.NET Core API that cached a fairly expensive aggregation query for 60 seconds. Under normal load, that was fine: one request rebuilds the cache, everyone else reads from it. Under peak load, dozens of requests would arrive in that same expiry window, all see a cache miss, and all fire the same expensive query in parallel. The database didn't like that. That was the moment when our caching layer stopped helping and started hurting. A burst of requests comes in at the same time, all miss the cache, and all go hammer the database or the downstream API at once. That's a cache stampede . The cache was supposed to protect our backend, and for a few hundred milliseconds it did the opposite. Why this happens IMemoryCache.GetOrCreate (and its async sibling) looks like it protects you, but it doesn't add any locking on its own. Look at the naive version: public async Task<Report> GetReportAsync(string key) ...
Read more

A complex system designed from scratch never works

A few years ago, I worked as an architect on a big mainframe rewrite. I still count it as one of my failures. Not because the technology was wrong, but because I couldn't convince the management team to simplify the approach. Years later, the organization is still struggling to get the new system up and running. I left the project at the time, because I couldn't put my name behind an approach that would take very long and cost a lot of money without a working system to show for it along the way. Gall’s Law That memory keeps coming back to me, because it's a textbook case of Gall's Law playing out in real life. Gall's Law , from John Gall's Systemantics , states it plainly: A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works, and it cannot be patched to make it work. You have to start over with a simple system that works. What does that mean in practice,...
Read more