How can I create a temp file with a specific extension with .NET? – Dev

The best answers to the question “How can I create a temp file with a specific extension with .NET?” in the category Dev.


I need to generate a unique temporary file with a .csv extension.

What I do right now is

string filepath = System.IO.Path.GetTempFileName().Replace(".tmp", ".csv");

However, this doesn’t guarantee that my .csv file will be unique.

I know the chances I ever got a collision are very low (especially if you consider that I don’t delete the .tmp files), but this code doesn’t looks good to me.

Of course I could manually generate random file names until I eventually find a unique one (which shouldn’t be a problem), but I’m curious to know if others have found a nice way to deal with this problem.


Try this function …

public static string GetTempFilePathWithExtension(string extension) {
  var path = Path.GetTempPath();
  var fileName = Guid.NewGuid().ToString() + extension;
  return Path.Combine(path, fileName);

It will return a full path with the extension of your choice.

Note, it’s not guaranteed to produce a unique file name since someone else could have technically already created that file. However the chances of someone guessing the next guid produced by your app and creating it is very very low. It’s pretty safe to assume this will be unique.


Guaranteed to be (statistically) unique:

string fileName = System.IO.Path.GetTempPath() + Guid.NewGuid().ToString() + ".csv"; 

(To quote from the wiki article on the probabilty of a collision:

…one’s annual risk of being hit by a
meteorite is estimated to be one
chance in 17 billion [19], that means
the probability is about 0.00000000006
(6 × 10−11), equivalent to the odds of
creating a few tens of trillions of
UUIDs in a year and having one
duplicate. In other words, only after
generating 1 billion UUIDs every
second for the next 100 years, the
probability of creating just one
duplicate would be about 50%. The
probability of one duplicate would be
about 50% if every person on earth
owns 600 million UUIDs

EDIT: Please also see JaredPar’s comments.


You can also alternatively use System.CodeDom.Compiler.TempFileCollection.

string tempDirectory = @"c:\\temp";
TempFileCollection coll = new TempFileCollection(tempDirectory, true);
string filename = coll.AddExtension("txt", true);
File.WriteAllText(Path.Combine(tempDirectory,filename),"Hello World");

Here I used a txt extension but you can specify whatever you want. I also set the keep flag to true so that the temp file is kept around after use. Unfortunately, TempFileCollection creates one random file per extension. If you need more temp files, you can create multiple instances of TempFileCollection.


public static string GetTempFileName(string extension)
  int attempt = 0;
  while (true)
    string fileName = Path.GetRandomFileName();
    fileName = Path.ChangeExtension(fileName, extension);
    fileName = Path.Combine(Path.GetTempPath(), fileName);

      using (new FileStream(fileName, FileMode.CreateNew)) { }
      return fileName;
    catch (IOException ex)
      if (++attempt == 10)
        throw new IOException("No unique temporary file name is available.", ex);

Note: this works like Path.GetTempFileName. An empty file is created to reserve the file name. It makes 10 attempts, in case of collisions generated by Path.GetRandomFileName();