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


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

What I do right now is

string filename = 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.

3/14/2016 11:38:23 AM

Accepted Answer

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:'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.

5/1/2009 2:21:59 PM

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.

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