Will path.getrandomfilename generate a unique filename every time?

Question

Will Path.GetRandomFileName generate a unique filename every single time? Also, what about Path.GetTempFileName - will that generate a unique name?

Answer 1

Copying from my fiddle at https://dotnetfiddle.net/bmFVSX the Path_SafeGetTempFilename method should give you a bit more safety (but still is prone to low probability data race that could be minimized by using your own temp subfolder):

//.NET's Path.GetTempFilename (https://docs.microsoft.com/en-us/dotnet/api/system.io.path.gettempfilename?view=net-5.0) raises an IOException if the system's temp folder has more than 65535 files

//Following a suggestion from https://github.com/dotnet/sdk/issues/8439
//to use Path.GetTempPath (https://docs.microsoft.com/en-us/dotnet/api/system.io.path.gettemppath?view=net-5.0) to get the system temp folder
//combined via Path.Combine (https://docs.microsoft.com/en-us/dotnet/api/system.io.path.combine?view=net-5.0)
//with GetRandomFilename (https://docs.microsoft.com/en-us/dotnet/api/system.io.path.getrandomfilename?view=net-5.0) instead

//For extra safety cross-checking (in a do/while loop) with File.Exists (https://docs.microsoft.com/en-us/dotnet/api/system.io.file.exists?view=net-5.0) before returning the file

//Using File.WriteAllLines (https://docs.microsoft.com/en-us/dotnet/api/system.io.file.writealllines?view=net-5.0) to create the temp file since that is the original behaviour of GetTempFilename in contrast to GetRandomFilename

//Note: "Path.GetRandomFileName() uses the RNGCryptoServiceProvider which is a cryptographically strong random number generator", quoting https://stackoverflow.com/questions/21684696/will-path-getrandomfilename-generate-a-unique-filename-every-time/21684881#21684881

using System;
using System.Diagnostics;
using System.IO;

public class Program
{
    public static string Path_SafeGetTempFileName() {
        string filepath; //no need to initialize with empty/null value, the do/while code flow construct will always set a value to filepath
        do
        {
            filepath = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName());
        } while (File.Exists(filepath));

        try
        {
            File.WriteAllLines(filepath, new string[]{}); //generate an empty file
        }
        catch
        {
            Debug.WriteLine($"Couldn't create empty temporary file: {filepath}");
            //TODO: log any failure to generate an empty temp file here
        }
        //Note: don't do a loop if it fails to create the temp file since it might be failing to access the filesystem for other reasons, which would cause infinite recursion

        return filepath;
    }

    public static void Main()
    {
        string tempFilePath1 = Path.GetTempFileName();
        string tempFilePath2 = Path_SafeGetTempFileName();

        Console.WriteLine(tempFilePath1);
        Console.WriteLine(tempFilePath2);       
    }
}