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

Try this function ...

public static string GetTempFilePathWithExtension(string extension) {
  var path = Path.GetTempPath();
  var fileName = Path.ChangeExtension(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.

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);

    try
    {
      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();

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.

The MSDN documentation for C++'s GetTempFileName discusses your concern and answers it:

GetTempFileName is not able to guarantee that the file name is unique.

Only the lower 16 bits of the uUnique parameter are used. This limits GetTempFileName to a maximum of 65,535 unique file names if the lpPathName and lpPrefixString parameters remain the same.

Due to the algorithm used to generate file names, GetTempFileName can perform poorly when creating a large number of files with the same prefix. In such cases, it is recommended that you construct unique file names based on GUIDs.

Why not checking if the file exists?

string fileName;
do
{
    fileName = System.IO.Path.GetTempPath() + Guid.NewGuid().ToString() + ".csv";
} while (System.IO.File.Exists(fileName));

You can also do the following

string filepath = Path.ChangeExtension(Path.GetTempFileName(), ".csv");

and this also works as expected

string filepath = Path.ChangeExtension(Path.GetTempPath() + Guid.NewGuid().ToString(), ".csv");

How about:

Path.Combine(Path.GetTempPath(), DateTime.Now.Ticks.ToString() + "_" + Guid.NewGuid().ToString() + ".csv")

It is highly improbable that the computer will generate the same Guid at the same instant of time. The only weakness i see here is the performance impact DateTime.Now.Ticks will add.

In my opinion, most answers proposed here as sub-optimal. The one coming closest is the original one proposed initially by Brann.

A Temp Filename must be

• Unique
• Conflict-free (not already exist)
• Atomic (Creation of Name & File in the same operation)
• Hard to guess

Because of these requirements, it is not a godd idea to program such a beast on your own. Smart People writing IO Libraries worry about things like locking (if needed) etc. Therefore, I see no need to rewrite System.IO.Path.GetTempFileName().

This, even if it looks clumsy, should do the job:

//Note that this already *creates* the file
string filename1 = System.IO.Path.GetTempFileName()
// Rename and move
filename = filename.Replace(".tmp", ".csv");
File.Move(filename1 , filename);

I mixed @Maxence and @Mitch Wheat answers keeping in mind I want the semantic of GetTempFileName method (the fileName is the name of a new file created) adding the extension preferred.

string GetNewTempFile(string extension)
{
    if (!extension.StartWith(".")) extension="." + extension;
    string fileName;
    bool bCollisions = false;
    do {
        fileName = Path.Combine(System.IO.Path.GetTempPath(), Guid.NewGuid().ToString() + extension);
        try
        {
            using (new FileStream(fileName, FileMode.CreateNew)) { }
            bCollisions = false;
        }
        catch (IOException)
        {
            bCollisions = true;
        }
    }
    while (bCollisions);
    return fileName;
}

This could be handy for you... It's to create a temp. folder and return it as a string in VB.NET.

Easily convertible to C#:

Public Function GetTempDirectory() As String
    Dim mpath As String
    Do
        mpath = System.IO.Path.Combine(System.IO.Path.GetTempPath, System.IO.Path.GetRandomFileName)
    Loop While System.IO.Directory.Exists(mpath) Or System.IO.File.Exists(mpath)
    System.IO.Directory.CreateDirectory(mpath)
    Return mpath
End Function

This seems to work fine for me: it checks for file existance and creates the file to be sure it's a writable location. Should work fine, you can change it to return directly the FileStream (which is normally what you need for a temp file):

private string GetTempFile(string fileExtension)
{
  string temp = System.IO.Path.GetTempPath();
  string res = string.Empty;
  while (true) {
    res = string.Format("{0}.{1}", Guid.NewGuid().ToString(), fileExtension);
    res = System.IO.Path.Combine(temp, res);
    if (!System.IO.File.Exists(res)) {
      try {
        System.IO.FileStream s = System.IO.File.Create(res);
        s.Close();
        break;
      }
      catch (Exception) {

      }
    }
  }
  return res;
} // GetTempFile

This is a simple but effective way to generate incremental filenames. It will look in the current directly (you can easily point that somewhere else) and search for files with the base YourApplicationName*.txt (again you can easily change that). It will start at 0000 so that the first file name will be YourApplicationName0000.txt. if for some reason there are file names with junk between (meaning not numbers) the left and right parts, those files will be ignored by virtue of the tryparse call.

    public static string CreateNewOutPutFile()
    {
        const string RemoveLeft = "YourApplicationName";
        const string RemoveRight = ".txt";
        const string searchString = RemoveLeft + "*" + RemoveRight;
        const string numberSpecifier = "0000";

        int maxTempNdx = -1;

        string fileName;
        string [] Files = Directory.GetFiles(Directory.GetCurrentDirectory(), searchString);
        foreach( string file in Files)
        {
            fileName = Path.GetFileName(file);
            string stripped = fileName.Remove(fileName.Length - RemoveRight.Length, RemoveRight.Length).Remove(0, RemoveLeft.Length);
            if( int.TryParse(stripped,out int current) )
            {
                if (current > maxTempNdx)
                    maxTempNdx = current;
            }
        }
        maxTempNdx++;
        fileName = RemoveLeft + maxTempNdx.ToString(numberSpecifier) + RemoveRight;
        File.CreateText(fileName); // optional
        return fileName;
    }

Based on answers I found from the internet, I come to my code as following:

public static string GetTemporaryFileName()
{       
    string tempFilePath = Path.Combine(Path.GetTempPath(), "SnapshotTemp");
    Directory.Delete(tempFilePath, true);
    Directory.CreateDirectory(tempFilePath);
    return Path.Combine(tempFilePath, DateTime.Now.ToString("MMddHHmm") + "-" + Guid.NewGuid().ToString() + ".png");
}

And as C# Cookbook by Jay Hilyard, Stephen Teilhet pointed in Using a Temporary File in Your Application:

• you should use a temporary file whenever you need to store information temporarily for later retrieval.

• The one thing you must remember is to delete this temporary file before the application that created it is terminated.

• If it is not deleted, it will remain in the user’s temporary directory until the user manually deletes it.

This is what I am doing:

string tStamp = String.Format("{0:yyyyMMdd.HHmmss}", DateTime.Now);
string ProcID = Process.GetCurrentProcess().Id.ToString();
string tmpFolder = System.IO.Path.GetTempPath();
string outFile = tmpFolder + ProcID + "_" + tStamp + ".txt";

Easy Function in C#:

public static string GetTempFileName(string extension = "csv")
{
    return Path.ChangeExtension(Path.GetTempFileName(), extension);
}

In this what we can do we can first find the extension of file which is coming from file and after finding its extension.Then we can create the temprary name of file and after that we can change extension by the previous one it will works.

var name = Path.GetTempFileName();
var changename = Path.GetFileName(name);
var fileName = Path.ChangeExtension(changename, fileExtension);

I think you should try this:

string path = Path.GetRandomFileName();
path = Path.Combine(@"c:\temp", path);
path = Path.ChangeExtension(path, ".tmp");
File.Create(path);

It generates a unique filename and creates a file with that file name at a specified location.