Creating a unique file name in C#

I find that I am often writing files where the names either:

• Need to be incremented (rather than overwriting a file that already exists in the directory with the same name)

• Need to be timestamped

And, so, I've created the following methods to do so:

```
public static string CreateDatedFileName(string directory, string filename, string DateTimeFormat = "yyyy-MM-dd", bool AppendNumberIfNameAlreadyExists = true)
{
if (!CheckIfDirectoryExists(directory))
throw new ArgumentException(string.Format("The specified directory - {0} - does not exist", directory));

string formatedTemplate = "{0} - {1}.{2}";

var fileNameWithoutExtension = filename.Split('.')[0];
var fileExtension = filename.Split('.')[1];
var DateValue = DateTime.Now.ToString(DateTimeFormat);

string newname = String.Format(formatedTemplate, fileNameWithoutExtension, DateValue, fileExtension);
string path = Path.Combine(directory, newname);

if (!AppendNumberIfNameAlreadyExists)
return Path.Combine(directory, newname);
else
return CreateFileName(directory, newname, true);
}

public static string CreateFileName(string directory, string filename, bool AppendNumberIfNameAlreadyExists = true)
{
if (!CheckIfDirectoryExists(directory))
throw new ArgumentException(string.Format("The specified directory - {0} - does not exist", directory));

string formatedTemplate = "{0}{1}.{2}";

var fileNameWithoutExtension = filename.Split('.')[0];
var fileExtension = filename.Split('.')[1];

string newname = String.Format(formatedTemplate, fileNameWithoutExtension, string.Empty, fileExtension);
string path = Path.Combine(directory, newname);
int AppendDigit = 0;

while (AppendNumberIfNameAlreadyExists && CheckIfFilePathExists(path))
{
++AppendDigit;
newname = String.Format(formatedTemplate, fileNameWithoutExtension, "_" + AppendDigit, fileExtension);
path = Path.Combine(directory, newname);
}

The only way to guarantee that the generated filename is truly unique is to actually create the file. Any other method is vulnerable to a race condition. Therefore, if you are serious about uniqueness, you should change

public static string CreateFileName(string directory, string filename, bool AppendNumberIfNameAlreadyExists = true)

to

/// Creates a file in the directory with the given filename
///
/// 
/// If true, then a numbered suffix (starting from 0) is appended
/// to the filename if necessary to avoid a name clash.
///
/// If false, then an IOException is thrown if the file already exists.
/// 
public static FileStream CreateUniqueFile(string directory, string filename, bool AppendNumberIfNameAlreadyExists = true)

StackExchange Code Review Q#116709, answer score: 11