Creating a unique file name in C#

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
///
/// <param name="AppendNumberIfNameAlreadyExists">
/// 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.
/// </param>
public static FileStream CreateUniqueFile(string directory, string filename, bool AppendNumberIfNameAlreadyExists = true)

You're assuming that filenames can't contain a period/full stop . other than to separate the name and the extension:

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

That's not true: "my.file.txt" is a perfectly valid filename - your code appears to incorrectly drop the file extension and changes the extension to ".file". That's a pretty big bug.

You should use the methods available on the System.Io.Path class e.g. GetFileNameWithoutExtension rather than using Split.

Here are some immediate responses to your post:

• Please follow name conventions for parameters – I was very confused trying to locate DateTimeFormat and some others, please do use camelCase all over unless we're talking about public properties.
• Why the CheckIfDirectoryExists()? – I'm kind of confused as to why you've made this functions, when the original System.IO.Directory.Exists() is almost the same size? If you add using System, you do the IO.Directory.Exists() or IO.File.Exists() and it is actually shorter than your extra functions...
• The two main functions are very similar – I would consider joining your two functions into one, and possibly have some sort of loop choosing what extension to use. One way to join these could be to add another parameter being the extension (which then would/could skip the splitting of the file name)
• Alternate extensions variants – In the loop trying to get unique file endings, I would consider looping on yyyy-MM-dd, and then yyyy-MM-ddTHH:mm, and then possibly yyyy-MM-ddTHH:mm::ss. This instead of simply adding numbers. This would enable the file list to be alphabetically sorted correctly, and makes sense if you create multiple files at nearly the same time.
• Simplify the first extension to origFilename_Extension.orgExt – I would suggest not to introduce spaces to filenames if it can be avoided, and I would suggest using underscore instead of the minus, -.

This does not directly relate to your question but I feel it's still relevant. The Windows API has a function for generating a guaranteed unique random file name (as in, a bunch of random characters and numbers).

This API is exposed in .NET via Path.GetRandomFileName: https://msdn.microsoft.com/en-us/library/system.io.path.getrandomfilename(v=vs.110).aspx

Example taken from said page:

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string result = Path.GetRandomFileName();
            Console.WriteLine("Random file name is " + result);
        }
    }
}

/*

 This code produces output similar to the following:

 Random file name is w143kxnu.idj
 Press any key to continue . . .

 */

Again, this doesn't help with your requirement for date and time stamping but I thought it was worth mentioning.