C#: How would you make a unique filename by adding a number?

Question

I would like to create a method which takes either a filename as a string or a FileInfo and adds an incremented number to the filename if the file

Answer 1

This is an answer to question in this Link, but they marked it as a duplicate, so I post my answer here.

I created this proof of concept class (may contain bugs). More explanation in code comments.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

namespace ConsoleApp
{
    class Program
    {
        static void Main( string[] args )
        {
            var testFilePaths = new List
            {
                @"c:\test\file.txt",
                @"c:\test\file(1).txt",
                @"c:\test\file(2).txt",
                @"c:\TEST2\file(3).txt",
                @"c:\test\file(5).txt",
                @"c:\test\file(5)abc.txt",
                @"c:\test\file(5).avi"
            };

            // inspect in debbuger for correct values
            var withSuffix      = new DecomposedFilePath( "c:\\files\\file(13).txt");
            var withoutSuffix   = new DecomposedFilePath( "c:\\files\\file(abc).txt");
            var withExtraNumber = new DecomposedFilePath( "c:\\files\\file(34)xyz(35).txt"); // "(34)" in the middle should be ignored

            DecomposedFilePath changedSuffix = withExtraNumber.ReplaceSuffix( 1999 ); // "file(34)xyz(35).txt" -> "file(34)xyz(1999).txt"
            DecomposedFilePath removedSuffix = changedSuffix.ReplaceSuffix( null ); // "file(34)xyz(1999).txt" -> "file(34)xyz.txt"

            var testPath = new DecomposedFilePath( "c:\\test\\file.txt");
            DecomposedFilePath nextPath1 = testPath.GetFirstFreeFilePath( testFilePaths );

            // update our list
            testFilePaths.Add( nextPath1.FullFilePath );
            DecomposedFilePath nextPath2 = testPath.GetFirstFreeFilePath( testFilePaths );
        
            testFilePaths.Add( nextPath2.FullFilePath );
            DecomposedFilePath nextPath3 = testPath.GetFirstFreeFilePath( testFilePaths );
        }
    }

    public sealed class DecomposedFilePath
    {
        public DecomposedFilePath( string filePath )
        {
            FullFilePath = Path.GetFullPath( filePath );
        }

        // "c:\myfiles\file(4).txt"
        public string FullFilePath { get; }

        // "file" or "file(1)"
        public string FileNameWithoutExt => Path.GetFileNameWithoutExtension( FullFilePath );

        // "file(13)" -> "file"
        public string FileNameWithoutExtAndSuffix => FileNameWithoutExt.Substring( 0, FileNameWithoutExt.Length - Suffix.Length ); // removes suffix

        // ".txt"
        public string Extenstion => Path.GetExtension( FullFilePath );

        // "c:\myfiles"
        public string DirectoryPath => Path.GetDirectoryName( FullFilePath );

        // "file(23)" -> "23", file -> stirng.Empty
        public string Suffix
        {
            get
            {
                // we want to extract suffix from file name, e.g. "(34)" from "file(34)"
                // I am not good at regex, but I hope it will work correctly

                var regex = new Regex( @"\([0-9]+\)$" );
                Match match = regex.Match( FileNameWithoutExt );

                if (!match.Success) return string.Empty; // suffix not found

                return match.Value; // return "(number)"
            }
        }

        // tranlates suffix "(33)" to 33. If suffix is does not exist (string.empty), returns null (int?)
        public int? SuffixAsInt
        {
            get
            {
                if (Suffix == string.Empty) return null;

                string numberOnly = Suffix.Substring( 1, Suffix.Length - 2 ); // remove '(' from beginning and ')' from end

                return int.Parse( numberOnly );
            }
        }

        // e.g. input is suffix: 56 then it changes file name from "file(34)" to "file(56)"
        public DecomposedFilePath ReplaceSuffix( int? suffix ) // null - removes suffix
        {
            string strSuffix = suffix is null ? string.Empty : $"({suffix})"; // add ( and )

            string path = Path.Combine( DirectoryPath, FileNameWithoutExtAndSuffix + strSuffix + Extenstion ); // build full path

            return new DecomposedFilePath( path );
        }

        public DecomposedFilePath GetFirstFreeFilePath( IEnumerable filesInDir )
        {
            var decomposed = filesInDir
                // convert all paths to our class
                .Select( x => new DecomposedFilePath( x ) )
                // pick files only with the same extensionm as our base file, ignore case
                .Where( x => string.Equals( Extenstion, x.Extenstion, StringComparison.OrdinalIgnoreCase) )
                // pick files only with the same name (ignoring suffix)
                .Where( x => string.Equals( FileNameWithoutExtAndSuffix, x.FileNameWithoutExtAndSuffix, StringComparison.OrdinalIgnoreCase) )
                // with the same directory
                .Where( x => string.Equals( DirectoryPath, x.DirectoryPath, StringComparison.OrdinalIgnoreCase) )
                .ToList(); // create copy for easier debugging

            if (decomposed.Count == 0) return this; // no name collision

            int? firstFreeSuffix = Enumerable.Range( 1, int.MaxValue) // start numbering duplicates from 1
                                  .Select( x => (int?) x) // change to int? because SuffixAsInt is of that type
                                  .Except( decomposed.Select( x => x.SuffixAsInt) ) // remove existing suffixes
                                  .First(); // get first free suffix

            return ReplaceSuffix( firstFreeSuffix );
        }

        public override string ToString() => FullFilePath;
    }
}