Windowsがファイルの名前を変更する方法に関するGiuseppeのコメントに関して、私は既存のインデックス、つまりファイル名に(2)を見つけ、それに応じてWindowsに従ってファイルの名前を変更するバージョンで作業しました。sourceFileNameは有効であると見なされ、ユーザーはこの時点で宛先フォルダーへの書き込み権限を持っていると見なされます。
using System.IO;
using System.Text.RegularExpressions;
    private void RenameDiskFileToMSUnique(string sourceFileName)
    {
        string destFileName = "";
        long n = 1;
        // ensure the full path is qualified
        if (!Path.IsPathRooted(sourceFileName)) { sourceFileName = Path.GetFullPath(sourceFileName); }
        string filepath = Path.GetDirectoryName(sourceFileName);
        string fileNameWOExt = Path.GetFileNameWithoutExtension(sourceFileName);
        string fileNameSuffix = "";
        string fileNameExt = Path.GetExtension(sourceFileName);
        // if the name includes the text "(0-9)" then we have a filename, instance number and suffix  
        Regex r = new Regex(@"\(\d+\)");
        Match match = r.Match(fileNameWOExt);
        if (match.Success) // the pattern (0-9) was found
        {
            // text after the match
            if (fileNameWOExt.Length > match.Index + match.Length) // remove the format and create the suffix
            {
                fileNameSuffix = fileNameWOExt.Substring(match.Index + match.Length, fileNameWOExt.Length - (match.Index + match.Length));
                fileNameWOExt = fileNameWOExt.Substring(0, match.Index);
            }
            else // remove the format at the end
            {
                fileNameWOExt = fileNameWOExt.Substring(0, fileNameWOExt.Length - match.Length);
            }
            // increment the numeric in the name
            n = Convert.ToInt64(match.Value.Substring(1, match.Length - 2)) + 1;
        }
        // format variation: indexed text retains the original layout, new suffixed text inserts a space!
        do
        {
            if (match.Success) // the text was already indexed
            {
                if (fileNameSuffix.Length > 0)
                {
                    destFileName = Path.Combine(filepath, String.Format("{0}({1}){2}{3}", fileNameWOExt, (n++), fileNameSuffix, fileNameExt));
                }
                else
                {
                    destFileName = Path.Combine(filepath, String.Format("{0}({1}){2}", fileNameWOExt, (n++), fileNameExt));
                }
            }
            else // we are adding a new index
            {
                destFileName = Path.Combine(filepath, String.Format("{0} ({1}){2}", fileNameWOExt, (n++), fileNameExt));
            }
        }
        while (File.Exists(destFileName));
        File.Copy(sourceFileName, destFileName);
    }