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public enum CopyOrRenameMode
{
    OnExistsRenameOld,
    OnExistsRenameNew,
    OnExistsReplace,
    OnExistsCancel
}



public static FileInfo FileCopyExt(FileInfo source, DirectoryInfo destDir, CopyOrRenameMode copyMode)
{
    var destFile = Path.Combine(destDir.FullName, source.Name);
    if (!File.Exists(destFile))
    {
        File.Copy(source.FullName, destFile, true);
        return new FileInfo(destFile);
    }
    else
    {
        if (copyMode == CopyOrRenameMode.OnExistsCancel)
            return null;
        if (copyMode == CopyOrRenameMode.OnExistsReplace)
        {
            File.Copy(source.FullName, destFile, true);
            return new FileInfo(destFile);
        }
        if (copyMode == CopyOrRenameMode.OnExistsRenameNew)
        {
            var newDestName = FindNextReplacingFilename(destFile);
            File.Copy(source.FullName, newDestName);
            return new FileInfo(newDestName);
        }
        if (copyMode == CopyOrRenameMode.OnExistsRenameOld)
        {
            var newExistingName = FindNextReplacingFilename(destFile);
            File.Move(destFile, newExistingName);
            File.Copy(source.FullName, destFile);
            return new FileInfo(destFile);
        }
    }
    return null;
}

public static string FindNextReplacingFilename(string filename)
{
    int counter = 0;
    string proposedDest = filename;
    while (counter < 5000)
    {
        if (File.Exists(proposedDest))
        {
            counter++;
            proposedDest = Path.Combine(Path.GetDirectoryName(filename),
                Path.GetFileNameWithoutExtension(filename) +
                "_" + counter + Path.GetExtension(filename));
        }
        else
        {
            return proposedDest;
        }
    }

    if (counter == 5000)
        throw new Exception($"Could not find available file name : too many retries");

    return "";
}