client/Services/PathHelper.cs

using System.IO;

namespace CloudPrint.Client.Services;

public static class PathHelper
{
    public static string CreateUniqueFilePath(string directory, string fileName)
    {
        Directory.CreateDirectory(directory);

        var safeFileName = SanitizeFileName(string.IsNullOrWhiteSpace(fileName) ? "untitled" : fileName);
        var name = Path.GetFileNameWithoutExtension(safeFileName);
        var extension = Path.GetExtension(safeFileName);
        var candidate = Path.Combine(directory, safeFileName);
        var index = 1;

        while (File.Exists(candidate))
        {
            candidate = Path.Combine(directory, $"{name}_{index++}{extension}");
        }

        return candidate;
    }

    public static string SanitizePathSegment(string value)
    {
        var invalidChars = Path.GetInvalidFileNameChars();
        return new string(value.Select(ch => invalidChars.Contains(ch) ? '_' : ch).ToArray());
    }

    private static string SanitizeFileName(string fileName)
    {
        return SanitizePathSegment(fileName);
    }
}