Why is it so complicated to multiply some numbers

Using this method when running TaskExecutor.UpdateChapters to get a list of all chapters that have not yet been downloaded.

Tranga/Chapter.cs

@@ -13,16 +13,21 @@ public struct Chapter
     public string? chapterNumber { get; }
     public string url { get; }
     public string fileName { get; }
+    public string sortNumber { get; }
 
     public Chapter(string? name, string? volumeNumber, string? chapterNumber, string url)
     {
         this.name = name;
-        this.volumeNumber = volumeNumber;
+        this.volumeNumber = volumeNumber is { Length: > 0 } ? volumeNumber : "1";
         this.chapterNumber = chapterNumber;
         this.url = url;
         string chapterName = string.Concat((name ?? "").Split(Path.GetInvalidFileNameChars()));
-        double multiplied = Convert.ToDouble(chapterNumber, new NumberFormatInfo() { NumberDecimalSeparator = "." }) *
-                            Convert.ToInt32(volumeNumber);
-        this.fileName = $"{chapterName} - V{volumeNumber}C{chapterNumber} - {multiplied.ToString(new NumberFormatInfo() { NumberDecimalSeparator = "." })}";
+        NumberFormatInfo nfi = new NumberFormatInfo()
+        {
+            NumberDecimalSeparator = "."
+        };
+        sortNumber = decimal.Round(Convert.ToDecimal(this.volumeNumber) * Convert.ToDecimal(this.chapterNumber, nfi), 1)
+            .ToString(nfi);
+        this.fileName = $"{chapterName} - V{volumeNumber}C{chapterNumber} - {sortNumber}";
     }
 }

Tranga/Connector.cs

@@ -73,10 +73,22 @@ public abstract class Connector
         XElement comicInfo = new XElement("ComicInfo",
             new XElement("Tags", string.Join(',',publication.tags)),
             new XElement("LanguageISO", publication.originalLanguage),
-            new XElement("Title", chapter.name)
+            new XElement("Title", chapter.name),
+            new XElement("Volume", chapter.volumeNumber),
+            new XElement("Number", chapter.sortNumber)
         );
         return comicInfo.ToString();
     }
+
+    public bool ChapterIsDownloaded(Publication publication, Chapter chapter)
+    {
+        return File.Exists(CreateFullFilepath(publication, chapter));
+    }
+
+    protected string CreateFullFilepath(Publication publication, Chapter chapter)
+    {
+        return Path.Join(downloadLocation, publication.folderName, chapter.fileName);
+    }
     
     /// <summary>
     /// Downloads Image from URL and saves it to the given path(incl. fileName)

Tranga/Connectors/MangaDex.cs

@@ -201,7 +201,7 @@ public class MangaDex : Connector
         File.WriteAllText(comicInfoPath, CreateComicInfo(publication, chapter));
         
         //Download Chapter-Images
-        DownloadChapterImages(imageUrls.ToArray(), Path.Join(downloadLocation, publication.folderName, chapter.fileName), this.downloadClient, comicInfoPath);
+        DownloadChapterImages(imageUrls.ToArray(), CreateFullFilepath(publication, chapter), downloadClient, comicInfoPath);
     }
 
     public override void DownloadCover(Publication publication)

Tranga/Publication.cs

@@ -33,7 +33,7 @@ public readonly struct Publication
         this.originalLanguage = originalLanguage;
         this.status = status;
         this.downloadUrl = downloadUrl;
-        this.folderName = string.Concat(sortName.Split(Path.GetInvalidPathChars()));
+        this.folderName = string.Concat(sortName.Split(Path.GetInvalidPathChars().Concat(Path.GetInvalidFileNameChars()).ToArray()));
     }
 
     /// <summary>

Tranga/TaskExecutor.cs

@@ -93,14 +93,11 @@ public static class TaskExecutor
     private static List<Chapter> UpdateChapters(Connector connector, Publication publication, string language, Dictionary<Publication, List<Chapter>> chapterCollection)
     {
         List<Chapter> newChaptersList = new();
-        if (!chapterCollection.ContainsKey(publication))
-            return newChaptersList;
+        chapterCollection.TryAdd(publication, newChaptersList); //To ensure publication is actually in collection
         
-        List<Chapter> currentChapters = chapterCollection[publication];
         Chapter[] newChapters = connector.GetChapters(publication, language);
+        newChaptersList = newChapters.Where(nChapter => !connector.ChapterIsDownloaded(publication, nChapter)).ToList();
         
-        newChaptersList = newChapters.ToList()
-            .ExceptBy(currentChapters.Select(cChapter => cChapter.url), nChapter => nChapter.url).ToList();
         return newChaptersList;
     }
 }

Tranga/TaskManager.cs

@@ -34,7 +34,7 @@ public class TaskManager
             foreach (TrangaTask task in _allTasks)
             {
                 if(task.ShouldExecute()) 
-                    TaskExecutor.Execute(this._connectors, task, this._chapterCollection);
+                    TaskExecutor.Execute(this._connectors, task, this._chapterCollection); //Might crash here, when adding new Task while another Task is running. Check later
             }
             Thread.Sleep(1000);
         }