Store Data of type SkiaSharp in LiteDB - System.InvalidCastException: Null object cannot be converted to a value type

Question

I am using SkiaSharp to develop a small note app in .net Maui. I want to store the data in LiteDB, which works. However, when loading the data from the database, I get the following error message.

System.InvalidCastException: Null object cannot be converted to a value type.

This is the class that I store in the database.

public class NSketch: NotebookItem
    {
        public List<Stroke> Strokes { get; set; }

        [BsonIgnoreAttribute]
        public override string Description
        {
            get { return "Skizze"; }
        }

        public NSketch()
        {
            Strokes = new List<Stroke>();
        }       
       
    }

    public class Stroke
    {
        public SKPaint Paint { get; set; }
        public List<SKPoint> Points { get; set; }

        public Stroke()
        {
            Paint = new SKPaint();
            Points = new List<SKPoint>();
        }
    }

and this is the code that causes the error.

//Methode um alle Noitzen in einem bestimmten Notizbuch zu laden und als liste zurückzugeben.
        public IList<NSketch> GetByNotebook(int notebookId)
        {
            using var db = new LiteDatabase(DatabaseName);
            var notes = db.GetCollection<NSketch>("nsketch");

            try
            {
                var documents = notes.Find(x => x.NotebookId == notebookId);
                // Handle the documents as needed
                return documents.ToList();
            }
            catch (Exception ex)
            {
                // Handle the exception thrown by the target method (Find)
                if (ex.InnerException != null)
                {
                    Console.WriteLine("Inner Exception Message: " + ex.InnerException.Message);
                    Console.WriteLine("Inner Exception Stack Trace: " + ex.InnerException.StackTrace);
                }

                // Optionally, rethrow the exception if needed
                // throw;

                return new List<NSketch>(); // Return an empty list or handle the exception appropriately
            }
        }

The problem is in this line return documents.ToList();

Can anyone tell me the cause and if possible a solution?

Thanks a lot

I test it with several other classes, and it works fine. I assume it is related to the SKiaSharp class.

You derive from `NotebookItem` - what's in there? – Fildor Aug 01 '23 at 07:59 — Fildor, Aug 01 '23 at 07:59

score 0 · Answer 1 · answered Aug 01 '23 at 07:35

It's probably because one of the element in the document list is null. So you have a few options to fix these:

Don't store null data in your db
Change you query to filter the null element
Filter the result with LINQ

return documents.Where(x => x != null).ToList();

Store Data of type SkiaSharp in LiteDB - System.InvalidCastException: Null object cannot be converted to a value type

1 Answers1