Catch DllNotFoundException doesn't work

Question

I am trying to log the connected scanner on a pc.

I am using NTwain.dll from https://bitbucket.org/soukoku/ntwain.

If I run my app on a server some dependency dlls from ntwain fail to load so I will load the dll at runtime and if it will fail I just want to return an empty list. There is no reference to NTwain in the project references anymore.

Problem: If I have the NTwain.dll in the folder with the exe and I run it on a server the app crashes. It doesn't return an empty list. If I delete the dll and run the app the empty list gets returned.

Code:

public class Scanner : IDB
    {

        private enum DataGroups : uint
        {
            None = 0,
            Control = 0x1,
            Image = 0x2,
            Audio = 0x4,
            Mask = 0xffff,
        }

        public string Name { get; private set; }
        public string ProductFamily { get; private set; }
        public string Version { get; private set; }

        public Scanner()
        {
            Name = String.Empty;
        }

        public static List<Scanner> getScanners()
        {

            List<Scanner> scanners = new List<Scanner>();

            try
            {
                Assembly assembly = Assembly.LoadFrom(Environment.CurrentDirectory + "\\NTwain.dll");
                Type tident = assembly.GetType("NTwain.Data.TWIdentity");
                Type tsession = assembly.GetType("NTwain.TwainSession");
                object appId = tident.GetMethod("CreateFromAssembly").Invoke(null, new object[] { DataGroups.Image, System.Reflection.Assembly.GetExecutingAssembly() });
                object session = Activator.CreateInstance(tsession, appId);
                tsession.GetMethod("Open", new Type[0]).Invoke(session, null);
                object sources = session.GetType().GetMethod("GetSources").Invoke(session, null);

                foreach (var item in (IEnumerable)sources)
                {
                    Scanner scanner = new Scanner();
                    scanner.Name = (string)item.GetType().GetProperty("Name").GetValue(item, null);
                    scanner.ProductFamily = (string)item.GetType().GetProperty("ProductFamily").GetValue(item, null);
                    object version = item.GetType().GetProperty("Version").GetValue(item, null);
                    scanner.Version = (string)version.GetType().GetProperty("Info").GetValue(version, null);
                    scanners.Add(scanner);
                }
                return scanners;
            }
            catch (Exception e)
            {
                return new List<Scanner>();
            }
        }
}

Answer 1

I would guess to catch a DllNotFoundException works in your code:

If u delete the dll then

Assembly assembly = Assembly.LoadFrom(Environment.CurrentDirectory + "\\NTwain.dll");

throws an DllNotFoundException which is catched by your catch block which says to return an empty list (which works as u say).

If u don't delete the dll then the code passes above line successfully. In case by the following lines a different thread is started and an error occurs that is not catched within that thread, then your catch block will not catch that error (whatever it might be) and the application crashes.