C# late binding and File exceptions

Question

Simple host app searching for assemblies by special Interface and importing from them list of delegates, now it's Func<string,string>. Then it can execute any Func<T,T> and there's no problem. Problems starts when any of this Func tries to access file, which doesn't exist.

No try-catch block, no File.Exists doesn't helps — when function tries to access a file (anyway, read, get stream, check, etc) — whole app just fails with FileNotFound in mscorlib.

How this can be fixed? App is really critical, and I can't perform file check in app, only just in assemblies.

UPD: Yes, that delegates contains async logic.

UPD2: Parts of code:

    try
    {
        if(!File.Exists(filePath)) return null;

                using (StreamWriter writer = new StreamWriter(destinationFilePath))
                {
                    using (StreamReader reader = new StreamReader(filePath))
                    {
                    //some logic there
                    }
                 }
     }
    catch 
    {

    }

Exception thrown at File.Exists().

This code used to import assemblies.

Commands = new Dictionary<string, Func<string, string>>(); 
foreach (string f in fileNames)
                {

                    Assembly asm = Assembly.LoadFrom(f);
                    var types = asm.GetTypes();
                    foreach(Type t in types)
                    {

                        if (t.GetInterface("IMountPoint") != null)
                        {

                            var obj = Activator.CreateInstance(t);
                            var cmds = ((IMountPoint)obj).Init(EntryPoint);
                            foreach (var cmd in cmds)
                            {
                                if (!Commands.ContainsKey(cmd.Key.Trim().ToUpper()))
                                {
                                    Commands.Add(cmd.Key.Trim().ToUpper(), cmd.Value);
                                }

                            }
                        }
                    }
                }

And this code to run delegates:

string input = Console.ReadLine();
string res = Commands[command_key](input);

Answer 1

That's shameful. I'm using late binding and forgot to copy assemblies manually, so assemblies with file existence checking are not loaded by app and it used old ones. Sorry, guys.