Dynamic Assembly Loading

Question

I'm trying to extend my CMS with a plugin interface to make extensibility a little easier to manage. I created 2 classes: an abstract base class (baseModule) containing a single abstract method Init() and a class that inherits from it (TestModule) and throws an exception when Init is called. In my Engine, I'm doing the following:

            if (!module_dir.Exists) throw new Exception("Modules directory does not exist.");
            foreach (FileInfo f in module_dir.GetFiles("*.dll"))
            {
                Type t = Assembly.LoadFile(f.FullName).GetType();
                baseModule m = Activator.CreateInstance(t) as baseModule;
                if (m == null)
                {
                    throw new Exception("dll is not of type IModule");
                }
                module_list.Add(m);
            }

Unfortunately when I hit Activator.CreateInstance, I get the error :

"No parameterless constructor defined for this object."

Any ideas what I'm doing wrong?

Alan · Answer

You probably had it as private before so that the abstract class couldn't be instantiated. However that would have made the base class constructor inaccessible to TestModule's constructor and caused the error.

Wes · Answer

I had tried it with constructors and without... the only thing that was different is that the abstract class didn't have a protected constructor. I added the protected modifier and it worked. Not quite sure what the deal was/is but it's working and that's cool for me. Thanks!

Alan · Answer

If you haven't explicitly defined any constructors then the system should provide TestModule with a public parameterless constructor and the abstract base class baseModule with a protected parameterless constructor. So as long as the classes are public and they're not in a namespace the code should work. Is there anything which you're doing which is different to the above?

Wes · Answer

I tried that and it didn't work :( No change... same error same line of code.

Alan · Answer

The Assembly.LoadFile() method actually returns an Assembly object which doesn't have a public parameterless constructor - hence the error when you try to instantiate it. Try this code instead: Assembly asm = Assembly.LoadFile(f.FullName); Type t = asm.GetType(TestModule); // assuming no namespace baseModule m = Activator.CreateInstance(t) as baseModule;