Question

I want to allow developers that use my product to be able to extend the product by implementing an interface and then just dropping the assembly into the executing folder.
How should I go about finding these types, do I have to run thru every single DLL in the folder or can I avoid those that are part of the original application?

Answer 1

In the end I elected to simply load up and search thru all dlls that I found in the bin.
Here is most of the code. the key function it the “IsAssignableToGenericType” function which finds the generic Interface I am looking for.

I beleive this is the link that provided most of the solution
Implementations of interface through Reflection

static AssemblyLocator()
    {
        AllDlls = GetAllDlls();
        SubscribersInBin = GetSubscribersInBin();
    }

    public static IEnumerable<Type> TypesImplementingInterface(Assembly[] assemblies, Type desiredType)
    {
        return assemblies
            .SelectMany(assembly => assembly.GetTypes())
            .Where(type => IsAssignableToGenericType(type, desiredType));
    }

    public static bool IsAssignableToGenericType(Type givenType, Type genericType)
    {
        if (givenType == null) throw new ArgumentNullException("givenType");
        if (genericType == null) throw new ArgumentNullException("genericType");

        var interfaceTypes = givenType.GetInterfaces();

        foreach (var it in interfaceTypes)
        {
            if (it.IsGenericType)
            {
                if (it.GetGenericArguments()[0].Name.Equals(genericType.GetGenericArguments()[0].Name))
                    return true;
            }
        }

        Type baseType = givenType.BaseType;
        if (baseType == null) return false;

        return (baseType.IsGenericType &&
            baseType.GetGenericTypeDefinition() == genericType) ||
            IsAssignableToGenericType(baseType, genericType);
    }

    private static ReadOnlyCollection<string> GetAllDlls()
    {
        string binFolder = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
        IList<string> dllFiles = Directory.GetFiles(binFolder, "*.dll", SearchOption.TopDirectoryOnly).ToList();
        return new ReadOnlyCollection<string>(dllFiles);
    }

    private static ReadOnlyCollection<Type> GetSubscribersInBin()
    {
        IList<Assembly> assembliesFoundInBin = new List<Assembly>();
        foreach (var item in AllDlls)
        {
            var assembly = System.Reflection.Assembly.LoadFrom(item);
            assembliesFoundInBin.Add(assembly);
        }

        var typesInBin = TypesImplementingInterface(assembliesFoundInBin.ToArray(), typeof(ISubscriber<T>));
        return new ReadOnlyCollection<Type>(typesInBin.ToList<Type>());
    }