How to create a generic factory returning different interface types and an abstract class

Question

I have an abstract base class which contains a good deal of shared code and configuration properties. Much of the shared code I've split up into logical interfaces which are also implemented by the base class. There are a series of implementations of the base class for each customer.

I currently have a factory for each of the interfaces. Each factory has identical switch statements. I would like to create a generic factory that will return a different subset of functionality based on how the class is declared.

My base class:

public abstract class BaseParser : IDatabaseCreation, IBulkImport, IAnalyticFeature
// ...a number of configuration fields, methods and abstract methods

Customer class:

class AParser : BaseParser
{
    private int _componentIndicatorColumn;        

    public AParser(ILogger log) : base (log) {
// ...configuration values and abstract method implementations

Current Base Factory:

class BaseFactory
{
    public BaseParser CreateParser(string key, ILogger log)
    {
        switch (key)
        {
            case "A":
                return new AParser(log);
            case "B":
                return new BParser(log);
            case "C":
                return new CParser(log);
            default:
                throw new NotImplementedException("Not Recognized or Not Registered in Factory");
        }
    }
}

Sample Interface Factory:

class BulkImportFactory
{
    public IBulkImport CreateDatabaseCreationObject(string key, ILogger log)
    {
        switch (key)
        {
            case "A":
                return new AParser(log);
            case "B":
                return new BParser(log);
            case "C":
                return new CParser(log);
            default:
                throw new NotImplementedException("Not Recognized or Not Registered in Factory");
        }
    }
}

This is my attempt at a GenericFactory that isn't working:

public class GenericFactory<T>
{
    public T CreateVariableInterfaceObject<T>(string key, ILogger log)  where T: BaseParser
    {
        switch (key)
        {
            case "A":               
                return new AParser(log);
            case "B":
                return new BParser(log);
            case "C":
                return new CParser(log);
            default:
                throw new NotImplementedException("Not Recognized or Not Registered in GenericFactory");
        }
    }
}

As you can see the logic in the factories is identical and repeated. However I can't get a generic parser to work. Not sure what syntax I'm missing.

What I'd like to do is allow all of these to be one factory:

ParserFactory parserFactory = new ParserFactory();
BaseParser parser = parserFactory.CreateParser(queueMessage.key,log);

BulkImportFactory bulkImportFactory = new BulkImportFactory();
IBulkImport bulkImporter = bulkImportFactory.CreateDatabaseCreationObject(key, log);

AnalyticFeatureFactory parserFactory = new AnalyticFeatureFactory();
IAnalyticFeature parser = parserFactory.CreateAnalyticFeatureObject(key, log);

Answer 1

Does something like this suit your needs?

sealed class GenericFactory<TKey, TOption, TObject>
{
    readonly IReadOnlyDictionary<TKey, Func<TKey, TOption, TObject>> _factories;

    public GenericFactory(
        IReadOnlyDictionary<TKey, Func<TKey, TOption, TObject>> factories)
    {
        _factories = factories;
    }

    public bool TryCreate(TKey key, TOption option, out TObject @object)
    {
        @object = default;
        if (!_factories.TryGetValue(key, out var factory))
            return false; // Cannot create; unknown key
        @object = factory(key, option);
        return true;
    }
}

static class GenericFactoryExtensions
{
    public static TObject CreateOrFail<TKey, TOption, TObject>(
        this GenericFactory<TKey, TOption, TObject> factory,
        TKey key,
        TOption option)
    {
        if (!factory.TryCreate(key, option, out var @object))
            throw new NotImplementedException($"Not Recognized or Not Registered in {nameof(GenericFactory<TKey, TOption, TObject>)}");
        return @object;
    }
}

void SimpleUseFactory()
{
    var baseParserFactory = new GenericFactory<string, ILogger, BaseParser>(new Dictionary<string, Func<string, ILogger, BaseParser>>
        {
            ["A"] = (key, logger) => new AParser(logger),
            ["B"] = (key, logger) => new BParser(logger)
        });

    var parser = baseParserFactory.CreateOrFail("A", logger);
    parser.DoStuff();
}

class Factories
{
    public Func<string, ILogger, BaseParser> BaseParserFactory { get; }
    public Func<string, ILogger, IBulkImport> BulkImportFactory { get; }
    public Func<string, ILogger, SomethingElse> SomethingElseFactory { get; }

    public Factories(
        Func<string, ILogger, BaseParser> baseParserFactory,
        Func<string, ILogger, IBulkImport> bulkImportFactory,
        Func<string, ILogger, SomethingElse> somethingElseFactory)
    {
        BaseParserFactory = baseParserFactory;
        BulkImportFactory = bulkImportFactory;
        SomethingElseFactory = somethingElseFactory;
    }
}

void ComplexUseFactory()
{
    var mappedFactories = new Dictionary<string, Factories>
    {
        ["A"] = new Factories(
            baseParserFactory: (key, logger) => new AParser(logger),
            bulkImportFactory: (key, logger) => new ABulkImport(logger),
            somethingElseFactory: (key, logger) => new ASomethingElse(logger)),
        ["B"] = new Factories(
            baseParserFactory: (key, logger) => new BParser(logger),
            bulkImportFactory: (key, logger) => new BBulkImport(logger),
            somethingElseFactory: (key, logger) => new BSomethingElse(logger))
    };

    var baseParserFactory = new GenericFactory<string, ILogger, BaseParser>(
        mappedFactories.ToDictionary(
            keySelector: kvp => kvp.Key,
            elementSelector: kvp => kvp.Value.BaseParserFactory));
    var bulkImportFactory = new GenericFactory<string, ILogger, IBulkImport>(
        mappedFactories.ToDictionary(
            keySelector: kvp => kvp.Key,
            elementSelector: kvp => kvp.Value.BulkImportFactory));
    var somethingElseFactory = new GenericFactory<string, ILogger, SomethingElse>(
        mappedFactories.ToDictionary(
            keySelector: kvp => kvp.Key,
            elementSelector: kvp => kvp.Value.SomethingElseFactory));

    var parser = baseParserFactory.CreateOrFail("A", logger);
    parser.DoStuff();
}

For the "complex" use case demonstrated:

The Factories class is what enforces that when there's a BaseParser for "A" then there's also a IBulkImport and a SomethingElse . When you want the compile-time guarantee that you can also create a YetAnotherThing for all cases then just add that as a required property of the Factories class and create a new GenericFactory according to the pattern.

When you want to add functionality for "C" then all you have to do is add another entry in the mappedFactories dictionary.

Note that the mappedFactories could be instantiated and then tossed around between different modules in order to populate it with all the necessary "A", "B", "C", etc cases before creating the GenericFactory s. Or instead of making the modules accept a Dictionary<string, Factories> object, maybe each module could have an implementation of an interface that generates just one Factories instance and you could gather the "A", "B", etc keys from module metadata. That way you could guarantee that the "B" module doesn't mess with "A" module's factories.

Can this be abstracted further? I think so, but I suspect it would come without the compile-time guarantee that when you can create a BaseParser then you can also create a IBulkImport .

For both cases:

You might be helped to develop a sense of smell for switch statements (that by definition are not Open for extension/Closed for modification, also known as the Open/Closed principle) which need to be modified to extend functionality. Composing with dictionaries is often the solution. Same for endless if statements.

Notice that the GenericFactory is sealed and missing the abstract keyword. That's intentional. Consumers of this factory should be composed of this factory instead of inheriting from it. Just like the UseFactory method composes instances of the factory instead of instances of things that inherit from it. That's another principle at play: favor composition over inheritance.

You'll also notice that the GenericFactory is really a factory that is composed of other factories—it delegates to other factories (each Func in the injected dictionary is itself a factory). If you truly need this then that signals to me that you probably aren't using an IoC container, because IoC containers typically give this mechanism of composing factories without you having to use this. In that case you might be helped to investigate IoC containers.

---

Edit: You and I both mentioned something about IoC.

If I had IoC, I would try really hard to get to the following scenario so that I wouldn't even need GenericFactory .

(My apologies in advance for making up pseudocode that doesn't work out of the box for any known IoC container)

ModuleA.cs

Register<AParser>().As<BaseParser>();
Register<ABulkImport>().As<IBulkImport>();

ModuleB.cs

Register<BParser>().As<BaseParser>();
Register<BBulkImport>().As<IBulkImport>();

CommonThing.cs

public class CommonThing
{
    readonly BaseParser _parser;
    readonly IBulkImport _bulkImport;

    public CommonThing(
        BaseParser parser,
        IBulkImport bulkImport)
    {
        _parser = parser;
        _bulkImport = bulkImport;
    }

    public void DoFancyStuff(string data)
    {
        var parsed = _parser.Parse(data);
        _bulkImport.Import(parsed);
    }
}

Single Composition Root

switch (module)
{
    case "A":
        RegisterModule<ModuleA>();
        break;
    case "B":
        RegisterModule<ModuleB>();
        break;
    default:
        throw new NotImplementedException($"Unexpected module {module}");
}
Register<CommonThing>();
Register<Application>();

Application.cs

public class Application
{
    readonly CommonThing _commonThing;

    public Application(
        CommonThing commonThing)
    {
        _commonThing = commonThing;
    }

    public void Run()
    {
        var json = "{\"key\":\"value\"}";
        _commonThing.DoFancyStuff(json);
    }
}

Program.cs (or entry point of your choice)

var containerBuilder = new IoCBuilder();
containerBuilder.RegisterModule<SingleCompositionRoot.cs>();
using (var container = containerBuilder.Build())
    container.Resolve<Application>().Run();

Note: the Single Composition Root often doesn't have to obey Open/Closed. But if you'd like the switch statement there to go away then one could move toward this kind of design:

ModuleNameAttribute.cs

public class ModuleNameAttribute : Attribute
{
    public string Name { get; }
    ...
}

ModuleA.cs

[ModuleName("A")]
public class ModuleA
{
    ...
}

ModuleB.cs

[ModuleName("B")]
public class ModuleB
{
    ...
}

Single Composition Root

var moduleType = GetAllTypesInAppDomain()
    .Select(type => (type, type.GetCustomAttribute<ModuleNameAttribute>()))
    .Where(tuple => tuple.Item2 != null)
    .Where(tuple => tuple.Item2.Name == module)
    .FirstOrDefault();
if (moduleType == null)
    throw new NotImplementedException($"No module has a {nameof(ModuleNameAttribute)} matching the requested module {module}");
RegisterModule(moduleType);

...

Note that one of the benefits of going all the way with Dependency Injection (meaning register/resolve the application itself like Program.cs does above) is that missing registrations cause very early runtime exceptions. This often eliminates the need for some kind of compile-time guarantee that all the correct things are in the correct places.

For example, if module were defined as "C" then that NotImplementedException in "Single Composition Root" would be thrown at application launch. Or if module C does exist but fails to register an implementation of IBulkImport then the IoC container would throw a runtime exception while trying to resolve CommonThing for Application , again at application launch. So if the application launches then you know that all dependencies either were resolved or can be resolved.

Answer 2

Let me see if I can clarify what you are trying to do.

You have a set of parsers (AParser, BParser, CParser) that have specific implementations of some of the functions of BaseParser . What you are trying to do is to give specific instances of AParser , BParser etc some special feature at runtime. So say that you want an AParser but the default AParser implementation doesn't support ParseFoo() but at runtime you want to give it ParseFoo() ability without defining this ahead of time?

If this is the case then I think you are going to have to consider a Decorator Design Pattern possibly along with your factory. With the Decorator, you will implement the functionality of the new features in their own classes and then your factory will return the decorator which simply wraps the BaseParser concrete class.

Answer 3

Seems like your code is breaking the Solid Principles, eg Single purpose class. If you want to fix this instead of your base class implementing each interface they could contain a property for each, eg parser, bulk importer, analytics But if you want to solve this an easy way with your current architecture you can do the following

You've defined that a base parser implements 3 interfaces. Obtain each interface from the same factory

//Note: you probably want to change the name of your factory
ParserFactory parserFactory = new ParserFactory();
BaseParser parser = parserFactory.CreateParser(queueMessage.key,log);
IBulkImport bulkImporter =  parserFactory.CreateParser(queueMessage.key,log);

If for some reason you need to add additional bulk importer that are not parsers then just make a single class which encapsulates the logic eg

in side your other factories you can have a base factory which you ask to resolve the interfaces first eg

//Note you need to change the code so it doesnt throw in the default
IBreakSolidPrinciplesFactory principlesFactory = new BreakSolidPrinciplesFactory();

var multipurposeClass = principlesFactory.GetImplentation(key, log);
if(multipurposeClass != null)
{
   reutrn multipurposeClass;
}
switch (key)
{
    case "bulkImporter":
        return new BulkImporterOnly(log);
    default:
        throw new NotImplementedException("Not Recognized or Not Registered in Factory");
}