How do you implement constructor injection in a factory?

Question

When you're using a factory pattern, how do you inject dependencies into constructors at runtime?

I'm building Foos with different formats - boolean, array, freetext, matrix, etc. That format list will grow as we find different uses for Foo. Here's my basic core domain:

public interface IFoo
{
    FooFormat Format { get; }
}

public class Foo : IFoo
{
    private FooFormat _format;

    internal Foo(FooFormat format)
    {
        _format = format;
    }

    public FooFormat Format { get { return _format; } }
}


public abstract class FooFormat
{
}

public class DefaultFooFormat : FooFormat
{
}

public class BooleanFooFormat : FooFormat
{
    public IList<bool> Values { get; set; }
}

public class ArrayFooFormat : FooFormat
{
    private IList<string> _values;

    public ArrayFooFormat(IList<string> values)
    {
        _values = values;
    }

    public IList<string> Values { get { return _values; } }
}

IFoo is decorated for the consumer context:

public abstract class FooDecorator : IFoo
{
    private IFoo _foo;

    protected FooDecorator(IFoo foo)
    {
        _foo = foo;
    }

    public FooFormat Format
    {
        get { return _foo.Format; }
    }

    protected IFoo foo
    {
        get { return _foo; }
    }
}

I don't want my consumer to instantiate a Foo directly, so I force them to use a factory:

public abstract class FooFactory
{
    protected IFoo Build<T>()
    {
        FooFormat format = GetFormat<T>();
        return new Foo(format);
    }

    private FooFormat GetFormat<T>()
    {
        if (typeof(T) == typeof(ArrayFooFormat)) return new ArrayFooFormat(new List<string>());
        if (typeof(T) == typeof(BooleanFooFormat)) return new BooleanFooFormat();
        return new DefaultFooFormat();
    }
}

And even then, they need to derive a factory from my abstract factory for their particular context.

I'm specifically building foos in an html context, like so:

public class HtmlFoo : FooDecorator
{
    public HtmlFoo(IFoo foo) : base(foo) { }

    public string ToHtml()
    {
        return "<div>" + this.Format.ToString() + "</div>";
    }
}


public class HtmlFooFactory : FooFactory
{
    public IFoo BuildFoo<T>()
    {
        IFoo foo = Build<T>();
        return new HtmlFoo(foo);
    }
}

public class HtmlFooConsumer
{
    public void DoSomeFoo()
    {
        var factory = new HtmlFooFactory();
        var htmlBooleanFoo = factory.BuildFoo<BooleanFooFormat>();
        var htmlArrayFoo = factory.BuildFoo<ArrayFooFormat>();
    }
}

My problem is in my abstract FooFactory: I'm always injecting an empty value list into my ArrayFooFormat. I want to be able to pass in a value list from the consumer. For other FooFormats, I want to pass in the right constructor arguments from the consumer. But I want to keep the public API dead simple - I don't want a bunch of overloads on BuildFoo().

So how do I pass a custom value list into the factory.BuildFoo<T>() call from inside HtmlFooConsumer.DoSomeFoo()? Any ideas, stackoverflow gurus?

Answer 1

Maybe you can do something along these lines where your abstract FooFormat becomes IFooFormat and a generic FooFormat provides an Init method that gets passed the parameter.

Then a single overload of Build lets you pass in the parameter.

public interface IFooFormat
{
}

public class FooFormat<TValue> : IFooFormat
{
    private TValue _value;

    public void Init(TValue value)
    {
        _value = value;
    }

    public TValue Value
    {
        get { return _value; }
    }
}

public class ArrayFooFormat : FooFormat<IList<string>> { }

public class BooleanFooFormat : FooFormat<bool> { }

public class DefaultFooFormat : IFooFormat { }

public interface IFoo { }

public class Foo : IFoo
{
    private IFooFormat _format;

    internal Foo(IFooFormat format)
    {
        _format = format;
    }

    public IFooFormat Format { get { return _format; } }
}

public class FooFactory
{
    protected IFoo Build<TFormat, TArg>(TArg arg) where TFormat : FooFormat<TArg>, new()
    {
        TFormat format = new TFormat();
        format.Init(arg);
        return new Foo(format);
    }

    protected IFoo Build<TFormat>() where TFormat : IFooFormat, new()
    {
        return new Foo(new TFormat());
    }
}

Answer 2

A factory is basically the object oriented version of a static variable. I'd avoid using one alltogether. Instead of forcing clients to use a factory, perhaps you can simply inject objects into their constructors, sidestepping the need for a factory.