Object factory which Creates objects that require dependencies

Question

Currently in code i have used an object factory to return me a processor based of a string tag, which has severed its purpose up until now.

using Core;
using Data;

public static class TagProcessorFactory
{
 public static ITagProcessor GetProcessor(string tag)
 {
    switch (tag)
    {
       case "gps0":
         return new GpsTagProcessor();
       case "analog_manager":
         return new AnalogManagerTagProcessor();
       case "input_manager":
         return new InputManagerTagProcessor();
       case "j1939":
         return new J1939TagProcessor(new MemcachedProvider(new[] {  "localhost" }, "DigiGateway"), new PgnRepository());
       default:
         return new UnknownTagProcessor();
    }
 }
}

Calling Code

var processor = TagProcessorFactory.GetProcessor(tag.Name);

if (!(processor is UnknownTagProcessor))
{
    var data = processor.Process(unitId, tag.Values);

    Trace.WriteLine("Tag <{0}> processed. # of IO Items => {1}".FormatWith(tag.Name, data.Count()));
}

as you can see one of my items has dependencies and im trying to execute testing code and i want to pass in mock repositories and cache providers but i can seem to think of a way to do this.

Is this a bad design or anyone have any ideas to fix it to make my factory testable?

Thanks

Is there some compelling reason that you decided not to use one of the many, many Free and excellent dependency injection containers available for C#? Any of them could handle this injection pattern gracefully. — anthony, Oct 28 '10 at 19:54
i am. i'm using autofac, my problem is i did not want to pass in the builder/container to resolve the dependency, iv read not to pass the container around. — Waterboy4800, Oct 28 '10 at 19:57
I would say passing a factory around is acceptable as the class is requesting a deferred creation of an object. — aqwert, Oct 28 '10 at 23:51
A bit unrelated, but why are you testing for the `UnknownTagProcessor`? Why not either: 1) thrown an exception instead of returning a value, or 2) return a Null Object that you can simply call `Process` on? This seems like a much cleaner design. — Steven, Oct 29 '10 at 09:20
it was at first throwing an exception, but there was a lot of unknown tags i did not want to bother with which was throwing a lot of exceptions. So I threw the unknown tag processor in there in which i was just calling process on which was meant to log that there was a tag without a processor, but then i lost the context of which tag unless i pass the tag name into the process method. Your right its not clean right now and I'm still thinking of the best way to wrap it up. — Waterboy4800, Oct 29 '10 at 13:44
Pass the tag name into the constructor of the UnknownTagProcessor. — WW., Nov 02 '10 at 07:00

score 3 · Accepted Answer · answered Oct 29 '10 at 16:03

Since you are using Autofac, you can take advantage of the lookup relationship type:

public class Foo
{
    private readonly IIndex<string, ITagProcessor> _tagProcessorIndex;

    public Foo(IIndex<string, ITagProvider> tagProcessorIndex)
    {
        _tagProcessorIndex = tagProcessorIndex;
    }

    public void Process(int unitId, Tag tag)
    {
        ITagProcessor processor;

        if(_tagProcessorIndex.TryGetValue(tag.Name, out processor))
        {
            var data = processor.Process(unitId, tag.Values);

            Trace.WriteLine("Tag <{0}> processed. # of IO Items => {1}".FormatWith(tag.Name, data.Count()));
        }
    }
}

See the TypedNamedAndKeysServices wiki article for more information. To register the various processors, you would associate each with its key:

builder.RegisterType<GpsTagProcessor>().Keyed<ITagProcessor>("gps0");
builder.RegisterType<AnalogManagerTagProcessor>().Keyed<ITagProcessor>("analog_manager");
builder.RegisterType<InputManagerTagProcessor>().Keyed<ITagProcessor>("input_manager");

builder
    .Register(c => new J1939TagProcessor(new MemcachedProvider(new[] {  "localhost" }, new PgnRepository()))
    .Keyed<ITagProcessor>("j1939");

Notice we don't register UnknownTagProcessor. That was a signal to the caller of the factory that no processor was found for the tag, which we express using TryGetValue instead.

i like this approach. Only think i cant figure out is how to resolve the IIndex from the builder to pass into the constructor of Foo. Help? — Waterboy4800, Oct 29 '10 at 19:36
Autofac's relationship types are automatic. If you just call `c.Resolve>()`, the container will interpret that request and create the appropriate implementation for you. This is true even when you use `builder.RegisterType()`. As another example, if you were to accept `Func`, the container will recognize the relationship type and generate a function which resolves `SomeType`. It is quite nifty. — Bryan Watts, Oct 29 '10 at 20:25

score 0 · Answer 2 · answered Oct 28 '10 at 20:01

0

Using something like StructureMap you could use the ObjectFactory which, when configured would return you a named concrete instance.

http://structuremap.net/structuremap/index.html

answered Oct 28 '10 at 20:01

brumScouse

3,166
1
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score 0 · Answer 3 · edited May 23 '17 at 11:47

0

I suggest you look through another SO post. It solves several problems at once, including how to replace contructor values - without a mess. Specifically, the parameters to the constructor simply become static fields of a "Context" class, which are read by the constructor of the interior class.

edited May 23 '17 at 11:47

Community

1
1

answered Oct 28 '10 at 20:12

Brent Arias

29,277
40
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Object factory which Creates objects that require dependencies

3 Answers3