C# Dependency Injection

Question

I'm trying to see if I understand depedency injection.

I have a project that is used as a parser. It can parse delimited text, key-value and will also regex.

The first way this was done was in one function with a switch. The next way was to put it in seperate functions and call it based on a switch

The next way I was thinking was to create an interface and than implement a class for each type. Is that to much work? Does the question come down to function or will doing this show benefits that I don't see yet.

I believe my problems stems that I was initially going to implement an interface and than for each time I needed a different parsing implement a new class. But than that would still require me going in and adding that to some type of logic flow as I don't see how to do that with injection frameworks.

So say I add in another way to parse that is based on tags or xml. Create the class implementing the interfaces and than I would need to add them to the flow logic to instantiate them as that interface if a user chooses to parse that type of text. Any clearner way to do that?

Answer 1

I think what you're really needing is a Factory. A Factory is a class that knows, given some information to work on, how to create objects of the proper type necessary. In your case, You'd create a parser interface, then separate classes implementing various parsers. Finally, create a parser factory that, given some ability to tell which kind of parser to create, creates and returns the kind needed. This is where your logic would go. The Factory provides a way to localize the creational logic for the items being created.

public interface IParser<T>
{
    T Parse<T>( string item );
}

public class KeyValueParser : IParser<KeyValue>
{
    KeyValuePair Parse<KeyValue>( string item );
}

...

public class ParserFactory
{
    public IParser<T> CreateParser<T>()
    {
        var type = typeof(T);
        if (type == typeof(KeyValuePair))
        {
            return new KeyValueParser();
        }
        ...
        throw new InvalidOperationException( "No matching parser type." );
    }
}

Some others have suggested a plugin model and, if appropriate, the factory can be adapted to read a plugin configuration, load the appropriate plugins, and create the instance types as needed. In that case it might be more appropriate to think of the factory as a "manager" since it does more than simply create instances.

Answer 2

I would say you on right track, but you are a little wrong. You should go in direction of Factory Method and Visitor patern, to solve parsing issue first.

1. Visitors that handle different types of input.
2. Visitors are created thought Factory Method.
3. Factory is injected as constructor accomplishing dependency inject pattern.

This work might be interesting for you - http://www.exciton.cs.rice.edu/research/sigcse05/dp4rdp.pdf