How to use a factory with Dependecy Injection without resorting to using Service Locator pattern

Question

I have a GUI application. In it I allow a user to select from a container-provided list of algorithms. Each algorithm will be kicked off as a background task in another view.

Answer 1

I always like to show general answers, without going to much into features of special containers. You say the following:

1. You need consumers to ask for new instances.
2. Instances must be returned (probably to be reused).

Using a factory is probably the best approach. The second requirement can be solved by returning IDisposable objects. You can then write code like this:

using (var algorithm = this.algoFactory.CreateNew(type))
{
    // use algorithm.
}

There are multiple ways of doing this, but you can let the algorithm interface implement IDisposable:

public interface IAlgorithm : IDisposable
{
}

Instead of returning the real algorithm, you can return an decorator, that allows that instance to be returned to the pool:

public sealed class PooledAlgorithmDecorator : IAlgorithm
{
    private readonly IAlgorithmPool pool;
    private IAlgorithm real;

    public PooledAlgorithmDecorator(IAlgorithm real,
        IAlgorithmPool pool)
    {
        this.real = real;
        this.pool = pool;
    }

    public void Dispose()
    {
        if (this.real != null)
        {
            this.Pool.ReturnToPool(this.real);
            this.real = null;
        }
    }
}

Now your factory can wrap the real algorithms with the decorator and return that to the consumer:

public class PooledAlgorithmFactory : IAlgorithmFactory
{
    private readonly IAlgorithmPool pool;

    public PooledAlgorithmFactory(IAlgorithmPool pool)
    {
        this.pool = pool;
    }

    public IAlgorithm CreateNew(string type)
    {
        var instance = this.pool.GetInstanceOrCreateNew(type);

        return new PooledAlgorithmDecorator(instance, this.pool);
    }
}

This is of course just an example. Your design will probably be different. Depending on the container you choose, you might get support for factories, object pooling and what not. I think it is important to find the correct design first and the next step is to see whether your container might be able to help you with this.