Factory Pattern with Open Generics

Question

In ASP.NET Core, one of the things you can do with Microsoft\'s dependency injection framework is bind \"open generics\" (generic types unbound to a concrete type) like so:<

Answer 1

The net.core dependency does not allow you to provide a factory method when registering an open generic type, but you can work around this by providing a type that will implement the requested interface, but internally it will act as a factory. A factory in disguise:

services.AddSingleton(typeof(IMongoCollection<>), typeof(MongoCollectionFactory<>)); //this is the important part
services.AddSingleton(typeof(IRepository<>), typeof(Repository<>))


public class Repository : IRepository {
    private readonly IMongoCollection _collection;
    public Repository(IMongoCollection collection)
    {
        _collection = collection;
    }

    // .. rest of the implementation
}

//and this is important as well
public class MongoCollectionFactory : IMongoCollection {
    private readonly _collection;

    public RepositoryFactoryAdapter(IMongoDatabase database) {
        // do the factory work here
        _collection = database.GetCollection(typeof(T).Name.ToLowerInvariant())
    }

    public T Find(string id) 
    {
        return collection.Find(id);
    }   
    // ... etc. all the remaining members of the IMongoCollection, 
    // you can generate this easily with ReSharper, by running 
    // delegate implementation to a new field refactoring
}

When the container resolves the MongoCollectionFactory ti will know what type T is and will create the collection correctly. Then we take that created collection save it internally, and delegate all calls to it. ( We are mimicking this=factory.Create() which is not allowed in csharp. :))

Update: As pointed out by Kristian Hellang the same pattern is used by ASP.NET Logging

public class Logger : ILogger
{
    private readonly ILogger _logger;

    public Logger(ILoggerFactory factory)
    {
        _logger = factory.CreateLogger(TypeNameHelper.GetTypeDisplayName(typeof(T)));
    }

    void ILogger.Log(...)
    {
        _logger.Log(logLevel, eventId, state, exception, formatter);
    }
}

https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Extensions.Logging.Abstractions/LoggerOfT.cs#L29

original discussion here:

https://twitter.com/khellang/status/839120286222012416