gistfile1.txt

//Psudo C#

public class Program(IDb db)
{
	_db = db;
	
	void Main()
	{
		var results = _db.Query(new FooQuery().From(DateTime.Today).ForUser("doe"));	
	}
}

public interface IDb
{
	IReadOnlyList<T> Query(IQuery<T> query)
	void Add<T>(T o)
	void Delete(T o)
	void SaveChanges()
}

public interface IQuery<T> // Can be extended to have asymetric in/out types to support chaining
{
	IQueryable<T> Build(IQueryable<TIn> source);
}


public class InMemoryDb : IDb
{
	Dictionary<Type, List<object>> _data = new Dictionary<Type, List<object>>();
	IReadOnlyList<T> Query(IQuery<T> query)
	{
		return query.Build(_data[typeof(T)].Cast<T>().AsQueryable()).ToArray();
	}
}

public class EFDb : DataContext, IDb
{
	IDbSet<Foo> Foos { get; set; }
	
	IReadOnlyList<T> Query(IQuery<T> query)
	{
		return query.Build(Set<T>()).ToArray();
	}
	
	void Add<T>(T entity)
	{
		Set<T>().Add(entity);
	}
}

public class FooQuery : IQuery<Foo>
{
	 IQueryable<Foo> Build(IQueryable<TIn> source)
	 {
	 	return source.Where(f => f.Foo == true);
	 }
}