通用类型约束构造函数的限制，需要咨询

Question

I'm trying to instantiate an object in the constructor of a class based on what type parameter was passed to create the instance of said class.

public class DefaultController<TEntity, TStrategy> : ApiController, IDisposable where TEntity : class, IEntity where TStrategy: BaseStrategy<TEntity>
{

    public IStrategy<TEntity> strategy { get; set; }


    public DefaultController()
    {
        //strategy = ??
    }
}

And here's BaseStrategy

public abstract class BaseStrategy<TEntity> : IStrategy<TEntity> where TEntity : class, IEntity
{

    private IRepository<TEntity> repository;

    public BaseStrategy(IRepository<TEntity> _repository)
    {
        repository = _repository;
    }

    //implements a bunch of methods...
}

As you can see, BaseStategy has no parameter-less constructor which makes it impossible to use the new() generic constraint in DefaultController

What I'm trying to achieve is, from a class extending DefaultController , configure which strategy to use

public class UserController : DefaultController<User, GreatStrategy<User>>
{
    //   GreatStrategy extends BaseStrategy
}

Any workaround to make the magic work or another approach perhaps ?

Thanks.

Answer 1

Is there a reason DefaultController needs a parameterless constructor? You could try:

public DefaultController(IStrategy<IEntity> strategy) : base(strategy)
{
    // Do stuff
}

And then in your UserController constructor:

public UserController() : base(new GreatStrategy<User>()) {
    // Do stuff
}

Are you using IoC in your application? If so, you can register your strategies and inject them into the controller constructors so that you don't have to spin up a new Strategy in the base constructor call.

Hope this gives you some guidance moving forward.

Answer 2

I ended up modifying the constructors of the default controller and of all controllers extending it as such:

    public DefaultController(IStrategy<TEntity> _strategy)
    {
        strategy = _strategy; // 'strategy' is a local member implementing IStrategy...
    }

And a controller extending default controller has a constructor like:

public class UserController : DefaultController<User>
{
    public UserController()
        : base (new GreatStrategy<User>(new Repository<User>()))
    {
        //GreatStrategy extends BaseStrategy...
    }

}