Is it a good practice to pass the Ninject kernel around?

Question

I'm writing a small framework that executed a couple of tasks. Some om the tasks needs specific properties which are injected through Ninject.

Let's say that I have the following constructor in my base class that represents a single Task:

protected DDPSchedulerTask(ILogger logger, List<string> platforms, IBackOfficeDataStore backOfficeDataStore, ICommonDataStore commonDataStore)
{
    _logger = logger;
    _platforms = platforms;
    _backOfficeDataStore = backOfficeDataStore;
    _commonDataStore = commonDataStore;
}

Those properties are needed for all the tasks, so I inject them using Ninject with the following Ninject module.

public class DDPDependencyInjectionBindingConfiguration : NinjectModule
{
    #region NinjectModule Members

    /// <summary>
    ///     Loads the module into the kernel.
    /// </summary>
    public override void Load()
    {
        Bind<Scheduler>().ToSelf(); // Make sure that the 'Scheduler' is resolved to itself.
        Bind<ILogger>().ToMethod(context => LogFactory.Create()); // Make sure that an instance of an ILogger is created through the LogFactory.

        // Straightforward binding.
        Bind<ICommonDataStore>().To<Common>();
        Bind<IBackOfficeDataStore>().To<BtDbInteract>();
        Bind<IDirectoryResolver>().To<Demo>();
    }

    #endregion
}

My scheduler object itself if the first entry in the chain which needs to be resolved by Ninject, so I'm resolving this manually through Ninject.

var schedulerInstance = kernel.Get<Scheduler>();

Now, my scheduler have a method which adds tasks to a list, so not by using Ninject:

var tasksList = new List<DDPSchedulerTask>
{
    new AWNFileGeneratorTask(_logger, availablePlatforms, _backOfficeDataStore, _commonDataStore)
};

Then, all those tasks are being executed. Now, some of those tasks does require additional dependencies which I would like to resolve through Ninject but how should I do this?

Inside a task, I've created a property with the Inject attribute, but the object stays nulls.

[Inject]
private IDirectoryResolver directoryResolver { get; set; }

Anyone has an idea on how this can be resolved?

I can pass the kernel around to the different tasks, but something's telling me that this isn't the correct approach.

Kind regards

Answer 1

You should take advantage of Ninject.Extensions.Factory.

Just create an interface that represents your tasks factory. Then pass information to Ninject that is a factory And he would create for you full implementations of this interface.

using System;
using System.Collections.Generic;
using Ninject;
using Ninject.Extensions.Factory;

internal class Program
{
    public static void Main(string[] args)
    {
        IKernel ninjectKernel = new StandardKernel();

        ninjectKernel.Bind<DDPSchedulerTask>().ToSelf();
        ninjectKernel.Bind<AWNFileGeneratorTask>().ToSelf();
        ninjectKernel.Bind<IDirectoryResolver>().To<DirectoryResolver>();
        ninjectKernel.Bind<ITaskFactory>().ToFactory();

        var mainTask = ninjectKernel.Get<DDPSchedulerTask>();
        mainTask.CreateDbSchedulerTask();
        mainTask.CreateAwnFileTask();

        Console.ReadLine();
    }
}

public interface ITaskFactory
{
    TTask CreateTask<TTask>() where TTask : DDPSchedulerTask;
}

public class DDPSchedulerTask
{
    private readonly ITaskFactory _tasksFactory;
    private readonly List<DDPSchedulerTask> _tasksList;

    public DDPSchedulerTask(ITaskFactory tasksFactory)
    {
        _tasksFactory = tasksFactory;

        _tasksList = new List<DDPSchedulerTask>();
    }

    public void CreateAwnFileTask()
    {
        var task = _tasksFactory.CreateTask<AWNFileGeneratorTask>();
        _tasksList.Add(task);
    }

    public void CreateDbSchedulerTask()
    {
        var task = _tasksFactory.CreateTask<DDPSchedulerTask>();
        _tasksList.Add(task);
    }
}

public class AWNFileGeneratorTask : DDPSchedulerTask
{
    [Inject]
    public IDirectoryResolver DirectoryResolver { get; set; }

    public AWNFileGeneratorTask(ITaskFactory tasksFactory)
        : base(tasksFactory)
    {
    }
}

public interface IDirectoryResolver
{
}

public class DirectoryResolver : IDirectoryResolver
{
}

@gisek As noted Giseke dependency injection via property is not the best solution. You can also use a constructor injection in this example.

public class AWNFileGeneratorTask : DDPSchedulerTask
{
    private IDirectoryResolver _directoryResolver;

    public AWNFileGeneratorTask(ITaskFactory tasksFactory, IDirectoryResolver directoryResolver)
        : base(tasksFactory)
    {
        _directoryResolver = directoryResolver;
    }
}

Extra params injection:

public interface ITaskFactory
{
    DDPSchedulerTask CreateDDPSchedulerTask();
    AWNFileGeneratorTask CreateAWNFileGeneratorTask(string extraParam);
}

public class AWNFileGeneratorTask : DDPSchedulerTask
{
    private IDirectoryResolver _directoryResolver;
    private string _extraParam;

    public AWNFileGeneratorTask(ITaskFactory tasksFactory, IDirectoryResolver directoryResolver,
        string extraParam)
        : base(tasksFactory)
    {
        _extraParam = extraParam;
        _directoryResolver = directoryResolver;
    }
}

public void CreateAwnFileTask()
{
    var task = _tasksFactory.CreateAWNFileGeneratorTask("extra");
    _tasksList.Add(task);
}

Answer 2

In such cases I usually use a Factory pattern. In the scheduler you can have the task factory as a dependency. This factory can also have multiple methods for creating different types of tasks.

class DDPSchedulerTaskFactory : IDDPSchedulerTaskFactory
{
    DDPSchedulerTaskFactory(ILogger logger, List<string> platforms, IBackOfficeDataStore backOfficeDataStore, ICommonDataStore commonDataStore)
    {
        _logger = logger;
        _platforms = platforms;
        _backOfficeDataStore = backOfficeDataStore;
        _commonDataStore = commonDataStore;
    }

    public IDDPSchedulerTask CreateNormal(){
        return new DDPSchedulerTask(
            _logger,
            _platforms,
            _backOfficeDataStore,
            _commonDataStore);
    }

    public IDDPSchedulerTask CreateSpecial(someAdditionalParameter){
        return new AnotherDDPSchedulerTask(
            _logger,
            _platforms,
            _backOfficeDataStore,
            _commonDataStore,
            someAdditionalParameter);
    }

    public IDDPSchedulerTask CreateTaskWithDifferentDependenties(yetAnotherParameter){
        return new AnotherDDPSchedulerTask(
            _logger,
            _platforms,
            yetAnotherParameter);
    }
}

Than in your scheduler you can add tasks like that:

class Scheduler{
    IDDPSchedulerTaskFactory _taskFactory;
    public Scheduler(IDDPSchedulerTaskFactory taskFactory){
        _taskFactory = taskFactory; // factory gets all the needed dependencies for all tasks from DI
    }   
    ...

    public void ConfigureTasks(){
        _tasks.Add(_taskFactory.CreateNormal());
        _tasks.Add(_taskFactory.CreateSpecial("Some important message"));
        _tasks.Add(_taskFactory.CreateTaskWithDifferentDependenties(123));
    }
}