Winforms - MVP Pattern: Using static ApplicationController to coordinate application?

Question

Background

I'm building a two-tiered C# .net application:

1. Tier 1: Winforms client application using the MVP (Model-View-Presenter) design pattern.
2. Tier 2: WebAPI RESTful service sitting on top of Entity Framework and SQL Server.

Currently, I have questions relating to the overall architecture of the Winforms client application. I'm new to programming (about a year) but I've made good progress with this application. I want to step back and re-evaluate my current approach to check that I'm generally heading in the right direction.

Application Domain

The Winforms application is a fairly simple security personnel tracking application. The main view (Form) is the focus of the application, and has different sections which group content into functional areas (e.g. a section for tracking personnel schedules, a section for tracking who is assigned where, etc.). A menu on the side of the application launches secondary views (e.g. history, statistics, contacts, etc.). The idea is that the app could be used by a security office to organize daily operations and then keep a detailed history of everything in a database for reporting on in the future.

Technical Details

As mentioned, the Winforms client is built using the MVP pattern (passive view), focusing on using dependency injection as much as possible (via SimpleInjector IoC container). Each view (form) is paired up with a single presenter. The views implement interfaces, allowing the presenter to control the view (regardless of the concrete implementation). The view raises events for the presenter to subscribe to. Currently, presenters are not allowed to directly communicate to another presenter.

An application controller is used to coordinate the application. This is the area of my application architecture where I'm the most shakey (hence the post title). The application controller is currently used to:

1. Open new views (forms) and manage open forms.
2. Facilitate communication between application components via an event aggregator. One presenter publishes an event and any number of presenter can subscribe to that event.
3. Host session information (i.e. security context/logon, config data, etc.)

The IoC container is registered into the application controller at application start-up. This allows the application controller, for example, to create a presenter from the container, and then have all subsequent dependencies (view, services, etc.) to be automatically handled by the container.

Question

In order to make the Application Controller accessible to all presenters, I have created the controller as a static class.

public static class ApplicationController
{
    private static Session _session;
    private static INavigationWorkflow _workflow;
    private static EventAggregator _aggregator;

    #region Registrations

    public static void RegisterSession(Session session) {}

    public static void RegisterWorkflow(INavigationWorkflow workflow) {}

    public static void RegisterAggregator(EventAggregator aggregator) {}

    #endregion

    #region Properties

    public static Session Session
    {
        get { return _session; }
    }

    #endregion

    #region Navigation

    public static void NavigateToView(Constants.View view) {}

    #endregion

    #region Events

    public static Subscription<TMessageType> Subscribe<TMessageType>(Action<TMessageType> action) {}

    public static void Publish<TMessageType>(TMessageType message) {}

    public static void Unsubscribe<TMessageType>(Subscription<TMessageType> subscription) {}

    #endregion
}

Is this considered an acceptable practice to make a static class like this? I mean, it certainly works. It just feels... off? Are there any other holes that you can see in my architecture based on what I have described?

-

** EDIT **

This edit is made in response to Ric .Net’s answer posted below.

I have read through all of your suggestions. As I am committed to utilizing dependency injection to the fullest extent I can, I’m onboard with all of your suggestions. That was my plan from the beginning, but when I ran into things I didn’t understand how to accomplish via injection, I turned to the global static controller class to solve my problems (A god class it is becoming, indeed. Yikes!). Some of those questions still exist:

Event Aggregator

The defining line here is what should be considered optional, I think. I’ll provide a bit more context about my app before outlining my problem. Using web terminology, my main form generally acts like a layout view, hosting navigation controls and a notification section in the left menu, and partial views being hosted in the center. Coming back to winforms terminology, the partial views are just custom made UserControls that I treat like views, and each of them are paired up with their own presenter. I have 6 of these partial views hosted on my main form, and they serve as the meat and potatoes of the application.

As an example, one partial view lists available security guards and another lists potential patrol areas. In a typical use case, a user would drag an available security guard from the available list to one of the potential patrol areas, effectively becoming assigned to that area. The patrol area view would then update to show the assigned security guard and the guard would be removed from the available list view. Utilizing drag-and-drop events, I can handle this interaction.

My questions come when I need to handle other types of interactivity between the various partial views. For example, double clicking on guard that is assigned to a location (as seen in one partial view) could highlight that guard’s name on another partial view showing all personnel schedules, or bring up employee details/history on another partial view. I could see the graph/matrix of what partial views are interested in events occurring in other partial views as becoming quite complex, and I’m not sure how to handle that via injection. With 6 partial views, I wouldn’t want to inject the other 5 partial views/presenters into each one. I was planning on accomplishing this via the event aggregator. Another example I could think of is needing to update data on a separate view (its own form) based off an event that occurs on one of the partial views on the main form.

Session & Form Opener

I really like your thoughts here. I’m going to take these ideas and run with them, and see where I end up!

Security

What are your thoughts on controlling user access to certain functionality based on what type of account they have? The recommendations I’ve been reading online say that security could be implemented by modifying the views based on their account type. The thought being, if a user can’t interact with a UI element to kick off a certain task, then the presenter will never be asked to perform that task. I’m curious if you inject the WindowsUserContext into each presenter and do additional checks, especially for http service bound requests?

I haven’t done too much development on the service side of things yet, but for http service bound requests, I imagine you need to send security information along with each request so that the service can authenticate the request. My plan was to inject the WindowsUserContext directly into the winforms service agents that end up making the service requests (i.e. the security validation would not be coming from the presenter). In that case, the service agents could potentially do a last minute security check before sending off a request.

Answer 1

A static class is of course handy in some cases but there are a lot of downsides to this approach.

• The tend to grow into something like a God class. You already see this happening. So this class violates SRP
• A static class cannot have dependencies and therefore it needs to use the Service Locator anti pattern to get it's dependencies. This is not a problem perse if you consider this class to be part of the composition root, but nevertheless, this often heads the wrong way.

In the supplied code I see three responsibilities of this class.

1. EventAggregator
2. What you call Session information
3. A service to open other views

Some feedback on this three parts:

EventAggregator

Although this is a widely used pattern and sometimes it can be very powerful I myself am not fond of this pattern. I see this pattern as something that provides optional runtime data where in most cases this runtime data is not optional at all. In other words, only use this pattern for truly optional data. For everything that is not really optional, use hard dependencies, using constructor injection.

The ones that need the information in that case depend upon IEventListener<TMessage>. The one that publish the event, depend upon IEventPublisher<TMessage>.

public interface IEventListener<TMessage> 
{
    event Action<TMessage> MessageReceived;
}

public interface IEventPublisher<TMessage> 
{
    void Publish(TMessage message);
}

public class EventPublisher<TMessage> : IEventPublisher<TMessage> 
{
    private readonly EventOrchestrator<TMessage> orchestrator;

    public EventPublisher(EventOrchestrator<TMessage> orchestrator) 
    {
        this.orchestrator = orchestrator;
    }

    public void Publish(TMessage message) => this.orchestrator.Publish(message);
}

public class EventListener<TMessage> : IEventListener<TMessage> 
{
    private readonly EventOrchestrator<TMessage> orchestrator;

    public EventListener(EventOrchestrator<TMessage> orchestrator) 
    {
        this.orchestrator = orchestrator;
    }

    public event Action<TMessage> MessageReceived 
    {
        add { orchestrator.MessageReceived += value; }
        remove { orchestrator.MessageReceived -= value; }
    }
}

public class EventOrchestrator<TMessage> 
{
    public void Publish(TMessage message) => this.MessageReceived(message);
    public event Action<TMessage> MessageReceived = (e) => { };
}

To be able to guarantee events are stored in one single location, we extract that storage (the event) into its own class, the EventOrchestrator.

The registration is as follows:

container.RegisterSingleton(typeof(IEventListener<>), typeof(EventListener<>));
container.RegisterSingleton(typeof(IEventPublisher<>), typeof(EventPublisher<>));
container.RegisterSingleton(typeof(EventOrchestrator<>), typeof(EventOrchestrator<>));

Usage is trivial:

public class SomeView
{
    private readonly IEventPublisher<GuardChanged> eventPublisher;

    public SomeView(IEventPublisher<GuardChanged> eventPublisher)
    {
        this.eventPublisher = eventPublisher;
    }

    public void GuardSelectionClick(Guard guard)
    {
        this.eventPublisher.Publish(new GuardChanged(guard));
    }
    // other code..
}

public class SomeOtherView
{
    public SomeOtherView(IEventListener<GuardChanged> eventListener)
    {
        eventListener.MessageReceived += this.GuardChanged;
    }

    private void GuardChanged(GuardChanged changedGuard)
    {
        this.CurrentGuard = changedGuard.SelectedGuard;
    }
    // other code..
}

If another view will receive a lot of events you could always wrap all IEventListeners of that View in a specific EventHandlerForViewX class which get all important IEventListener<> injected.

Session

In the question you define several ambient context variables as Session information. Exposing this kind of information through a static class promotes tight coupling to this static class and thus makes it more difficult to unit test parts of your application. IMO all information provided by Session is static (in the sense that it doesn't change throughout the lifetime of the application) data that could just as easily be injected into those parts that actually need this data. So Session should completely be removed from the static class. Some examples how to solve this in a SOLID manner:

Configuration values

The composition root is in charge of reading all information from the configuration source (e.g. your app.config file). This information can there be stored in a POCO class crafted for its usage.

public interface IMailSettings
{
    string MailAddress { get; }
    string DefaultMailSubject { get; }
}

public interface IFtpInformation
{
    int FtpPort { get; }
}

public interface IFlowerServiceInformation
{
    string FlowerShopAddress { get; }
}

public class ConfigValues :
    IMailSettings, IFtpInformation, IFlowerServiceInformation
{
    public string MailAddress { get; set; }
    public string DefaultMailSubject { get; set; }

    public int FtpPort { get; set; }

    public string FlowerShopAddress { get; set; }
}
// Register as
public static void RegisterConfig(this Container container)
{
    var config = new ConfigValues
    {
        MailAddress = ConfigurationManager.AppSettings["MailAddress"],
        DefaultMailSubject = ConfigurationManager.AppSettings["DefaultMailSubject"],
        FtpPort = Convert.ToInt32(ConfigurationManager.AppSettings["FtpPort"]),
        FlowerShopAddress = ConfigurationManager.AppSettings["FlowerShopAddress"],
    };

    var registration = Lifestyle.Singleton.CreateRegistration<ConfigValues>(() => 
                                config, container);
    container.AddRegistration(typeof(IMailSettings),registration);
    container.AddRegistration(typeof(IFtpInformation),registration);
    container.AddRegistration(typeof(IFlowerServiceInformation),registration);
}

And where you need some specific information, e.g. information to send an email you can just put IMailSettings in the constructor of the type needing the information.

This will also give you the possibility to test a component using different config values, which would be harder to do if all config information had to come from the static ApplicationController.

For security information, e.g. the logged on User the same pattern can be used. Define an IUserContext abstraction, create a WindowsUserContext implementation and fill this with the logged on user in the composition root. Because the component now depends on IUserContext instead of getting the user at runtime from the static class, the same component could also be used in an MVC application, where you would replace the WindowsUserContext with an HttpUserContext implementation.

Opening other forms

This is actually the hard part. I normally also use some big static class with all kinds of methods to open other forms. I don't expose the IFormOpener from this answer to my other forms, because they only need to know, what to do, not which form does that task for them. So my static class exposes this kinds of methods:

public SomeReturnValue OpenCustomerForEdit(Customer customer)
{ 
     var form = MyStaticClass.FormOpener.GetForm<EditCustomerForm>();
     form.SetCustomer(customer);
     var result = MyStaticClass.FormOpener.ShowModalForm(form);
     return (SomeReturnValue) result;
}

However....

I'm not at all happy with this approach, because over time this class grows and grows. With WPF I use another mechanism, which I think could also be used with WinForms. This approach is based on a message based architecture described in this and this awesome blogposts. Although at first the information looks as it is not at all related, it is the message based concept that let these patterns rock!

All my WPF windows implement an open generic interface, e.g. IEditView. And if some view needs to edit a customer, it just get's this IEditView injected. A decorator is used to actually show the view in pretty much the same way as the forementioned FormOpener does it. In this case I make use of a specific Simple Injector feature, called decorate factory decorator, which you can use to create forms whenever it is needed, just as the FormOpener used the container directly to create forms whenever it needs to.

So I did not really test this, so there could be some pitfalls with WinForms, but this code seems to work on a first and single run..

public class EditViewShowerDecorator<TEntity> : IEditView<TEntity>
{
    private readonly Func<IEditView<TEntity>> viewCreator;

    public EditViewShowerDecorator(Func<IEditView<TEntity>> viewCreator)
    {
        this.viewCreator = viewCreator;
    }

    public void EditEntity(TEntity entity)
    {
        // get view from container
        var view = this.viewCreator.Invoke();
        // initview with information
        view.EditEntity(entity);
        using (var form = (Form)view)
        {
            // show the view
            form.ShowDialog();
        }
    }
}

The forms and decorator should be registered as:

container.Register(typeof(IEditView<>), new[] { Assembly.GetExecutingAssembly() });
container.RegisterDecorator(typeof(IEditView<>), typeof(EditViewShowerDecorator<>), 
                             Lifestyle.Singleton);

Security

The IUserContext must the base for all security.

For the userinterface I normally hide all controls/buttons that a certain userrole doesn't have access to. The best place is to perform this in the Load event.

Because I use the command/handler pattern as described here for my all actions external of my forms/views I use a decorator to check if a user has permission to perform this certain command (or query).

I would advise you to read this post a few times until you really get the hang of it. Once you get familiar with this pattern you won't do anything else!

If you have any questions about these patterns and how to apply a (permission)decorator, add a comment!