ASP.NET MVC - Generic Controller Model Issue and Polymorphism

Question

Setup:

I have designed a Wizard (based initially on Steve Sanderson's wizard in Pro ASP.NET MVC 2).

Essentially, the base wizard controller is declared as follows:

public abstract class WizardController<TModel> : Controller where TModel : class, IWizardModel, new()
{
   // Loads-n-loadsa-code
}

To implement my wizard, therefore, I need to declare my Wizard controller as follows:

[WizardOptions(StartLabel="Edit >>")]
public partial class EditFeeEarnerController : WizardController<MyApp.Models.MySpecificWizardModel>
{
     // small amounts of highly intuitive code
}

Where MyViewModel implements IWizardModel.

So far so good. The wizard works beautifully and I am right chuffed with it, so don't get hung up on that.

The issue follows:

Issue:

The only issue is that my Wizard uses partial views for each step, that get "stitched" together with a view (Wizard.cshtml).

Wizard.cshtml is ALWAYS the same for ANY wizard EXCEPT for the @model declaration at the top, in my example:

@model MyApp.Models.MySpecificWizardModel

As a result, in an app with 20 wizards, I have the very same file appearing 20 times.

Definitely not DRY.

Question:

I would like to put a file in ~/Views/Shared/Wizard.cshtml and use that for all my wizards. The reason I can't do that is because I only know in advance that my wizard viewmodels will inherit from IWizardModel.

And I can't do this:

@model IWizardModel

So what is the best way to do this, or is it just not possible?

I suppose I could have a base Wizard viewmodel that all my wizard viewmodels inherit from (instead of directly implementing IWizardModel).

Would this work?

EDIT (With the benefit of hindsight):

For the record, my problem was that I was misguided in thinking that using an interface as my model, ie, @model IWizardModel, wouldn't work. As Iain Galloway pointed out in his answer, it does and that solved my problem.

The reason I thought I couldn't use an interface as my model is that I did the following:

@model IWizardModel // MyNamespace.MySpecificWizardModel

The problem is the comment to the right (I commented out the old model).

For some reason, the comment generated an error (something to do with the ViewEngine). In my haste, I just presumed you couldn't use interfaces for the model.

As olde Will said: Great is thy power, oh delay.

See Iain Galloway's answer and my Addendum to it.

Instead of using ViewPartials would you benefit from using @Html.Action("MyAction", "MyController")? — user1477388
– user1477388, Commented Nov 1, 2012 at 16:51
No, I would need to go into the detail, but that doesn't work. It would also spoil the beauty of how this wizard works. — awrigley
– awrigley, Commented Nov 1, 2012 at 16:53
What's preventing you doing @model IWizardModel? I've got a razor view accepting an interface type as its model and it's working for me. — Iain Galloway
– Iain Galloway, Commented Nov 1, 2012 at 16:58
I've used a base model type before...are you sure interfaces don't work? — Lee Gunn
– Lee Gunn, Commented Nov 1, 2012 at 17:11

Iain Galloway · Accepted Answer · 2012-11-01 17:58:30Z

2

I've just tested the following:-

public interface IModel
{
    string Value { get; }
}

public class Foo : IModel
{
    public string Value { get; set; }
}

public class HomeController : Controller
{
    public ActionResult GenericView()
    {
        return View(new Foo() { Value = "Foo" });
    }
}

Along with GenericView.cshtml:-

@model MvcApplication1.Models.IModel

<h2>@Model.Value</h2>

This appears to be working for me. Am I missing something important in your requirements?

answered Nov 1, 2012 at 17:58

Iain Galloway

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2 Comments

awrigley Over a year ago

It would be better to test this with a more complex model. IWizardModel has only one property, that tracks the step the wizard is on. A real life model for a wizard will include many more properties. It is access to those properties that worried me. Initial tests show these concerns to be unfounded, so I have upvoted your post. And will mark it as the answer if all tests show the system working.

awrigley Over a year ago

It seems that all works fine when using IWizardModel in the wizard.cshtml file, so I can remove all these instances (except in special cases) and move it all to the ~/Views/Shared folder. As this was the purpose of the question, I have marked this response as the answer.

awrigley · Accepted Answer · 2012-11-03 12:58:33Z

Addendum to Iain Galloway's answer:

Iain Galloway's answer is correct, and there is no problem with using the interface in the view.

Thinking it through, the process is as follows:

The Model:

Lets say I have an interface ISomeInterface:

public interface ISomeInterface
{
    public int Step { get;set; }
}

And I have a model class MyModel that implements ISomeInterface:

public class MyModel : ISomeInterface
{
    public int Step { get;set; }
    public string Name { get;set; }
    public string Address { get;set; }
}

Controller and Action:

You can pass the model to the view as follows:

return View("MyView", MyModel);

View:

As MyModel implements ISomeInterface, I can then write in the view:

@model ISomeInterface

Polymorphism applies and so the view has access to all properties on the MyModel that was passed in by the code in the action, not just the ISomeInterface properties of the view declaration.

This is what I wanted. It works and so Iain Galloway's answer is marked as correct.

In the case of my Wizard:

As using @model ISomeInterface works in the general case, it also works for my wizard.

For clarity, however, in the partial views (and only in the partial views) that hold the markup for each step, I have left the declaration for the specific model, not just the interface it implements. Ie:

@model MyModel

As a result of polymorphism working with @model, I can now move the wizard.cshtml file to the shared folder which is a great result for the overall DRYness of my wizards.

I still have the flexibility to further customise my wizard.cshtml for each wizard by including an implementation in the folder for the controller. I might want to do this if, for example, I had a captcha control on my wizard, and it requires specific javascript and style files.

Erik Funkenbusch · Accepted Answer · 2012-11-01 16:54:26Z

0

To my knowledge, you can't do this. The reason is that the view engine statically compiles the strongly typed view. There's probably some highly advanced things you could do by implementing your own view engine, but it's unlikely you would want to go to that much work to save a few lines of template code.

answered Nov 1, 2012 at 16:54

Erik Funkenbusch

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5 Comments

awrigley Over a year ago

Sorry, seems you can! See other replies to this post.

awrigley Over a year ago

In the context of my question, this answer is incorrect. Do you want to remove the question? Otherwise, I will vote it down to remove any ambiguity in people viewing this thread who have the same doubts as I had.

Erik Funkenbusch Over a year ago

@awrigley - My answer is relating to the question you asked, not what it mutated into. As such, it's still completely relevant. You can't dynamically change the type of the view template, and that answer is correct. There are other things you can do to work around it, but that doesn't change the fact that the question you asked the answer is no.

awrigley Over a year ago

I get where you are coming from now. For the record, I'm not saying the answer to "Would this work?" isn't "No." But... Your answer ignores the detailed CONTEXT (Setup, Issue AND Question), and it doesn't highlight that my assumption was WRONG: Ie, that using @model IWizardModel was NOT possible. It is and that solved my issue. Please don't think I want to polemicise with you. I just want any future readers not to be mislead by your answer, as I was initially. These comments provide enough context so no need to vote down.

awrigley Over a year ago

I will of course be more careful with how I phrase my questions in future. I can see how someone just finds the question and answers that. My fault.

Kirill Bestemyanov · Accepted Answer · 2012-11-01 17:12:56Z

0

You can do what you want with trick: You can create wrapper model WizardModel that implement IWizardModel and contains property InheritedWizardModel of type IWizardModel.

public class WizardModel: IWizardModel
{
    private IWizardModel _inheritedWizardModel{get;set;}

    //implementation IWizardModel
    public SomeType SomeIWizardProperty
    {
        get{return _inheritedWizardModel.SomeIWizardProperty;} 
        set{_inheritedWizardModel.SomeIWizardProperty = value;}
    }
}

And use it

answered Nov 1, 2012 at 17:12

Kirill Bestemyanov

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2 Comments

Erik Funkenbusch Over a year ago

That's a great idea. To be clear, you would always use WizardModel in your view, but the property gets set to your real model.

awrigley Over a year ago

In the context of my question, this is way too complicated. Polymorphism is enough to solve the issue. See my answer to my question for details.

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ASP.NET MVC - Generic Controller Model Issue and Polymorphism

Setup:

Issue:

Question:

EDIT (With the benefit of hindsight):

4 Answers 4

2 Comments

Addendum to Iain Galloway's answer:

The Model:

Controller and Action:

View:

In the case of my Wizard:

Comments

5 Comments

2 Comments

Your Answer

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Collectives™ on Stack Overflow

Setup:

Issue:

Question:

EDIT (With the benefit of hindsight):

4 Answers 4

2 Comments

Addendum to Iain Galloway's answer:

The Model:

Controller and Action:

View:

In the case of my Wizard:

Comments

5 Comments

2 Comments

Your Answer

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