Years ago, ex-Googler Doug Edwards wrote a blog post to explain the meaning behind a few favorite words in the software developer’s vocabulary: orthogonal, cruft, canonical, and the big one - non-trivial.
Non-trivial
It means impossible. Since no engineer is going to admit something is impossible, they use this word instead.
I’ve spent the bulk of my career developing commercial software, and it’s amazing how many sales people, executives, and marketing managers ask if something is possible. Example:
Is it possible to reverse the flow of time when we click this button?
The requests are more realistic, of course, but the stock reply is:
Given enough resources, anything is possible.
The response doesn’t say the feature is impossible, or even non-trivial. You might think the response is the habitual reply of yet another passive-aggressive software developer (the canonical developer, to use Mr. Edward’s lexicon), but I much prefer to think of the reply as a zen-like answer that points the questioner to the realities of software development. Despite what the pointy haired boss hears from tool vendors and analysts, creating software still requires resources – both time and mental effort.
Building commercial software for a vertical market is a … non-trivial endeavor. But, not non-trivial in the impossible sense. The problem is you don’t know precisely what features will provide enough value to attract new customers until you’ve done some work.
I’ve generally found that if someone inside an ISV is asking if a feature is possible, it’s only a feature they want if it comes for free. They haven't done the homework to understand how the feature would work to provide value for the mainstream customer, and the idea is still in an incubation phase. It will be difficult to even estimate the amount of work required.
When they turn the question into:
What does it take to get this into our software?
Then you know they are serious and passionate about the idea, and it’s time to start talking.
After my last post on model binding tips I’ve had a number of questions about the nuances of model binding. Let’s work through a sample.
Imagine you have a Recipe class that can hold all the information you need to make Love Mussels, and now you've decided to build a model binder to bind and validate recipes.
Let’s start with the naive approach. This code has a number of problems.
public class RecipeModelBinder : IModelBinder { public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) { var form = controllerContext.HttpContext.Request.Form; var recipe = new Recipe(); recipe.Name = form["Name"]; // ... and so on for all properties if (String.IsNullOrEmpty(recipe.Name)) { bindingContext.ModelState.AddModelError("Name", "..."); } return recipe; } }
Problem
The code works directly off the HttpContext.Request.Form.
There are a couple difficulties working with the Form collection directly. One problem is how your tests will require more setup to create an HttpContext, Request, and Form objects.
The second problem is related to culture. Values you need from the Form collection are culture sensitive because the user will type a date and time value into their browser using a local convention. However, the URL is another place you might need to check when binding values (the query string and routing data in general), and these values are culture invariant.
Instead of worrying about all these details, it’s better to use the ValueProvider given to us by the incoming binding context. The value provider is easy to populate in a unit test, and takes care of culture sensitive conversions.
We’ll add a GetValue method to help fetch values from the ValueProvider. At runtime the MVC framework populates the provider with values it finds in the request’s form, route, and query string collections.
public class RecipeModelBinder : IModelBinder { public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) { var recipe = new Recipe(); recipe.Name = GetValue<string>(bindingContext, "Name"); // ... and so on for all properties if (String.IsNullOrEmpty(recipe.Name)) { bindingContext.ModelState.AddModelError("Name", "..."); } return recipe; } private T GetValue<T>(ModelBindingContext bindingContext, string key) { ValueProviderResult valueResult; bindingContext.ValueProvider.TryGetValue(key, out valueResult); return (T)valueResult.ConvertTo(typeof(T)); } }
Problem:
If you use any HTML Helpers (like Html.TextBox), you’ll see null reference exceptions when validation errors are present.
One of the side-effects of model binding is that binding the model should put model values into ModelState. When an HTML helper sees there is a ModelState error for “Name”, it assumes it will also find the “attempted value” that the user entered. The helper uses attempted values to repopulate inputs and allow the user to fix any errors.
The only change is to set the model value inside GetValue.
private T GetValue<T>(ModelBindingContext bindingContext, string key) { ValueProviderResult valueResult; bindingContext.ValueProvider.TryGetValue(key, out valueResult); bindingContext.ModelState.SetModelValue(key, valueResult); return (T)valueResult.ConvertTo(typeof(T)); }
Problem:
The model binder we’ve written so far will work with controller actions like the following:
[AcceptVerbs(HttpVerbs.Post)] public ActionResult Create(Recipe newRecipe) { // ... return View(newRecipe); }
But it won’t work in this scenario:
[AcceptVerbs(HttpVerbs.Post)] public ActionResult Create(FormCollection formCollection) { var recipe = RecipeFactory.Create(); TryUpdateModel(recipe); if (ModelState.IsValid) { // save it ... } return View(recipe); }
The recipe in the controller action will never see any changes when calling TryUpdateModel, because the model binder is creating and binding data into its own recipe object. Perhaps you never use UpdateModel or TryUpdateModel, but if you expect your model binder to work in any possible situation, you need to make sure the model binder works when someone else creates the model.
The only change is to check bindingContext.Model to see if we already have a model. Only when this property is null will we go to the trouble of creating a new model.
public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) { var form = controllerContext.HttpContext.Request.Form; var recipe = (Recipe)(bindingContext.Model ?? new Recipe()); recipe.Name = GetValue<string>(bindingContext, "Name"); // ... return recipe; }
Problem:
Congratulations! We just re-implemented the behavior of the DefaultModelBinder - only our model binder is stupid and only works with Recipes.
If all we need is validation for a specific type of model, we can derive from the built-in binder and override OnModelUpdated or OnPropertyValidating and provide our custom logic.
OnModelUpdated is easy to work with, so what follows is the entire listing for our custom model binder.
public class RecipeModelBinder : DefaultModelBinder { protected override void OnModelUpdated(ControllerContext controllerContext, ModelBindingContext bindingContext) { var recipe = bindingContext.Model as Recipe; if (String.IsNullOrEmpty(recipe.Name)) { bindingContext.ModelState.AddModelError("Name", "..."); } } }
It turns out we didn’t need any code from those first four iterations, but I hope you find them useful because they demonstrate some common problems I’m seeing in custom model binders. Of course we could take this example even further and eliminate the magic strings, but we’ll leave the work for another day.
In software development – every iteration is a learning opportunity!
Model binding in the ASP.NET MVC framework is simple. Your action methods need data, and the incoming HTTP request carries the data you need. The catch is that the data is embedded into POST-ed form values, and possibly the URL itself. Enter the DefaultModelBinder, which can magically convert form values and route data into objects. Model binders allow your controller code to remain cleanly separated from the dirtiness of interrogating the request and its associated environment.
Here are some tips on how to take advantage of model binding in your MVC projects.
Tip #1: Prefer Binding Over Request.Form
If you are writing your actions like this ..
[AcceptVerbs(HttpVerbs.Post)] public ActionResult Create() { Recipe recipe = new Recipe(); recipe.Name = Request.Form["Name"]; // ... return View(); }
.. then you are doing it all wrong. The model binder can save you from using the Request and HttpContext properties – those properties make the action harder to read and harder to test. One step up would be to use a FormCollection parameter instead:
public ActionResult Create(FormCollection values) { Recipe recipe = new Recipe(); recipe.Name = values["Name"]; // ... return View(); }
With the FormCollection you don’t have to dig into the Request object, and sometimes you need this low level of control. But, if all of your data is in Request.Form, route data, or the URL query string, then you can let model binding work its magic:
[AcceptVerbs(HttpVerbs.Post)] public ActionResult Create(Recipe newRecipe) { // ... return View(); }
In this example, the model binder will create your newRecipe object and populate it with data it finds in the request (by matching up data with the recipe’s property names). It’s pure auto-magic. There are many ways to customize the binding process with “white lists”, “black lists”, prefixes, and marker interfaces. For more control over when the binding takes place you can use the UpdateModel and TryUpdateModel methods. Just beware of unintentional binding – see Justin Etheredge’s Think Before You Bind.
Tip #2: Custom model binders
Model binding is also one of the extensibility points in the MVC framework. If you can’t use the default binding behavior you can provide your own model binders, and mix and match binders. To implement a custom model binder you need to implement the IModelBinder interface. There is only method involved - how hard can it be?
public interface IModelBinder { object BindModel(ControllerContext controllerContext,
ModelBindingContext bindingContext); }
Once you get neck deep into model binding, however, you’ll discover that the simple IModelBinder interface doesn’t fully describe all the implicit contracts and side-effects inside the framework. If you take a step back and look at the bigger picture you’ll see that model binding is but one move in a carefully orchestrated dance between the model binder, the ModelState, and the HtmlHelpers. You can pick up on some of these implicit behaviors by reading the unit tests for the default model binder.
Scott Hanselman demonstrates a non-trivial model binder in his post “Splitting DateTime – Unit Testing ASP.NET MVC Custom Model Binders”. One detail I want to call out about Scott’s DateTime splitter is how you still don’t need to use Request.Form when building the model. Inside the GetA<T> method you’ll see how Scott uses the binding context’s ValueProvider property to fetch data. The ValueProvider represents an amalgamation of all data from the request’s posted form values, routing data, and query string. Scott’s example is great, but it is missing one detail – the propagation of binding errors.
If the default model binder has problems putting data into your object, it will place the error messages and the erroneous data value into ModelState. You can check ModelState.IsValid to see if binding problems are present, and use ModelState.AddModelError to inject your own error messages. See this very simple tutorial for more information on how ModelState and HtmlHelpers can work together to present validation errors to the user.
If you scroll down the comments to Scott’s post you’ll see Sebastien Crocquesel’s patch for Scott’s code. If a conversion fails, Sebastien’s code will use ModelState.AddModelError to propagate the error. Both the controller action and the view can look in ModelState to see if there was a binding problem. The controller would need to check ModelState for errors before saving stuff into the database, while the view can check ModelState for errors to give the user validation feedback. One important note is that the HtmlHelpers you use in a view will require ModelState to hold both a value (via ModelState.SetModelValue) and the error (via AddModelError) or you’ll have runtime errors (null reference exceptions). The following code can demonstrate the problem:
[AcceptVerbs(HttpVerbs.Post)] public ActionResult Create(FormCollection Form) { // this is the wrong approach ... if (Form["Name"].Trim().Length == 0) ModelState.AddModelError("Name", "Name is required"); return View(); }
The above code creates a model error without ever setting a model value. It has other problems, too, but it will create exceptions if you render the following view.
<%= Html.TextBox("Name", Model.Name) %>Even though you’ve specified Model.Name as the value for the textbox, the textbox helper will see the model error and attempt to display the “attempted value” that the user tried to put in the model. If you didn’t set the model value in model state you’ll see a null reference exception.
Tip #3: Custom Model Binding via Inheritance
If you’ve decided to implement a custom model binder, you might be able to cut down on the amount of work required by inheriting from DefaultModelBinder and adding some custom logic. In fact, this should be your default plan until you are certain you can’t subclass the default binder to achieve the functionality you need. For example, suppose you just want to have some control over the creation of your model object. The DefaultModelBinder will create object’s using Activator.CreateInstance and the model’s default constructor. If you don’t have a default constructor for your model, you can subclass the DefaultModelBinder and override the CreateModel method.
Jimmy Bogard has an example of sub classing the DefaultModelBinder in his post titled “A Better Model Binder”.
Tip #4: Using Data Annotations for Validation
Brad Wilson explains everything beautifully in this post: DataAnnotations and ASP.NET MVC.
I encourage you to go read Brad’s post, but if you are in a hurry, here is a summary:
.NET 3.5 SP1 shipped a System.ComponentModel.DataAnnotations assembly that looks to play a central role as we move forward with the .NET framework. By using data annotations and the DataAnnotationsModelBinder, you can take care of most of your server-side validation by simply decorating your model with attributes.
public class Recipe { [Required(ErrorMessage="We need a name for this dish.")] [RegularExpression("^Bacon")] public string Name { get; set; } // ... }
The DataAnnotationsModelBinder is also a great sample to read and understand how to effectively subclass the default model binder.
Tip #5 : Recognize Binding and Validation As Two Phases
Binding is about taking data from the environment and shoving it into the model, while validation is checking the model to make sure it meets our expectations. These are different different operations, but model binding tends to blur the distinction. If you want to perform validation and binding together in a model binder, you can – it’s exactly what the DataAnnotationsModelBinder will do. You can also find samples like Automatic Model Validation with ASP.NET MVC, xVal, Castle, and a Custom Binder (John McDowall), and Enterprise Library Validation Application Block with MVC Binders (Steve Michelotti). However, one thing that is often overlooked is how the DefaultModelBinder itself separates the binding and validation phases. If all you need is simple property validation, then all you need to do is override the OnPropertyValidating method of the DefaultModelBinder.
Tip #6: Binders Are About The Environment
Earlier I said that “model binders allow your controller code to remain cleanly separated from the dirtiness of interrogating the request and its associated environment”. Generally, when we think of binder we think of moving data from the routing data and posted form values into the model. However, there is no restriction of where you find data for your model. The context of a web request is rich with information about the client. A good example is another Scott Hanselman post on automatically binding the user’s identity into a model see: IPrincipal (User) ModelBinder in ASP.NET MVC for easier testing.
In Conclusion
Model binding is beautiful magic, so take advantage of the built-in magic when you can. I think the topic of model binding could use it’s own dedicated web site. It would be a very boring web site with lots of boring code, but model binding has many subtleties. For instance, we never even got to the topic of culture in this post.
Do you have any model binding tips?
The May issue of MSDN Magazine is now on-line with my article “The Life and Times of an ASP.NET MVC Controller”.
In this article, I will dissect the ASP.NET MVC framework and look at how controllers work. I'll explain how the framework interacts with your controllers and how you can influence those interactions. I'll look at controller factories, controller actions, action filters, and action results as well.
I don’t like programming by coincidence, and I hope this article gives you everything you need to work confidently with MVC controllers. In addition to the technical details, I also believe you have to properly apply a technology to make great software – thus the next installment of Extreme ASP.NET in July will cover some guiding principles to keep in mind when developing applications with the ASP.NET MVC framework.
Looking even further down the road (September seems so far away), I’m not sure what I’ll dig into next. Will it be web forms? More ASP.NET MVC? If there is something you want to hear about that is related to the ASP.NET server-side runtime - drop me a line or leave a comment.
Luis and David recently posted about the controls that appear in the ASP.NET MVC Futures 1.0 release.
I’ve seen some discussions where people positively erupt at any mention of the word “control” in an MVC setting. These are the people who consider ASP.NET Web Forms as the ultimate source of evil in the universe – a cross between a Sith lord and a velociraptor. The idea of introducing controls, with un-testable event handling code and giant gobs of view state, is a travesty that will corrupt the minds of all developers before devouring their children. They must be stopped.
Let’s take the simple scenario of rendering a text input in the browser. If you are using the ASP.NET MVC framework release with no additional libraries, you’ll be looking to use one of the 4 Html helper TextBox methods (excerpted for space below):
string TextBox(... string name); string TextBox(... string name, object value); string TextBox(... string name, object value, IDictionary<string, object> htmlAttributes); string TextBox(... string name, object value, object htmlAttributes);
A text input is a pretty trivial control, but even these 4 options don’t give you a clean path to the entire universe of things you might want to do when displaying a value in a text input, like specifying a formatting string to use when converting the model value to a string. One solution is to write additional helper methods, or include an additional library with the methods already written, but you are only adding to the number of method overloads a developer has to parse when they just want to display a simple text input. In the end, you’ll still be looking at code similar to the following…
<%= Html.TextBox("ReleaseDate", String.Format("{0:d}", Model.ReleaseDate), new { @class = "special" })%>
…versus the declarative syntax of the MVC futures TextBox control…
<mvc:TextBox Name="ReleaseDate" Format="{0:d}" class="special" runat="server" />
Note that the MVC control does not inject view state into the rendered output, but that’s not to say that the MVC controls don’t have some issues. One issue is that they still inherit properties, behaviors, and events from System.Web.UI.Control, and some of those inherited features don’t make sense in an MVC view.
I think I'm still happier using Html helpers.
Here are some advantages I see to using controls:
And the disadvantages:
It’s also worth pointing out that there are other solutions, besides controls, to the often clumsy Html helpers. By sticking to view-specific models, conventions, and effective CSS with JavaScript, you can remove many of the concerns that the HtmlHelpers burden themselves with. Using a view engine other than the web forms view engine can also solve some of these issues.
What do you think?
Will MVC controls be the spawn of Satan, or the blessing of a saint?
Just a quick note to let you know that the first three modules of my ASP.NET MVC course are ready for consumption at Pluralsight! On-Demand. Pluralsight is building a fantastic library of online content you can view as a subscriber, or see in person via instructor led training at a classroom or on-site at your place.
There is also a heap of free screencasts and previews to wet your appetite.
A single subscription can get you access to training for WPF, C#, .NET, ASP.NET, AJAX, WF, WCF, Silverlight, BizTalk, with more on the way.
P.S. I’ve heard the LINQ class is awesome, too…
In the last post we talked about using entities as the models in an MVC application. This approach works well until an application reaches a certain level of complexity, at which point entities as the M in MVC can become painful.
I believe entities exist to serve the business layer. They also have a role in the data access layer, but only because the data access layer has to be acutely aware of entities. The reverse is not true – entities don’t need to know details about the data access layer nor how they are saved in persistent storage. Entities aren’t owned by the UI, or the data access layer. Entities are business objects and are owned by the business layer.
An application that supports a complex business needs a fine-tuned layer of business logic. You know an application is growing in complexity when you uncover scenarios like these:
These types of requirements have a significant impact on the design and behavior of the business layer and its entities. You want all this complicated business stuff in a layer where it is testable, maintainable, and free from the influence of all the infrastructure that surrounds it – databases, data grids, message queues, communication protocols, and the technology du jour. It’s a business layer built with business objects that encapsulate business state and business behavior. It’s the secret sauce that makes money for your company. It’s a rich model of your domain.
So - why wouldn’t we want to use this rich domain model inside of our views? Isn’t it every young view’s dream to grow up and marry an extraordinarily rich, fat model?
Views can grow complex just as quickly as business logic can grow complex. Complex views might exhibit some of the following conditions.
Perhaps you don’t consider these views as super-complex because you build them all the time, but the types of views we are talking about in the above three bullet points do place requirements and constraints on the model. For example, the UI may require a model that can serialize into a JSON or RSS format.
If you share your entities or domain model with the UI layer, you’ll find your business objects have to serve two masters. The requirements from these two masters will pull your business objects in different directions, and they won’t be optimized to fit in either role.
Also, ask yourself these questions about the model for your views:
Answers:
Complex applications often require multiple models. There is the domain model that encapsulates your company’s secret money-making business sauce, and then there are multiple view models that the UI layer consumes. Somewhere in between is logic that maps data between the various models. This isn’t a new idea, and it was at one time known as the Model Model View Controller pattern.
You might create a model class for each type of view, like a – MovieReviewViewModel, and perhaps all the UI models derive from a base class. The model classes will hold just the state that their respective views require. These classes don’t need behavior – they are essentially just data transfer objects. In a sense, the model classes also become contracts that explicitly describe what the controller needs to put together for the UI, and the view author sees a model with just the information they need.
Of course, building these additional models comes with a price.
It’s up to you to decide if the benefits are worth the price:
It was:
“I have created domain entities like Customer, Order, etc. They come out of repository classes backed by NHibernate. I’m wondering if I send them directly to the view or if I create a ViewModel class to hold data. I’m confused by all the terminology”.
I haven’t given a definitive answer, but I hope I’ve given you enough of my opinion to see that the answer depends on the complexity of your application and its long term goals. For forms-over-data applications, passing your entities can be a simple and effective solution. For more complex applications you may need models built specifically for your views to maintain the integrity and maintainability of your business objects.
OdeToCode by K. Scott Allen
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