ASP.NET MVC Preview 3 Release
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by Scott Guthrie
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Improved HTML Helper Methods

The HTML helper methods have been updated with ASP.NET MVC Preview 3.  In addition to a bunch of bug fixes, they also include a number of nice usability improvements.

Automatic Value Lookup

With previous preview releases you needed to always explicitly pass in the value to render when calling the Html helpers.  For example: to include a value within a <input type="text" value="some value"/> element you would write:

Figure 7

The above code continues to work - although now you can also just write:

Figure 8

The HTML helpers will now by default check both the ViewData dictionary and any Model object passed to the view for a ProductName key or property value to use.

SelectList and MultiSelectList ViewModels

New SelectList and MultiSelectList View-Model classes are now included that provide a cleaner way to populate HTML dropdowns and multi-select listboxes (and manage things like current selection, etc).  One approach that can make form scenarios cleaner is to instantiate and setup these View-Model objects in a controller action, and then pass them in the ViewData dictionary to the View to format/render. 

For example, below I'm creating a SelectList view-model class over the set of unique category objects in our database.  I'm indicating that I want to use the "CategoryID" property as the value of each item in the list, and the "CategoryName" as the display text.  I'm also setting the list selection to the current CategoryId of the Product we are editing:

Figure 9

Within our view we then just have to write the below code to indicate that we want to create a drop-downlist against the SelectList we put into ViewData:

Figure 10

This will then render the appropriate drop down with items and selection for us at runtime:

Figure 11

Figure 12

Built-in error validation support isn't included with our HTML helpers yet (you currently need to write code for this) - but will show up in the future, which will make form editing scenarios even easier.

You'll also start to see ASP.NET AJAX helper methods show up in future preview releases as well, which will make it easier to integrate AJAX into MVC applications with a minimum of code.

URL Routing Improvements

ASP.NET MVC Preview 3 includes a number of improvements to the URL routing system.  URL routing is one of the most "fundamental" components of a web MVC framework to get right, hence the reason we've spent a lot of focus the first few previews getting this area nailed.  Our new URL routing engine will ship in .NET 3.5 SP1 this summer, and will support both Web Forms and MVC requests.  ASP.NET MVC will be able to use the built-in .NET 3.5 SP1 routing engine when running on .NET 3.5 SP1. ASP.NET MVC will also include its own copy of the assembly so that it can also work on non-SP1 systems.

Some of the URL Routing Improvements in the Preview 3 release include:

MapRoute() and IgnoreRoute() helper methods

ASP.NET MVC Preview 3 includes new "MapRoute" and "IgnoreRoute" helper methods that you can use to more easily register routing rules.  MapRoute() provides an easy way to add a new MVC Route rule to the Routes collection.  IgnoreRoute() provides an easy way to tell the URL routing system to stop processing certain URL patterns (for example: handler .axd resources in ASP.NET that are used to serve up JavaScript, images, etc). 

Below is an example of the default RegisterRoutes() method within Global.asax when you create a new ASP.NET MVC project where you can see both of these new helper methods in action.

Figure 13

The MapRoute() helper method is overloaded and takes two, three or four parameters (route name, URL syntax, URL parameter default, and optional URL parameter regular expression constraints). 

You can call MapRoute() as many times as you want to register multiple named routes in the system.  For example, in addition to the default convention rule, we could add a "Products-Browse" named routing rule like below:

Figure 14

We can then refer to this "Products-Browse" rule explicitly within our Controllers and Views when we want to generate a URL to it.  For example, we could use the Html.RouteLink view helper to indicate that we want to link to our "Products-Browse" route and pass it a "Food" category parameter using code in our view template like below:

Figure 15

This view helper would then access the routing system and output an appropriate HTML hyperlink URL like below (note: how it did automatic parameter substitution of the category parameter into the URL using the route rule):

Figure 16

We could alternatively use the new Url.RouteUrl(routeName, values) within views if we wanted to just retrieve the URL for a named route (and not output the <a> html element). 

We could also use the new RedirectToRoute(routeName, values) helper method on the Controller base class to issues browser redirects based on named routing rules. 

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