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:
The above code continues to work - although now you can also
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:
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
This will then render the appropriate drop down with items
and selection for us at runtime:
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
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:
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.
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:
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:
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):
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.