MVC Route Constraint to Exclude Values

For Album Credits I wanted to allow personalized urls of the format http://albumcredits.com/yournamehere.  This turned out to be quite an interesting routing exercise.

Since this is an MVC app, our standard url format is of the usual http://albumcredits.com/{controller}/{action}/{index} kind, and for some pages, I need to allow the url to simply specify the controller, defaulting the action to index – again, the usual ASP.NET MVC pattern.

I was familiar with the constraint parameter option for the AddRoute method, but had never studied it in much detail – we’d used it to limit certain indexes to be numeric, but that was all.  For the root-level personalized urls we needed a more robust constraint – specifically we needed to exclude any controller from the list of valid personalized Urls.

I first spent more time than I cared to trying to come up with a regular expression pattern that would NOT match the list of controller names – it looked something like this:
^(?:(?!\b(foo|bar)\b).)*$
(thanks to Justin Poliey/stackoverflow.com) where foo and bar, etc were the controller names to NOT match.

Not until after I got that to work did I think to google “mvc custom route constraint”.  Of course the MS MVC team was smarter than that – custom route constraints are really very straight forward…

For my purposes, I went with David Hayden’s approach – the code below is essentially the same as his, just with the logic reversed.

using System;
using System.Linq;
using System.Web;
using System.Web.Routing;

namespace AlbumCredits.Web
{
	/// <summary>
	/// Route constraint that returns true if the parameter value is not one of the excluded values.
	/// </summary>
	/// <example>A controller constraint like 
	/// <code>new { controller = new ExcludeValuesConstraint("foo", "bar") }</code>
	/// will match "blah" or "snort" but will not match "foo" or "bar".
	/// </example>
	public class ExcludeValuesConstraint : IRouteConstraint
	{
		private readonly string[] _excludeValues;
		/// <summary>
		/// Initializes a new instance of the <see cref="ExcludeValuesConstraint"/> class.
		/// Example: <code>new { controller = new ExcludeValuesConstraint("foo", "bar") }</code>
		/// will match "blah" or "snort" but will not match "foo" or "bar".
		/// </summary>
		/// <param name="excludeValues">The excluded values.</param>
		public ExcludeValuesConstraint(params string[] excludeValues)
		{
			_excludeValues = excludeValues;
		}

		/// <summary>
		/// Determines whether the URL parameter contains a valid value for this constraint.
		/// </summary>
		/// <param name="httpContext">An object that encapsulates information about the HTTP request.</param>
		/// <param name="route">The object that this constraint belongs to.</param>
		/// <param name="parameterName">The name of the parameter that is being checked.</param>
		/// <param name="values">An object that contains the parameters for the URL.</param>
		/// <param name="routeDirection">An object that indicates whether the constraint check is being performed when an incoming request is being handled or when a URL is being generated.</param>
		/// <returns>
		/// true if the URL parameter contains a valid value; otherwise, false.
		/// </returns>
		public bool Match(HttpContextBase httpContext, Route route, string parameterName, 
			RouteValueDictionary values, RouteDirection routeDirection)
		{
			return !(_excludeValues.Contains(values[parameterName].ToString(), StringComparer.InvariantCultureIgnoreCase));
		}
	}
}

I can now use this when setting up my Route Table like this:

	routes.MapRoute("PersonalizedUrl",
		/* for urls like  */ "{personalizedUrl}",
		/* route defaults */ new { controller = MVC.Profile.Name, action = MVC.Profile.Actions.IndexByPersonalizedUrl, personalizedUrl = string.Empty },
		/* where          */ new { personalizedUrl = new ExcludeValuesConstraint(ControllerNameArray) }
	);