The default URL routing setup in an ASP.NET application (MVC or web forms), using the System.Web.Routing framework, is very forgiving. For instance, it does not care about upper vs lower case, about trailing slashes or host names. This can be a problem if you care about search engine optimization (SEO), as the relaxed URL handling yields duplicate content.

Canonicalize complements the ASP.NET routing engine with a configurable CanonicalizeRoute, which will handle requests to non-canonical URLs by permanently redirecting (HTTP 301) to the canonical URL.

Install the package via NuGet:

Install-Package Canonicalize

In your route registration (typically in Global.asax.cs) add the following, before any existing registrations:

routes.Canonicalize().Www().Lowercase().NoTrailingSlash();

The line above adds a CanonicalizeRoute to your routing table and adds three canonicalization strategies to its configuration. Remove any strategy you do not require, and use IntelliSense to explore what other strategies are available or consult the list below.

• Lowercase will ensure that all characters in the path are lowercase.
• Www / NoWww will redirect from example.com to www.example.com or vice versa.
• TrailingSlash / NoTrailingSlash will add or remove a trailing slash from the path.
• Host will redirect all requests to a specific DNS host name or IP address.
• Map will lookup the path in a dictionary to find a new path.
• Pattern performs a search/replace on the path against a regular expression.

Defining your own URL canonicalization strategies requires you to implement the one-method IUrlStrategy interface. Then use the more verbose route setup syntax:

routes.Add(new CanonicalizeRoute(new LowercaseStrategy(), new CustomStrategy()));

In order to enable fluent configuration with your new strategy, you must also add an extension method to CanonicalizeRouteBuilder. If you find that your strategy might be useful for others, consider sending in a patch.

While Canonicalize ensures that all incoming requests matches your applications URL scheme, the responsibility to output internal links in their canonical form is left with you. The AppendTrailingSlash and LowercaseUrls options – introduced in .NET 4.5 – can be useful in this regard.

Canonicalize is released under the MIT license.