ContentType route constraint in ASP.NET MVC

Update: The functionality of this constraint is incorrect, and should use AcceptTypes, rather than ContentType. I will write an update to it.

The kudos for this goes to ‘Daniel T‘ who wrote the original JSON implementation.

I was helping out on StackOverflow, and I came across a question which asked why a users custom route constraint didn’t work. Their implementation was for JSON only, but there’s no reason why it couldn’t be extended further to encompass any ContentType.

And that’s what I’ve done…

First let’s the ConstraintContentType enum and add a few example MIME types into the description attributes:

  public enum ConstraintContentType
  {
    [Description("text/xml")]
    XML,
    [Description("application/json")]
    JSON,
    [Description("text/html")]
    HTML
  }

Now, let’s create something so that we can get the description attribute:

  public static class Enum
  {
    public static string Description(T value)
    {
      DescriptionAttribute[] da = (DescriptionAttribute[])(typeof(T).GetField(value.ToString()).GetCustomAttributes(typeof(DescriptionAttribute), false));
      return da.Length > 0 ? da[0].Description : value.ToString();
    }
  }

And now the ContentTypeConstraint:

namespace MyWebsite
{
  public class ContentTypeConstraint : IRouteConstraint
  {
    private ConstraintContentType contentType;

    public ContentTypeConstraint(ConstraintContentType constraintType)
    {
      contentType = constraintType;
    }

    public bool Match(HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values, RouteDirection routeDirection)
    {
      if (routeDirection == RouteDirection.UrlGeneration)
        return true;

        return httpContext.Request.ContentType == Enum.Description(contentType);
    }
  }
}

So how do you use it?

Well…

  routes.MapRoute(
    "JsonOnlyRoute",
    "json/{controller}/{action}/{id}",
    new { controller = "Home", action = "Index" },
    new { contentType = new ContentTypeConstraint(ConstraintContentType.JSON) }
  );

It’s that simple! Now the only content type that matches this route is JSON. Of course, you could easily expand this to include a whitelist/blacklist approach, and also add in more MIME types, but that’s just another reason why the ASP.NET MVC routing system is so great – it’s so extensible!

Tags: asp.net mvc, routeconstraint