How do I use a custom constraint with a HttpMethodConstraint in ASP.NET MVC routing?

Question

I have a controller that only accepts a POST on this URL:

POST http://server/stores/123/products

The POST should be of content-type application/json, so this is what I have in my routing table:

routes.MapRoute(null,
                "stores/{storeId}/products",
                new { controller = "Store", action = "Save" },
                new {
                      httpMethod = new HttpMethodConstraint("POST"),
                      json = new JsonConstraint()
                    }
               );

Where JsonConstraint is:

public class JsonConstraint : IRouteConstraint
{
    public bool Match(HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values, RouteDirection routeDirection)
    {
        return httpContext.Request.ContentType == "application/json";
    }
}

When I use the route, I get a 405 Forbidden:

The HTTP verb POST used to access path '/stores/123/products' is not allowed

However, if I remove the json = new JsonConstraint() constraint, it works fine. Does anybody know what I'm doing wrong?

Answer 1

I'd put this in a comment but there isn't enough space.

When writing a custom constraint it is very important to inspect the routeDirection parameter and make sure that your logic runs only at the right time.

That parameter tells you whether your constraint is being run while processing an incoming request or being run while someone is generating a URL (such as when they call Html.ActionLink).

In your case I think you want to put all your matching code in a giant "if":

public bool Match(HttpContextBase httpContext, Route route,
    string parameterName, RouteValueDictionary values,
    RouteDirection routeDirection) 
{
    if (routeDirection == RouteDirection.IncomingRequest) {
        // Only check the content type for incoming requests
        return httpContext.Request.ContentType == mimeType; 
    }
    else {
        // Always match when generating URLs
        return true;
    }
}