using mvc route constraints so a url can only be mapped to one of three possible params

Question

Here is my route:

routes.MapRoute(null, "myaccount/monitor/{category}", // Matches
                new { controller = "MyAccount", action = "Monitor", category = (string)null }
);

I would like to add a constraint so the category can only match a null or one of three params (i.e. overview, projection, history)

Answer 1

You can use UrlParameter.Optional to allow nulls, and also use the constraints parameter of the MapRoute method..

 routes.MapRoute(null,
                      "myaccount/monitor/{category}", // Matches
                      new { controller = "MyAccount", action = "Monitor", category = UrlParameter.Optional  },
                      new { category = "overview|projection|history"}
            );

Answer 2

The inline Regex Gaby posted would work. Another way to do it is to define a custom IRouteConstraint:

public class FromValuesListConstraint : IRouteConstraint
{
    private List<string> _values;

    public FromValuesListConstraint(params string[] values)
    {
        this._values = values.Select(x => x.ToLower()).ToList();
    }

    public bool Match(HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values, RouteDirection routeDirection)
    {
        string value = values[parameterName].ToString();

        if (string.IsNullOrWhiteSpace(value))
        {
            return _values.Contains(string.Empty);
        }

        return _values.Contains(value.ToLower());
    }
}

Then pass in an instance of the constraint into your MapRoute call:

routes.MapRoute(null,
    "myaccount/monitor/{category}", // Matches
new { controller = "MyAccount", action = "Monitor", category = UrlParameter.Optional  },
new { category = new FromValuesListConstraint ("overview", "projection", "history", string.Empty) }
        );