这里有一个简单的约束，它在一个虚构的存储库中查找一个文章slug：

public class SlugRouteConstraint : IRouteConstraint
{
    private readonly ISlugRepository slugRepository = new SlugRepository();

    public bool Match(HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values, RouteDirection routeDirection)
    {
        if (!values.TryGetValue(parameterName, out object slug))
            return false;

        return slugRepository.Exists(slug as string);
    }
}

可以像这样关联约束：

routes.MapRoute("Slugs", "{slug}",
    new { controller = "Articles", action = "View" },
    new { slug = new SlugConstraint() });