Add custom query parameter to action URL in ASP.NET Core MVC

Question

In ASP.NET Core MVC, I'd like to make it so that URLs created with Url.Action and action-based tag helpers include a custom query parameter in the URL. I want to apply this globally, regardless of the controller or action.

I tried overriding the default route handler, which worked at one time, but broke with an ASP.NET Core update. What am I doing wrong? Is there a better way?

Answer 1

Try adding it to the collection instead of overriding the DefaultHandler. The following worked for me on version 1.1.2:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    // ... other configuration
    app.UseMvc(routes =>
    {
        routes.Routes.Add(new HostPropagationRouter(routes.DefaultHandler));
        routes.MapRoute(
            name: "default",
            template: "{controller=Home}/{action=Index}/{id?}");
    });
    // ... other configuration
}

Here's the router, just for completeness.

public class HostPropagationRouter : IRouter
{
    readonly IRouter router;

    public HostPropagationRouter(IRouter router)
    {
        this.router = router;
    }

    public VirtualPathData GetVirtualPath(VirtualPathContext context)
    {
        if (context.HttpContext.Request.Query.TryGetValue("host", out var host))
            context.Values["host"] = host;
        return router.GetVirtualPath(context);
    }

    public Task RouteAsync(RouteContext context) => router.RouteAsync(context);
}