ASP.NET CORE SubdomainRedirect

Startup.cs

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            // add this somewhere in your configure method
            app.UseRewriter(new RewriteOptions()
               .Add(new SubdomainRedirectRule())
               .AddRedirectToWwwPermanent()
              );

        }

SubdomainRedirectRule.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Rewrite;
using Microsoft.Net.Http.Headers;

namespace Redirect.library
{
    public class SubdomainRedirectRule : IRule
    {
        public void ApplyRule(RewriteContext context)
        {
            var request = context.HttpContext.Request;

            // if host contains m., redirect;
            var subdomain = GetSubDomain(request);


            if (!string.IsNullOrEmpty(subdomain) && subdomain == "m")
            {
                var response = context.HttpContext.Response;
                response.StatusCode = StatusCodes.Status301MovedPermanently;
                response.Headers[HeaderNames.Location] = "https://www.yourdomain.com" + request.PathBase + request.Path + request.QueryString;
                context.Result = RuleResult.EndResponse;
            }
        }

        private static string GetSubDomain(HttpRequest request)
        {
            string host = request.Host.Value;
            if (host.Split('.').Length > 1)
            {
                int index = host.IndexOf(".");
                return host.Substring(0, index);
            }
            return null;
        }
    }
}