How to determine which port ASP.NET Core 2 is listening on when a dynamic port (0) was specified

Question

I've got an ASP.NET Core 2.0 app which I intend to run as a stand-alone application. The app should start up and bind to an available port. To achieve this I configure the WebHostBuilder to listen on "http://127.0.0.1:0" and to use the Kestrel server. Once the web host starts listening I want to save the url with the actual port in a file. I want to do this as early as possible, since another application will read the file to interact with my app.

How can I determine the port the web host is listening on?

Answer 1

You can use the Start() method instead of Run() to access IServerAddressesFeature at the right moment:

IWebHost webHost = new WebHostBuilder()
    .UseKestrel(options => 
         options.Listen(IPAddress.Loopback, 0)) // dynamic port
    .Build();

webHost.Start();

string address = webHost.ServerFeatures
    .Get<IServerAddressesFeature>()
    .Addresses
    .First();
int port = int.Parse(address.Split(':').Last());

webHost.WaitForShutdown();