全世界只有我遇到？ASP.NET Core localhost 動態 Port 繫結錯誤

用 Visual Studio 建了一個 ASP.NET Core 專案跑不起來，試了用 dotnet new 新建也是如此，錯誤訊如下：

X:\MyTest\EmptyWeb>dotnet run
Building...
Unhandled exception. System.InvalidOperationException: Dynamic port binding is not supported when binding to localhost. You must either bind to 127.0.0.1:0 or [::1]:0, or both.
   at Microsoft.AspNetCore.Server.Kestrel.Core.LocalhostListenOptions..ctor(Int32 port)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.ParseAddress(String address, Boolean& https)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.AddressesStrategy.BindAsync(AddressBindContext context, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.AddressBinder.BindAsync(IEnumerable`1 listenOptions, AddressBindContext context, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.BindAsync(CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Server.Kestrel.Core.KestrelServerImpl.StartAsync[TContext](IHttpApplication`1 application, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Hosting.GenericWebHostService.StartAsync(CancellationToken cancellationToken)
   at Microsoft.Extensions.Hosting.Internal.Host.StartAsync(CancellationToken cancellationToken)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
   at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.Run(IHost host)
   at Microsoft.AspNetCore.Builder.WebApplication.Run(String url)
   at Program.<Main>$(String[] args) in X:\MyTest\EmptyWeb\Program.cs:line 6

Dynamic port binding is not supported when binding to localhost. You must either bind to 127.0.0.1:0 or [::1]:0, or both. 錯誤訊息無比明確，但 Google 結果只有一筆，還是 Github 某個 Kooboo.HttpServer 專案原始碼中的錯誤訊息定義：

第一次遇到這種詭異狀況，難道全世界只有我一個人遇到這個問題？

查詢 ASP.NET Core 原始碼，在 aspnetcore/src/Servers/Kestrel/Core/src/CoreStrings.resx 找到 DynamicPortOnLocalhostNotSupported 定義相同訊息的字串(筆記：所以 Google 不是所有公開連結都搜得到)，再進一步追到內部物件 LocalhostListenOptions 會在傳入 localhost:0 時噴錯：

internal sealed class LocalhostListenOptions : ListenOptions
{
    internal LocalhostListenOptions(int port)
        : base(new IPEndPoint(IPAddress.Loopback, port))
    {
        if (port == 0)
        {
            throw new InvalidOperationException(CoreStrings.DynamicPortOnLocalhostNotSupported);
        }
    }
   //...略....

回頭檢查 EmptyWeb\Properties\launchSettings.json 果真出現 ℎttp://localhost:0! 比對正常專案應該要類似 "applicationUrl": "ℎttps://localhost:7238;ℎttp://localhost:5000"，為什麼會出現 localhost:0 是個謎：

接著一陣胡亂操作，更新 Visual Studio 版本初試沒用，但再重開機問題便消失。由於無法重現錯誤，也就無法再調查，筆記備忘。

至於為什麼 localhost 不支援動態 Port，但 127.0.0.1、::1 可以？我猜測是因為 localhost 同時代表 127.0.0.1 跟 ::1，各自的可用 Port 不同，難以動態決定 Port (理論上還是可能，但複雜度較高)。在 AddressBinder.ParseAddress 有一段註解，就當成佐證吧：

        else if (string.Equals(parsedAddress.Host, "localhost", StringComparison.OrdinalIgnoreCase))
        {
            // "localhost" for both IPv4 and IPv6 can't be represented as an IPEndPoint.
            options = new LocalhostListenOptions(parsedAddress.Port);
        }