Fix ignored exceptions in MqttHostedServer startup

Source/MQTTnet.AspnetCore/MQTTnet.AspNetCore.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
         <TargetFramework>net8.0</TargetFramework>

Source/MQTTnet.AspnetCore/MqttHostedServer.cs

@@ -12,37 +12,26 @@
 
 namespace MQTTnet.AspNetCore;
 
-public sealed class MqttHostedServer : MqttServer, IHostedService
+public sealed class MqttHostedServer : BackgroundService
 {
-    readonly IHostApplicationLifetime _hostApplicationLifetime;
     readonly MqttServerFactory _mqttFactory;
-
     public MqttHostedServer(
-        IHostApplicationLifetime hostApplicationLifetime,
         MqttServerFactory mqttFactory,
         MqttServerOptions options,
         IEnumerable<IMqttServerAdapter> adapters,
-        IMqttNetLogger logger) : base(options, adapters, logger)
+        IMqttNetLogger logger
+        )
     {
+        MqttServer = new(options, adapters, logger);
         _mqttFactory = mqttFactory ?? throw new ArgumentNullException(nameof(mqttFactory));
-        _hostApplicationLifetime = hostApplicationLifetime;
-    }
-
-    public async Task StartAsync(CancellationToken cancellationToken)
-    {
-        // The yield makes sure that the hosted service is considered up and running.
-        await Task.Yield();
-
-        _hostApplicationLifetime.ApplicationStarted.Register(OnStarted);
-    }
-
-    public Task StopAsync(CancellationToken cancellationToken)
-    {
-        return StopAsync(_mqttFactory.CreateMqttServerStopOptionsBuilder().Build());
     }
 
-    void OnStarted()
+    public MqttServer MqttServer { get; }
+    protected override Task ExecuteAsync(CancellationToken stoppingToken)
+        => MqttServer.StartAsync();
+    public override async Task StopAsync(CancellationToken cancellationToken)
     {
-        _ = StartAsync();
+        await MqttServer.StopAsync(_mqttFactory.CreateMqttServerStopOptionsBuilder().Build());
+        await base.StopAsync(cancellationToken);
     }
-}
+}

Source/MQTTnet.AspnetCore/ServiceCollectionExtensions.cs

@@ -45,8 +45,9 @@ public static void AddHostedMqttServer(this IServiceCollection services)
         services.TryAddSingleton(new MqttServerFactory());
 
         services.AddSingleton<MqttHostedServer>();
-        services.AddSingleton<IHostedService>(s => s.GetService<MqttHostedServer>());
-        services.AddSingleton<MqttServer>(s => s.GetService<MqttHostedServer>());
+        services.AddHostedService(s => s.GetService<MqttHostedServer>());
+        services.AddSingleton(s => s.GetService<MqttHostedServer>().MqttServer);
+
     }
 
     public static IServiceCollection AddHostedMqttServerWithServices(this IServiceCollection services, Action<AspNetMqttServerOptionsBuilder> configure)

Source/MQTTnet.Tests/ASP/MqttHostedServerStartup.cs

@@ -0,0 +1,66 @@
+using Microsoft.AspNetCore.Builder;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.AspNetCore;
+using System.Net.Sockets;
+using System.Net;
+using System.Threading.Tasks;
+using Microsoft.Extensions.DependencyInjection;
+using MQTTnet.Server;
+using Microsoft.AspNetCore.Hosting;
+
+namespace MQTTnet.Tests.ASP
+{
+    [TestClass]
+    public class MqttHostedServerStartup
+    {
+        private async Task TestStartup(bool useOccupiedPort)
+        {
+            using TcpListener l = new TcpListener(IPAddress.Any, 0);
+            l.Start();
+            int port = ((IPEndPoint)l.LocalEndpoint).Port;
+
+            if(!useOccupiedPort)
+                l.Stop();
+
+
+            var builder = WebApplication.CreateBuilder();
+            builder.WebHost.UseUrls("http://127.0.0.1:0");
+
+            builder.Services.AddMqttTcpServerAdapter();
+            builder.Services.AddHostedMqttServer(cfg =>
+            {
+                cfg
+                    .WithDefaultEndpoint()
+                    .WithDefaultEndpointPort(port);
+            });
+
+
+            var app = builder.Build();
+
+            if(!useOccupiedPort)
+            {
+                await app.StartAsync();
+                var server = app.Services.GetRequiredService<MqttServer>();
+                Assert.IsTrue(server.IsStarted);
+                await app.StopAsync();
+            }
+            else
+            {
+                await Assert.ThrowsExceptionAsync<SocketException>(() =>
+                    app.StartAsync()
+                    );
+            }
+        }
+
+        [TestMethod]
+        [DoNotParallelize]
+        public Task TestSuccessfullyStartup()
+            => TestStartup(useOccupiedPort: false);
+
+        [TestMethod]
+        [DoNotParallelize]
+        public Task TestFailedStartup()
+            => TestStartup(useOccupiedPort: true);
+
+    }
+}