fixed core 3.1 startup and verified it still works with 2.1 and net461

Source/MQTTnet.AspnetCore/Client/Tcp/TcpConnection.cs

@@ -40,8 +40,11 @@ namespace MQTTnet.AspNetCore.Client.Tcp
             _sender = new SocketSender(_socket, PipeScheduler.ThreadPool);
             _receiver = new SocketReceiver(_socket, PipeScheduler.ThreadPool);
         }
-
+#if NETCOREAPP3_1
+        public override ValueTask DisposeAsync()
+#else
         public Task DisposeAsync()
+#endif
         {
             IsConnected = false;
 
@@ -50,8 +53,15 @@ namespace MQTTnet.AspNetCore.Client.Tcp
 
             _socket?.Dispose();
 
+#if NETCOREAPP3_1
+
+            return base.DisposeAsync();
+        }
+#else
+
             return Task.CompletedTask;
         }
+#endif
 
         public async Task StartAsync()
         {

Source/MQTTnet.AspnetCore/Extensions/ApplicationBuilderExtensions.cs

@@ -1,14 +1,13 @@
 using System;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
-using System.Linq;
 using MQTTnet.Server;
-using System.Collections.Generic;
 
 namespace MQTTnet.AspNetCore
 {
     public static class ApplicationBuilderExtensions
     {
+        [Obsolete("This class is obsolete and will be removed in a future version. The recommended alternative is to use MapMqtt inside Microsoft.AspNetCore.Builder.UseEndpoints(...).")]
         public static IApplicationBuilder UseMqttEndpoint(this IApplicationBuilder app, string path = "/mqtt")
         {
             app.UseWebSockets();
@@ -24,7 +23,7 @@ namespace MQTTnet.AspNetCore
 
                 if (context.Request.Headers.TryGetValue("Sec-WebSocket-Protocol", out var requestedSubProtocolValues))
                 {
-                    subProtocol = SelectSubProtocol(requestedSubProtocolValues);
+                    subProtocol = MqttSubProtocolSelector.SelectSubProtocol(requestedSubProtocolValues);
                 }
 
                 var adapter = app.ApplicationServices.GetRequiredService<MqttWebSocketServerAdapter>();
@@ -37,14 +36,6 @@ namespace MQTTnet.AspNetCore
             return app;
         }
 
-        public static string SelectSubProtocol(IList<string> requestedSubProtocolValues)
-        {
-            // Order the protocols to also match "mqtt", "mqttv-3.1", "mqttv-3.11" etc.
-            return requestedSubProtocolValues
-                .OrderByDescending(p => p.Length)
-                .FirstOrDefault(p => p.ToLower().StartsWith("mqtt"));
-        }
-
         public static IApplicationBuilder UseMqttServer(this IApplicationBuilder app, Action<IMqttServer> configure)
         {
             var server = app.ApplicationServices.GetRequiredService<IMqttServer>();

Source/MQTTnet.AspnetCore/Extensions/ConnectionRouteBuilderExtensions.cs

@@ -0,0 +1,20 @@
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Connections;
+using System;
+
+namespace MQTTnet.AspNetCore
+{
+    public static class ConnectionRouteBuilderExtensions
+    {
+#if NETCOREAPP3_1
+        [Obsolete("This class is obsolete and will be removed in a future version. The recommended alternative is to use MapMqtt inside Microsoft.AspNetCore.Builder.UseEndpoints(...).")]
+#endif
+        public static void MapMqtt(this ConnectionsRouteBuilder connection, PathString path)
+        {
+            connection.MapConnectionHandler<MqttConnectionHandler>(path, options =>
+            {
+                options.WebSockets.SubProtocolSelector = MqttSubProtocolSelector.SelectSubProtocol;
+            });
+        }
+    }
+}

Source/MQTTnet.AspnetCore/Extensions/EndpointRouterExtensions.cs

@@ -0,0 +1,21 @@
+
+#if NETCOREAPP3_1
+
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Routing;
+
+namespace MQTTnet.AspNetCore
+{
+    public static class EndpointRouterExtensions
+    {
+        public static void MapMqtt(this IEndpointRouteBuilder endpoints, string pattern) 
+        {
+            endpoints.MapConnectionHandler<MqttConnectionHandler>(pattern, options =>
+            {
+                options.WebSockets.SubProtocolSelector = MqttSubProtocolSelector.SelectSubProtocol;
+            });
+        }
+    }
+}
+
+#endif

Source/MQTTnet.AspnetCore/Extensions/ServiceCollectionExtensions.cs

@@ -1,10 +1,10 @@
 using System;
-using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
 using MQTTnet.Server;
 using MQTTnet.Implementations;
+using Microsoft.Extensions.DependencyInjection;
 
 namespace MQTTnet.AspNetCore
 {

Source/MQTTnet.AspnetCore/MqttHostedServer.cs

@@ -21,7 +21,8 @@ namespace MQTTnet.AspNetCore
 
         public Task StartAsync(CancellationToken cancellationToken)
         {
-            return StartAsync(_options);
+            _ = StartAsync(_options);
+            return Task.CompletedTask;
         }
 
         public Task StopAsync(CancellationToken cancellationToken)

Source/MQTTnet.AspnetCore/MqttSubProtocolSelector.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace MQTTnet.AspNetCore
+{
+    public static class MqttSubProtocolSelector
+    {
+        public static string SelectSubProtocol(IList<string> requestedSubProtocolValues)
+        {
+            // Order the protocols to also match "mqtt", "mqttv-3.1", "mqttv-3.11" etc.
+            return requestedSubProtocolValues
+                .OrderByDescending(p => p.Length)
+                .FirstOrDefault(p => p.ToLower().StartsWith("mqtt"));
+        }
+    }
+}