Squashed commit of the following:

commit 2d94abdde3
Author: Christian <christian.kratky@googlemail.com>
Date:   Sat Nov 13 10:09:59 2021 +0100

    Update MQTTnet.nuspec

    Update docs.

commit f0ea13c07f
Author: logicaloud <47704763+logicaloud@users.noreply.github.com>
Date:   Sat Nov 13 22:05:54 2021 +1300

    Fix MQTT 3.1.1 session persistence when CleanSession=1 (#1294)

commit 47cd5aa3cd
Author: Volker Milbrandt <milbrandt@users.noreply.github.com>
Date:   Sat Nov 13 10:02:55 2021 +0100

    test ResultCode for invalid credentials (no userName but password) (#1270)

    fix order of parameters in exceptions AreEqual(expected, actual)

commit 258df6bb14
Author: HansM <HansM2013@gmx.de>
Date:   Thu Nov 11 20:10:35 2021 +0100

    Removed IDisposable from MqttClientConnection class. (#1288)

commit 85c3c0b352
Author: SeppPenner <HansM2013@gmx.de>
Date:   Mon Nov 8 08:50:59 2021 +0100

    MQTTnet.noUWP.sln: Removed no longer present MQTTnet.Server project.

Tests/MQTTnet.Core.Tests/Server/General.cs

@@ -1021,7 +1021,7 @@ namespace MQTTnet.Tests.Server
                 var server = await testEnvironment.StartServer(new MqttServerOptionsBuilder().WithPersistentSessions());
 
                 // Create the session including the subscription.
-                var client1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("a"));
+                var client1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("a").WithCleanSession(false));
                 await client1.SubscribeAsync("x");
                 await client1.DisconnectAsync();
                 await Task.Delay(500);
@@ -1029,7 +1029,7 @@ namespace MQTTnet.Tests.Server
                 var clientStatus = await server.GetClientsAsync();
                 Assert.AreEqual(0, clientStatus.Count);
 
-                var client2 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("b"));
+                var client2 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("b").WithCleanSession(false));
                 await client2.PublishAsync("x", "1");
                 await client2.PublishAsync("x", "2");
                 await client2.PublishAsync("x", "3");

Tests/MQTTnet.Core.Tests/Server/Security_Tests.cs

@@ -1,4 +1,4 @@
-using System.Threading.Tasks;
+using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Adapter;
 using MQTTnet.Client;
@@ -32,6 +32,7 @@ namespace MQTTnet.Tests.Server
                 var ex = await Assert.ThrowsExceptionAsync<MqttConnectingFailedException>(async () => await client.ConnectAsync(clientOptions));
                 Assert.IsInstanceOfType(ex.InnerException, typeof(MqttProtocolViolationException));
                 Assert.AreEqual("Error while authenticating. If the User Name Flag is set to 0, the Password Flag MUST be set to 0 [MQTT-3.1.2-22].", ex.Message, false);
+                Assert.AreEqual(MqttClientConnectResultCode.UnspecifiedError, ex.ResultCode);
             }
         }
 
@@ -53,7 +54,7 @@ namespace MQTTnet.Tests.Server
             return TestCredentials("x", "x");
         }
 
-        async Task TestCredentials(string userName, string password)
+        private async Task TestCredentials(string userName, string password)
         {
             using (var testEnvironment = CreateTestEnvironment())
             {
@@ -84,7 +85,7 @@ namespace MQTTnet.Tests.Server
                     .Build();
 
                 var ex = await Assert.ThrowsExceptionAsync<MqttConnectingFailedException>(async () => await client.ConnectAsync(clientOptions));
-                Assert.AreEqual(ex.Result.ResultCode, MqttClientConnectResultCode.BadUserNameOrPassword);
+                Assert.AreEqual(MqttClientConnectResultCode.BadUserNameOrPassword, ex.Result.ResultCode);
             }
         }
     }

Tests/MQTTnet.Core.Tests/Server/Session_Tests.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
@@ -213,6 +212,58 @@ namespace MQTTnet.Tests.Server
             }
         }
 
+        [TestMethod]
+        public async Task Clean_Session_Persistence()
+        {
+            using (var testEnvironment = new TestEnvironment(TestContext))
+            {
+                // Create server with persistent sessions enabled
+
+                await testEnvironment.StartServer(o => o.WithPersistentSessions());
+
+                const string ClientId = "Client1";
+
+                // Create client with clean session and long session expiry interval
+
+                var client1 = await testEnvironment.ConnectClient(o => o
+                    .WithProtocolVersion(Formatter.MqttProtocolVersion.V311)
+                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                    .WithSessionExpiryInterval(9999) // not relevant for v311 but testing impact
+                    .WithCleanSession(true) // start and end with clean session
+                    .WithClientId(ClientId)
+                    .Build()
+                );
+
+                // Disconnect; empty session should be removed from server
+
+                await client1.DisconnectAsync();
+
+                // Simulate some time delay between connections
+
+                await Task.Delay(1000);
+
+                // Reconnect the same client ID without clean session
+
+                var client2 = testEnvironment.CreateClient();
+                var options = testEnvironment.Factory.CreateClientOptionsBuilder()
+                    .WithProtocolVersion(Formatter.MqttProtocolVersion.V311)
+                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                    .WithSessionExpiryInterval(9999) // not relevant for v311 but testing impact
+                    .WithCleanSession(false) // see if there is a session
+                    .WithClientId(ClientId)
+                    .Build();
+
+
+                var result = await client2.ConnectAsync(options).ConfigureAwait(false);
+
+                await client2.DisconnectAsync();
+
+                // Session should NOT be present for MQTT v311 and initial CleanSession == true
+
+                Assert.IsTrue(!result.IsSessionPresent, "Session present");
+            }
+        }
+
         async Task<IMqttClient> TryConnect(TestEnvironment testEnvironment, MqttClientOptionsBuilder options)
         {
             try

Tests/MQTTnet.Core.Tests/Server/Status_Tests.cs

@@ -1,4 +1,4 @@
-using System.Linq;
+using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
@@ -80,8 +80,8 @@ namespace MQTTnet.Tests.Server
             {
                 var server = await testEnvironment.StartServer(new MqttServerOptionsBuilder().WithPersistentSessions());
 
-                var c1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("client1"));
-                var c2 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("client2"));
+                var c1 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("client1").WithCleanSession(false));
+                var c2 = await testEnvironment.ConnectClient(new MqttClientOptionsBuilder().WithClientId("client2").WithCleanSession(false));
 
                 await c1.DisconnectAsync();