Added certificate validation callback for .NET framework 4.x (#1806)

* Added certificate validation callback for .NET framework 4.x

* Update ReleaseNotes.md

* Also expose sender to event arguments

---------

Co-authored-by: Christian <6939810+chkr1011@users.noreply.github.com>

.github/workflows/ReleaseNotes.md

@@ -1,4 +1,5 @@
 * [Client] Fixed _PlatformNotSupportedException_ when using Blazor (#1755, thanks to @Nickztar).
+* [Client] Added support for _RemoteCertificateValidationCallback_ for .NET 4.5.2, 4.6.1 and 4.8 (#1806, thanks to @troky).
 * [Client] Fixed wrong logging of obsolete feature when connection was not successful (#1801, thanks to @ramonsmits).
 * [Client] Fixed _NullReferenceException_ when performing several actions when not connected (#1800, thanks to @ramonsmits).
 * [Server] Fixed _NullReferenceException_ in retained messages management (#1762, thanks to @logicaloud).

Source/MQTTnet/Client/Options/MqttClientCertificateValidationEventArgs.cs

@@ -14,8 +14,14 @@ namespace MQTTnet.Client
 
         public X509Chain Chain { get; set; }
 
-        public SslPolicyErrors SslPolicyErrors { get; set; }
-
         public IMqttClientChannelOptions ClientOptions { get; set; }
+#if NET452 || NET461 || NET48
+        /// <summary>
+        ///     Can be a host string name or an object derived from WebRequest.
+        /// </summary>
+        public object Sender { get; set; }
+#endif
+
+        public SslPolicyErrors SslPolicyErrors { get; set; }
     }
-}
+}

Source/MQTTnet/Implementations/MqttWebSocketChannel.cs

@@ -248,12 +248,20 @@ namespace MQTTnet.Implementations
                 throw new NotSupportedException("Remote certificate validation callback is not supported when using 'netstandard2.0'.");
 #elif WINDOWS_UWP
                 throw new NotSupportedException("Remote certificate validation callback is not supported when using 'uap10.0'.");
-#elif NET452
-                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'net452'.");
-#elif NET461
-                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'net461'.");
-#elif NET48
-                throw new NotSupportedException("Remote certificate validation callback is not supported when using 'net48'.");
+#elif NET452 || NET461 || NET48
+                ServicePointManager.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) => 
+                {
+                    var context = new MqttClientCertificateValidationEventArgs
+                    {
+                        Sender = sender,
+                        Certificate = certificate,
+                        Chain = chain,
+                        SslPolicyErrors = sslPolicyErrors,
+                        ClientOptions = _options
+                    };
+
+                    return certificateValidationHandler(context);
+                };
 #else
                 clientWebSocket.Options.RemoteCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) =>
                 {
@@ -272,4 +280,4 @@ namespace MQTTnet.Implementations
             }
         }
     }
-}
+}