CSHARP-2632: Don't send $readPreference to standalone servers

Author: ksadoff

src/MongoDB.Driver.Core/Core/WireProtocol/CommandUsingCommandMessageWireProtocol.cs

@@ -27,6 +27,7 @@
 using MongoDB.Driver.Core.Connections;
 using MongoDB.Driver.Core.Misc;
 using MongoDB.Driver.Core.Operations;
+using MongoDB.Driver.Core.Servers;
 using MongoDB.Driver.Core.WireProtocol.Messages;
 using MongoDB.Driver.Core.WireProtocol.Messages.Encoders;
 
@@ -190,7 +191,9 @@ private Type0CommandMessageSection<BsonDocument> CreateType0Section(ConnectionDe
             var dbElement = new BsonElement("$db", _databaseNamespace.DatabaseName);
             extraElements.Add(dbElement);
 
-            if (_readPreference != null && _readPreference != ReadPreference.Primary)
+            if (connectionDescription.IsMasterResult.ServerType != ServerType.Standalone
+                && _readPreference != null
+                && _readPreference != ReadPreference.Primary)
             {
                 var readPreferenceDocument = QueryHelper.CreateReadPreferenceDocument(_readPreference);
                 var readPreferenceElement = new BsonElement("$readPreference", readPreferenceDocument);

tests/MongoDB.Driver.Tests/ReadPreferenceOnStandaloneTests.cs

@@ -0,0 +1,77 @@
+/* Copyright 2019-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using FluentAssertions;
+using MongoDB.Bson;
+using MongoDB.Bson.TestHelpers.XunitExtensions;
+using MongoDB.Driver.Core;
+using MongoDB.Driver.Core.Clusters;
+using MongoDB.Driver.Core.Events;
+using MongoDB.Driver.Core.Misc;
+using MongoDB.Driver.TestHelpers;
+using Xunit;
+
+namespace MongoDB.Driver.Tests
+{
+    public class ReadPreferenceOnStandaloneTests
+    {
+        [Theory]
+        [ParameterAttributeData]
+        public void ReadPreference_should_not_be_sent_to_standalone_server(
+            [Values(false, true)] bool async)
+        {
+            var eventCapturer = new EventCapturer().Capture<CommandStartedEvent>(e =>
+                e.CommandName.Equals("find") || e.CommandName.Equals("$query"));
+            using (var client = CreateDisposableClient(eventCapturer, ReadPreference.PrimaryPreferred))
+            {
+                var database = client.GetDatabase(DriverTestConfiguration.DatabaseNamespace.DatabaseName);
+                var collection = database.GetCollection<BsonDocument>(DriverTestConfiguration.CollectionNamespace.CollectionName);
+
+                if (async)
+                {
+                    var _ = collection.FindAsync("{ x : 2 }").GetAwaiter().GetResult();
+                }
+                else
+                {
+                    var _ = collection.FindSync("{ x : 2 }");
+                }
+
+                CommandStartedEvent sentCommand = ((CommandStartedEvent) eventCapturer.Events[0]);
+                var serverVersion = client.Cluster.Description.Servers[0].Version;
+                var clusterType = client.Cluster.Description.Type;
+
+                var expectedContainsReadPreference = clusterType == ClusterType.Standalone ||
+                    (clusterType == ClusterType.ReplicaSet && serverVersion < Feature.CommandMessage.FirstSupportedVersion)
+                    ? false
+                    : true;
+                var readPreferenceFieldName = sentCommand.Command.Contains("$readPreference")
+                    ? "$readPreference"
+                    : "readPreference";
+
+                sentCommand.Command.Contains(readPreferenceFieldName).Should().Be(expectedContainsReadPreference);
+            }
+        }
+
+        // private methods
+        private DisposableMongoClient CreateDisposableClient(EventCapturer eventCapturer, ReadPreference readPreference)
+        {
+            return DriverTestConfiguration.CreateDisposableClient((MongoClientSettings settings) =>
+            {
+                settings.ClusterConfigurator = c => c.Subscribe(eventCapturer);
+                settings.ReadPreference = readPreference;
+            });
+        }
+    }
+}