CSHARP-5349: Fix discriminator convention inheritance.

src/MongoDB.Bson/Serialization/BsonClassMap.cs

@@ -1323,11 +1323,40 @@ internal IDiscriminatorConvention GetDiscriminatorConvention()
             var discriminatorConvention = _discriminatorConvention;
             if (discriminatorConvention == null)
             {
-                // it's possible but harmless for multiple threads to do the field initialization at the same time
-                discriminatorConvention = _hasRootClass ? StandardDiscriminatorConvention.Hierarchical : StandardDiscriminatorConvention.Scalar;
+                // it's possible but harmless for multiple threads to do the discriminator convention lookukp at the same time
+                discriminatorConvention = LookupDiscriminatorConvention();
                 _discriminatorConvention = discriminatorConvention;
             }
             return discriminatorConvention;
+
+            IDiscriminatorConvention LookupDiscriminatorConvention()
+            {
+                var classMap = this;
+                while (classMap != null)
+                {
+                    if (classMap._discriminatorConvention != null)
+                    {
+                        return classMap._discriminatorConvention;
+                    }
+
+                    if (BsonSerializer.IsDiscriminatorConventionRegisteredAtThisLevel(classMap._classType))
+                    {
+                        // in this case LookupDiscriminatorConvention below will find it
+                        break;
+                    }
+
+                    if (classMap._isRootClass)
+                    {
+                        // in this case auto-register a hierarchical convention for the root class and look it up as usual below
+                        BsonSerializer.GetOrRegisterDiscriminatorConvention(classMap._classType, StandardDiscriminatorConvention.Hierarchical);
+                        break;
+                    }
+
+                    classMap = classMap._baseClassMap;
+                }
+
+                return BsonSerializer.LookupDiscriminatorConvention(_classType);
+            }
         }
 
         // private methods

src/MongoDB.Bson/Serialization/BsonSerializer.cs

@@ -269,6 +269,51 @@ public static object Deserialize(TextReader textReader, Type nominalType, Action
             }
         }
 
+        internal static IDiscriminatorConvention GetOrRegisterDiscriminatorConvention(Type type, IDiscriminatorConvention discriminatorConvention)
+        {
+            __configLock.EnterReadLock();
+            try
+            {
+                if (__discriminatorConventions.TryGetValue(type, out var registeredDiscriminatorConvention))
+                {
+                    return registeredDiscriminatorConvention;
+                }
+            }
+            finally
+            {
+                __configLock.ExitReadLock();
+            }
+
+            __configLock.EnterWriteLock();
+            try
+            {
+                if (__discriminatorConventions.TryGetValue(type, out var registeredDiscrimantorConvention))
+                {
+                    return registeredDiscrimantorConvention;
+                }
+
+                RegisterDiscriminatorConvention(type, discriminatorConvention);
+                return discriminatorConvention;
+            }
+            finally
+            {
+                __configLock.ExitWriteLock();
+            }
+        }
+
+        internal static bool IsDiscriminatorConventionRegisteredAtThisLevel(Type type)
+        {
+            __configLock.EnterReadLock();
+            try
+            {
+                return __discriminatorConventions.ContainsKey(type);
+            }
+            finally
+            {
+                __configLock.ExitReadLock();
+            }
+        }
+
         /// <summary>
         /// Returns whether the given type has any discriminators registered for any of its subclasses.
         /// </summary>

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp5349Tests.cs

@@ -0,0 +1,93 @@
+/* Copyright 2010-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 System.Linq;
+using FluentAssertions;
+using MongoDB.Bson.Serialization;
+using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Bson.Serialization.Conventions;
+using MongoDB.Bson.Serialization.Serializers;
+using MongoDB.Driver.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira
+{
+    public class CSharp5349Tests : Linq3IntegrationTest
+    {
+        static CSharp5349Tests()
+        {
+            BsonSerializer.RegisterDiscriminatorConvention(typeof(BaseNoRoot), new ScalarDiscriminatorConvention("__type"));
+            BsonSerializer.RegisterDiscriminatorConvention(typeof(BaseWithRoot), new HierarchicalDiscriminatorConvention("__type"));
+        }
+
+        [Fact]
+        public void InsertOne_BaseNoRoot_should_use_the_configured_discriminator()
+        {
+            var collection = GetCollectionOfBaseNoRoot();
+
+            var documents = collection.AsQueryable().As(BsonDocumentSerializer.Instance).ToList();
+
+            documents.Single().Should().Be("{ _id : 1, __type : 'DerivedNoRoot' }");
+        }
+
+        [Fact]
+        public void InsertOne_BaseWithRoot_should_use_the_configured_discriminator()
+        {
+            var collection = GetCollectionOfBaseWithRoot();
+
+            var documents = collection.AsQueryable().As(BsonDocumentSerializer.Instance).ToList();
+
+            documents.Single().Should().Be("{ _id : 1, __type : ['BaseWithRoot', 'DerivedWithRoot'] }");
+        }
+
+        private IMongoCollection<BaseNoRoot> GetCollectionOfBaseNoRoot()
+        {
+            var collection = GetCollection<BaseNoRoot>("test");
+            CreateCollection(
+                collection,
+                new DerivedNoRoot { Id = 1 });
+            return collection;
+        }
+
+        private IMongoCollection<BaseWithRoot> GetCollectionOfBaseWithRoot()
+        {
+            var collection = GetCollection<BaseWithRoot>("test");
+            CreateCollection(
+                collection,
+                new DerivedWithRoot { Id = 1 });
+            return collection;
+        }
+
+        private class BaseNoRoot
+        {
+            public int Id { get; set; }
+        }
+
+        private class DerivedNoRoot : BaseNoRoot
+        {
+        }
+
+        [BsonDiscriminator(RootClass = true)]
+        [BsonKnownTypes(typeof(DerivedWithRoot))]
+        private class BaseWithRoot
+        {
+            public int Id { get; set; }
+        }
+
+        private class DerivedWithRoot : BaseWithRoot
+        {
+        }
+    }
+}