CSHARP-3494: Fix discriminator for generic types

src/MongoDB.Bson/Serialization/BsonClassMap.cs

@@ -1057,7 +1057,7 @@ public void Reset()
             _creatorMaps.Clear();
             _creator = null;
             _declaredMemberMaps = new List<BsonMemberMap>();
-            _discriminator = _classType.Name;
+            _discriminator = BsonUtils.GetFriendlyTypeName(_classType);
             _discriminatorIsRequired = false;
             _extraElementsMemberMap = null;
             _idMemberMap = null;

tests/MongoDB.Driver.Tests/Jira/CSharp3494Tests.cs

@@ -0,0 +1,66 @@
+/* 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 FluentAssertions;
+using MongoDB.Bson;
+using MongoDB.Bson.Serialization;
+using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Driver.Tests.Linq.Linq3Implementation;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Jira
+{
+    public class CSharp3494Tests : Linq3IntegrationTest
+    {
+        abstract class BaseDocument;
+
+        class DerivedDocument<T> : BaseDocument
+        {
+            [BsonId]
+            public int Id { get; set; }
+
+            public T Value { get; set; }
+        }
+
+        [Fact]
+        public void Correct_discriminator_should_be_used_for_generic_type()
+        {
+            var document = new DerivedDocument<int> { Id = 1, Value = 42 };
+            var serialized = document.ToJson(typeof(BaseDocument));
+            serialized.Should().Be("""{ "_t" : "DerivedDocument<Int32>", "_id" : 1, "Value" : 42 }""");
+        }
+
+        [BsonKnownTypes(typeof(DerivedDocument2<int>))]
+        abstract class BaseDocument2 {}
+
+        class DerivedDocument2<T> : BaseDocument2
+        {
+            [BsonId]
+            public int Id { get; set; }
+
+            public T Value { get; set; }
+        }
+
+        [Fact]
+        public void Correct_type_should_be_instantiated_with_discriminator_for_generic_type()
+        {
+            //This test needs to use a different set of classes than the previous one, otherwise the discriminators could have been already
+            //registered, depending on the order of the tests. We need BsonKnownTypes for this to work.
+            var serialized = """{ "_t" : "DerivedDocument2<Int32>", "_id" : 1, "Value" : 42 }""";
+            var rehydrated = BsonSerializer.Deserialize<BaseDocument2>(serialized);
+            rehydrated.Should().BeOfType<DerivedDocument2<int>>();
+        }
+    }
+}