CSHARP4040: Fix bug when using field with same element name as discriminator

src/MongoDB.Bson/Serialization/BsonClassMap.cs

@@ -1317,16 +1317,31 @@ public void UnmapProperty(string propertyName)
         /// Gets the discriminator convention for the class.
         /// </summary>
         /// <returns>The discriminator convention for the class.</returns>
-        internal IDiscriminatorConvention GetDiscriminatorConvention()
+        internal IDiscriminatorConvention GetDiscriminatorConvention(bool checkConflicts = false)
         {
             // return a cached discriminator convention when possible
             var discriminatorConvention = _discriminatorConvention;
             if (discriminatorConvention == null)
             {
-                // it's possible but harmless for multiple threads to do the discriminator convention lookukp at the same time
+                // it's possible but harmless for multiple threads to do the discriminator convention lookup at the same time
                 discriminatorConvention = LookupDiscriminatorConvention();
                 _discriminatorConvention = discriminatorConvention;
             }
+
+            if (checkConflicts && discriminatorConvention != null)
+            {
+                var conflictingMemberMap = _allMemberMaps.FirstOrDefault(memberMap => memberMap.ElementName == discriminatorConvention.ElementName);
+
+                if (conflictingMemberMap != null)
+                {
+                    var fieldOrProperty = conflictingMemberMap.MemberInfo is FieldInfo ? "field" : "property";
+
+                    throw new BsonSerializationException(
+                        $"The {fieldOrProperty} {conflictingMemberMap.MemberName} of type '{_classType.FullName}' cannot use element name '{discriminatorConvention.ElementName}' " +
+                        $"because it is already being used by the discriminator convention '{discriminatorConvention.GetType().Name}'.");
+                }
+            }
+
             return discriminatorConvention;
 
             IDiscriminatorConvention LookupDiscriminatorConvention()

src/MongoDB.Bson/Serialization/Conventions/StandardDiscriminatorConvention.cs

@@ -39,14 +39,19 @@ public abstract class StandardDiscriminatorConvention : IDiscriminatorConvention
         /// <param name="elementName">The element name.</param>
         protected StandardDiscriminatorConvention(string elementName)
         {
-            if (elementName == null)
+            if (string.IsNullOrEmpty(elementName))
             {
-                throw new ArgumentNullException("elementName");
+                throw new ArgumentNullException(nameof(elementName));
             }
             if (elementName.IndexOf('\0') != -1)
             {
-                throw new ArgumentException("Element names cannot contain nulls.", "elementName");
+                throw new ArgumentException("Element names cannot be null or empty.", nameof(elementName));
             }
+            if (elementName == "_id")
+            {
+                throw new ArgumentException("Element names cannot be '_id'.", nameof(elementName));
+            }
+
             _elementName = elementName;
         }
 

src/MongoDB.Bson/Serialization/Serializers/BsonClassMapSerializer.cs

@@ -637,7 +637,7 @@ private void SerializeExtraElements(BsonSerializationContext context, object obj
 
         private void SerializeDiscriminator(BsonSerializationContext context, Type nominalType, object obj)
         {
-            var discriminatorConvention = _classMap.GetDiscriminatorConvention();
+            var discriminatorConvention = _classMap.GetDiscriminatorConvention(true);
             if (discriminatorConvention != null)
             {
                 var actualType = obj.GetType();

tests/MongoDB.Bson.Tests/Serialization/Conventions/StandardDiscriminatorConventionTests.cs

@@ -28,6 +28,12 @@ public void TestConstructorThrowsWhenElementNameContainsNulls()
             Assert.Throws<ArgumentException>(() => new ScalarDiscriminatorConvention("a\0b"));
         }
 
+        [Fact]
+        public void TestConstructorThrowsWhenElementNameIsId()
+        {
+            Assert.Throws<ArgumentException>(() => new ScalarDiscriminatorConvention("_id"));
+        }
+
         [Fact]
         public void TestConstructorThrowsWhenElementNameIsNull()
         {

tests/MongoDB.Driver.Tests/Jira/Csharp4040Tests.cs

@@ -0,0 +1,50 @@
+/* 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.Bson.Serialization.Conventions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Jira
+{
+    public class Csharp4040Tests
+    {
+        private class BaseDocument
+        {
+            [BsonId] public ObjectId Id { get; set; } = ObjectId.GenerateNewId();
+
+            [BsonElement("_t")]
+            public string Field1 { get; set; }
+        }
+
+        private class DerivedDocument : BaseDocument {}
+
+        [Fact]
+        public void BsonClassMapSerializer_serialization_when_using_field_with_same_element_name_as_discriminator_should_throw()
+        {
+            var obj = new DerivedDocument { Field1 = "field1" };
+
+            var recordedException = Record.Exception(() => obj.ToJson(typeof(BaseDocument)));
+            recordedException.Should().NotBeNull();
+            recordedException.Should().BeOfType<BsonSerializationException>();
+            recordedException.Message.Should().Be("The property Field1 of type 'MongoDB.Driver.Tests.Jira.Csharp4040Tests+DerivedDocument' " +
+                                                  "cannot use element name '_t' because it is already being used by " +
+                                                  "the discriminator convention 'ScalarDiscriminatorConvention'.");
+        }
+    }
+}