Prototype using public read-only property in POCO as discriminator

src/MongoDB.Bson/Serialization/Attributes/BsonDiscriminatorMemberAttribute.cs

@@ -0,0 +1,55 @@
+/* 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;
+
+namespace MongoDB.Bson.Serialization.Attributes;
+
+/// <summary>
+/// Identifies a class member to be used as the discriminator.
+/// </summary>
+[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
+public class BsonDiscriminatorMemberAttribute : Attribute, IBsonMemberMapAttribute
+{
+    private readonly string _elementName;
+
+    /// <summary>
+    /// Initializes a new instance of BsonDiscriminatorMemberAttribute.
+    /// </summary>
+    public BsonDiscriminatorMemberAttribute()
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of BsonDiscriminatorMemberAttribute.
+    /// </summary>
+    /// <param name="elementName">The element name.</param>
+    public BsonDiscriminatorMemberAttribute(string elementName)
+    {
+        _elementName = elementName;
+    }
+
+    /// <inheritdoc/>
+    public void Apply(BsonMemberMap memberMap)
+    {
+        var classMap = memberMap.ClassMap;
+        classMap.SetDiscriminatorMember(memberMap);
+
+        if (_elementName != null)
+        {
+            memberMap.SetElementName(_elementName);
+        }
+    }
+}

src/MongoDB.Bson/Serialization/BsonClassMap.cs

@@ -52,6 +52,7 @@ public class BsonClassMap
         private Func<object> _creator;
         private string _discriminator;
         private bool _discriminatorIsRequired;
+        private BsonMemberMap _discriminatorMemberMap;
         private bool _hasRootClass;
         private bool _isRootClass;
         private BsonMemberMap _idMemberMap;
@@ -156,6 +157,14 @@ public bool DiscriminatorIsRequired
             get { return _discriminatorIsRequired; }
         }
 
+        /// <summary>
+        /// Gets the discriminator member map (null if none).
+        /// </summary>
+        public BsonMemberMap DiscriminatorMemberMap
+        {
+            get { return _discriminatorMemberMap; }
+        }
+
         /// <summary>
         /// Gets the member map of the member used to hold extra elements.
         /// </summary>
@@ -637,6 +646,15 @@ public BsonClassMap Freeze()
                             }
                         }
 
+                        if (_discriminatorMemberMap == null)
+                        {
+                            // see if we can inherit the discriminatorMemberMap from our base class
+                            if (_baseClassMap != null)
+                            {
+                                _discriminatorMemberMap = _baseClassMap.DiscriminatorMemberMap;
+                            }
+                        }
+
                         if (_extraElementsMemberMap == null)
                         {
                             // see if we can inherit the extraElementsMemberMap from our base class
@@ -1118,6 +1136,29 @@ public void SetDiscriminatorIsRequired(bool discriminatorIsRequired)
             _discriminatorIsRequired = discriminatorIsRequired;
         }
 
+        /// <summary>
+        /// Sets the discriminator member.
+        /// </summary>
+        /// <param name="memberMap">The discriminator member (null if none).</param>
+        public void SetDiscriminatorMember(BsonMemberMap memberMap)
+        {
+            if (memberMap != null)
+            {
+                EnsureMemberMapIsForThisClass(memberMap);
+
+                if (memberMap.MemberInfo is not PropertyInfo propertyInfo ||
+                    propertyInfo.CanWrite ||
+                    !propertyInfo.GetMethod.IsVirtual)
+                {
+                    throw new ArgumentException("The discriminator member must be a virtual read-only property.", nameof(memberMap));
+                }
+            }
+
+            if (_frozen) { ThrowFrozenException(); }
+
+            _discriminatorMemberMap = memberMap;
+        }
+
         /// <summary>
         /// Sets the member map of the member used to hold extra elements.
         /// </summary>
@@ -1329,7 +1370,17 @@ internal IDiscriminatorConvention GetDiscriminatorConvention()
 
                 if (discriminatorConvention != null)
                 {
-                    EnsureNoMemberMapConflicts(discriminatorConvention.ElementName);
+                    if (_discriminatorMemberMap == null)
+                    {
+                        EnsureNoMemberMapConflicts(discriminatorConvention.ElementName);
+                    }
+                    else
+                    {
+                        if (_discriminatorMemberMap.ElementName != discriminatorConvention.ElementName)
+                        {
+                            throw new BsonSerializationException("Discriminator convention and disciminator map element names do not match.");
+                        }
+                    }
                 }
             }
 

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

@@ -581,7 +581,7 @@ private void SerializeClass(BsonSerializationContext context, BsonSerializationA
 
             if (ShouldSerializeDiscriminator(args.NominalType))
             {
-                SerializeDiscriminator(context, args.NominalType, document);
+                SerializeDiscriminator(context, args.NominalType, document.GetType());
             }
 
             foreach (var memberMap in _classMap.AllMemberMaps)
@@ -625,12 +625,11 @@ private void SerializeExtraElements(BsonSerializationContext context, object obj
             }
         }
 
-        private void SerializeDiscriminator(BsonSerializationContext context, Type nominalType, object obj)
+        private void SerializeDiscriminator(BsonSerializationContext context, Type nominalType, Type actualType)
         {
             var discriminatorConvention = _classMap.GetDiscriminatorConvention();
             if (discriminatorConvention != null)
             {
-                var actualType = obj.GetType();
                 var discriminator = discriminatorConvention.GetDiscriminator(nominalType, actualType);
                 if (discriminator != null)
                 {
@@ -642,6 +641,14 @@ private void SerializeDiscriminator(BsonSerializationContext context, Type nomin
 
         private void SerializeMember(BsonSerializationContext context, object obj, BsonMemberMap memberMap)
         {
+            if (memberMap == _classMap.DiscriminatorMemberMap)
+            {
+                // the discriminator has already been serialized
+                // verify that the value of the read-only property matches the discriminator value returned by the discriminator convention
+                VerifyDiscriminatorMemberValue(obj, memberMap);
+                return;
+            }
+
             try
             {
                 if (memberMap != _classMap.ExtraElementsMemberMap)
@@ -682,6 +689,26 @@ private bool ShouldSerializeDiscriminator(Type nominalType)
             return (nominalType != _classMap.ClassType || _classMap.DiscriminatorIsRequired || _classMap.HasRootClass) && !_classMap.IsAnonymous;
         }
 
+        private void VerifyDiscriminatorMemberValue(object obj, BsonMemberMap discriminatorMemberMap)
+        {
+            var discriminatorConvention = _classMap.GetDiscriminatorConvention();
+            if (discriminatorConvention != null)
+            {
+                var discriminatorMemberSerializer = discriminatorMemberMap.GetSerializer();
+                var discriminatorMemberValue = discriminatorMemberMap.Getter(obj);
+                var serializedDiscriminatorMemberValue = discriminatorMemberSerializer.ToBsonValue(discriminatorMemberValue);
+
+                var nominalType = _classMap.ClassType;
+                var actualType = obj.GetType();
+                var conventionDiscriminatorValue = discriminatorConvention.GetDiscriminator(nominalType, actualType);
+
+                if (!object.Equals(serializedDiscriminatorMemberValue, conventionDiscriminatorValue))
+                {
+                    throw new BsonSerializationException("Discriminator member value does not match discriminator convention GetDiscriminator return value.");
+                }
+            }
+        }
+
         // nested classes
         // helper class that implements member map bit array helper functions
         internal static class FastMemberMapHelper

tests/MongoDB.Bson.Tests/Jira/PublicDiscriminatorTests.cs

@@ -0,0 +1,62 @@
+/* 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;
+using System.Collections.Generic;
+using FluentAssertions;
+using MongoDB.Bson.Serialization;
+using MongoDB.Bson.Serialization.Attributes;
+using Xunit;
+
+namespace MongoDB.Bson.Tests.Jira;
+
+public class PublicDiscriminatorTests
+{
+    [Fact]
+    public void Deserialize_with_public_discriminator_should_work()
+    {
+        var json = "{ _id : 1, _t : ['C', 'D'] }";
+        var serializer = BsonSerializer.LookupSerializer<C>();
+
+        var document = BsonSerializer.Deserialize<C>(json);
+
+        document.Should().BeOfType<D>();
+        document.Id.Should().Be(1);
+        document.Discriminator.Should().Equal("C", "D");
+    }
+
+    [Fact]
+    public void Serialize_with_public_discriminator_should_work()
+    {
+        var document = new D { Id = 1 };
+
+        var serializedDocument = document.ToBsonDocument<C>(args: new BsonSerializationArgs { SerializeIdFirst = true });
+
+        serializedDocument.Should().Be("{ _id : 1, _t : ['C', 'D'] }");
+    }
+
+    [BsonDiscriminator(RootClass = true)]
+    [BsonKnownTypes(typeof(D))]
+    public abstract class C
+    {
+        public int Id { get; set; }
+        [BsonDiscriminatorMember("_t")] public virtual IReadOnlyList<string> Discriminator => ["C"];
+    }
+
+    public sealed class D : C
+    {
+        public override IReadOnlyList<string> Discriminator => ["C", "D"];
+    }
+}