From 30ac64aecc439ec82dd3f4d6a4a1b03940ae0ab4 Mon Sep 17 00:00:00 2001
From: Ferdinando Papale <4850119+papafe@users.noreply.github.com>
Date: Thu, 7 May 2026 14:54:17 +0200
Subject: [PATCH 1/2] CSHARP-5816: Insonsistent behavior of
 `EnsureNoMemberMapConflicts` for discriminator convention

---
 .../Serialization/BsonClassMap.cs             |  4 ++-
 .../Serialization/BsonClassMapTests.cs        | 30 +++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/src/MongoDB.Bson/Serialization/BsonClassMap.cs b/src/MongoDB.Bson/Serialization/BsonClassMap.cs
index 3e257d2259f..2f2810fc369 100644
--- a/src/MongoDB.Bson/Serialization/BsonClassMap.cs
+++ b/src/MongoDB.Bson/Serialization/BsonClassMap.cs
@@ -1325,12 +1325,14 @@ internal IDiscriminatorConvention GetDiscriminatorConvention()
             {
                 // it's possible but harmless for multiple threads to do the discriminator convention lookup at the same time
                 discriminatorConvention = LookupDiscriminatorConvention();
-                _discriminatorConvention = discriminatorConvention;
 
                 if (discriminatorConvention != null)
                 {
                     EnsureNoMemberMapConflicts(discriminatorConvention.ElementName);
                 }
+
+                // only cache if validation succeeds
+                _discriminatorConvention = discriminatorConvention;
             }
 
             return discriminatorConvention;
diff --git a/tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs b/tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs
index 337aae069c6..339e711930c 100644
--- a/tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs
+++ b/tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs
@@ -22,6 +22,7 @@
 using MongoDB.Bson.IO;
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Bson.Serialization.Conventions;
 using MongoDB.Bson.TestHelpers;
 using Xunit;
 
@@ -735,4 +736,33 @@ private class D : C
         {
         }
     }
+
+    public class BsonClassMapGetDiscriminatorConventionTests
+    {
+        [Fact]
+        public void GetDiscriminatorConvention_should_throw_consistently_when_member_conflicts_with_discriminator_element_name()
+        {
+            BsonSerializer.RegisterDiscriminatorConvention(typeof(Foo), new FooDiscriminatorConvention5816());
+
+            var classMap = new BsonClassMap<Foo>();
+            classMap.AutoMap();
+            classMap.Freeze();
+
+            Assert.Throws<BsonSerializationException>(() => classMap.GetDiscriminatorConvention());
+            Assert.Throws<BsonSerializationException>(() => classMap.GetDiscriminatorConvention());
+        }
+
+        private class Foo
+        {
+            [BsonElement("type")]
+            public string Type { get; set; }
+        }
+
+        private class FooDiscriminatorConvention5816 : IDiscriminatorConvention
+        {
+            public string ElementName => "type";
+            public Type GetActualType(IBsonReader bsonReader, Type nominalType) => nominalType;
+            public BsonValue GetDiscriminator(Type nominalType, Type actualType) => nominalType.Name;
+        }
+    }
 }

From 92070fd903c4e116e81eb3702e757b05c2a3d869 Mon Sep 17 00:00:00 2001
From: Ferdinando Papale <4850119+papafe@users.noreply.github.com>
Date: Thu, 7 May 2026 15:10:22 +0200
Subject: [PATCH 2/2] Small correction

---
 tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs b/tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs
index 339e711930c..eedf3b3f302 100644
--- a/tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs
+++ b/tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs
@@ -742,7 +742,7 @@ public class BsonClassMapGetDiscriminatorConventionTests
         [Fact]
         public void GetDiscriminatorConvention_should_throw_consistently_when_member_conflicts_with_discriminator_element_name()
         {
-            BsonSerializer.RegisterDiscriminatorConvention(typeof(Foo), new FooDiscriminatorConvention5816());
+            BsonSerializer.RegisterDiscriminatorConvention(typeof(Foo), new FooDiscriminatorConvention());
 
             var classMap = new BsonClassMap<Foo>();
             classMap.AutoMap();
@@ -758,7 +758,7 @@ private class Foo
             public string Type { get; set; }
         }
 
-        private class FooDiscriminatorConvention5816 : IDiscriminatorConvention
+        private class FooDiscriminatorConvention : IDiscriminatorConvention
         {
             public string ElementName => "type";
             public Type GetActualType(IBsonReader bsonReader, Type nominalType) => nominalType;