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CSHARP-5816: Insonsistent behavior of EnsureNoMemberMapConflicts for discriminator convention #1991

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Merged
papafe merged 2 commits into from
May 28, 2026
Merged

CSHARP-5816: Insonsistent behavior of EnsureNoMemberMapConflicts for discriminator convention #1991

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CSHARP-5816: Insonsistent behavior of `EnsureNoMemberMapConflicts` fo…
papafe May 7, 2026
92070fd
Small correction
papafe May 7, 2026
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4 changes: 3 additions & 1 deletion src/MongoDB.Bson/Serialization/BsonClassMap.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -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

@ajcvickers ajcvickers May 18, 2026

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Claude: When LookupDiscriminatorConvention() returns null, the new code assigns null back to _discriminatorConvention, so no caching of the null result occurs and every call re-runs the lookup walk — this is pre-existing behavior preserved by the fix, not a regression, but the comment "only cache if validation succeeds" is slightly misleading since caching null is also suppressed; consider guarding the null case separately (e.g. a dedicated sentinel or a bool _discriminatorConventionResolved flag) if the repeated lookup ever becomes a performance concern in hot paths.

@papafe papafe May 19, 2026

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Reasonable comment, but I wanted to do the minimum amount of changes to fix this.

_discriminatorConvention = discriminatorConvention;
}

return discriminatorConvention;
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30 changes: 30 additions & 0 deletions tests/MongoDB.Bson.Tests/Serialization/BsonClassMapTests.cs
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Expand Up @@ -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;

Expand Down Expand Up @@ -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 FooDiscriminatorConvention());

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 FooDiscriminatorConvention : IDiscriminatorConvention
{
public string ElementName => "type";
public Type GetActualType(IBsonReader bsonReader, Type nominalType) => nominalType;
public BsonValue GetDiscriminator(Type nominalType, Type actualType) => nominalType.Name;
}
}
}
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