NullReferenceException in IEnumerableSerializerBase.Serialize when projecting nullable array with conditional Select

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    • Type: Bug
    • Resolution: Fixed
    • Priority: Major - P3
    • 3.9.0
    • Affects Version/s: None
    • Component/s: None
    • None
    • 🔴 Roadblock
    • Fully Compatible
    • C Drivers
    • Not Needed
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      Summary:

       NullReferenceException in IEnumerableSerializerBase.Serialize when projecting nullable array with conditional Select

      Environment:

       - MongoDB.Driver
        3.8.1 (NuGet, regression from 3.1.0)
       - Windows 11 64-bit x86_64
       - .NET 10 SDK
       - MongoDB
        7.0 standalone (docker)

      How to Reproduce:

      Full repro repo with two single-file .NET 10 code: https://github.com/realTeddy/mongo-bug-report/tree/main/MongoDriverNreRepro

       1. Start MongoDB: docker compose up -d
       2. Run dotnet run repro_old.cs (driver
        3.1.0) — both queries succeed
       3. Run dotnet run repro_new.cs (driver
        3.8.1) — broken query throws NRE

      The repro inserts three documents:

       1. Loan with Attachments = null
       2. Loan with Attachments = [] (empty list)
       3. Loan with two populated Attachment items

      The following LINQ projection throws NullReferenceException on document #1 (null Attachments) with driver 3.8.1:

       

       var results = collection.AsQueryable()
     .Select(l => new LoanDto
     {
         Id = l.Id,
         Attachments = l.Attachments == null
             ? null
             : l.Attachments.Select(a => new Attachment
             {
                 AttachmentId = a.AttachmentId,
                 Title = a.Title
             })
     })
     .ToList();

       

      Stack trace:

       

      System.NullReferenceException: Object reference not set to an instance of an object.
    at MongoDB.Driver.Linq.Linq3Implementation.Serializers.IEnumerableSerializerBase`2.Serialize(BsonSerializationContext context, BsonSerializationArgs args, TEnumerable value)
    at MongoDB.Bson.Serialization.IBsonSerializerExtensions.ToBsonValue(IBsonSerializer serializer, Object value)
    at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.ConstantExpressionToAggregationExpressionTranslator.Translate(...)
    at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.ConditionalExpressionToAggregationExpressionTranslator.Translate(...)

       

      Additional Background:

      The NRE occurs during query translation (not execution), when the LINQ3 provider tries to serialize the null constant in the ternary's true-branch through 
      IEnumerableSerializerBase.Serialize, which doesn't handle null.

      Workaround — replace the ternary with null-coalesce:

       

       Attachments = (l.Attachments ?? new List<Attachment>())
     .Select(a => new Attachment { AttachmentId = a.AttachmentId, Title = a.Title })

       

            Assignee:
            Ferdinando Papale
            Reporter:
            Tewodros Assefa
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              Created:
              Updated:
              Resolved: