Summary

After upgrading from MongoDB.Driver 3.7.0 to 3.7.1+, applications that register an EnumRepresentationConvention and serialize types containing JToken (or any self-referencing IEnumerable<T>) crash with a StackOverflowException during convention application.

Affects Version/s: 3.7.1, 3.8.0

Component: BSON

How to Reproduce

1. Register a convention pack containing EnumRepresentationConvention:

var conventionPack = new ConventionPack
{
    new EnumRepresentationConvention(BsonType.String)
};
ConventionRegistry.Register("conventions", conventionPack, _ => true);

2. Have a document class with a JObject or JToken property:

public class MyDocument
{
    public string Id { get; set; }
    public JObject Manifest { get; set; }
}

3. Perform any MongoDB operation that triggers class map auto-mapping for MyDocument.
4. The process crashes with exit code 134 (SIGABRT) due to a StackOverflowException.

Root Cause

EnumRepresentationConvention.CouldApply (introduced by the fix for CSHARP-5905) recursively walks the generic arguments of IEnumerable<T> to check whether any nested type is an enum. JToken implements IEnumerable<JToken>, creating a cycle: JToken → IEnumerable<JToken> → JToken → … — infinite recursion with no visited-type guard.

Stack trace (repeating segment):

at MongoDB.Bson.Serialization.Conventions.EnumRepresentationConvention.CouldApply(System.Type)
at MongoDB.Bson.Serialization.Conventions.EnumRepresentationConvention.CouldApply(System.Type)
at MongoDB.Bson.Serialization.Conventions.EnumRepresentationConvention.CouldApply(System.Type)
...

Expected Behavior

CouldApply should track visited types (e.g. via a HashSet<Type>) to detect cycles and return false when encountering a previously seen type.

Workaround

Pin MongoDB.Driver to 3.7.0 and remove EnumRepresentationConvention from the convention pack, registering explicit EnumSerializer<T>(BsonType.String) for each enum type instead.

Related Issues

• CSHARP-5493 — Same StackOverflow bug in 3.2.0, fixed in 3.2.1 (the fix was reverted/regressed)
• CSHARP-5905 — The fix that introduced CouldApply, which caused this regression
• CSHARP-5508 — "Revisit Conventions.Apply" (BACKLOG)