When attempting to use indexOf(-1) in a bulk operation update, the produced mongo query does not contain the "$" positional operator resulting in the query throwing an error.

Example

bulk.Find(Query<Users>.Where(user => user.FavoriteColors.Any(f => f.Name == "green")))
.UpdateOne(Update<Users>.Set(user => user.FavoriteColors.IndexOf(-1).Name, "blue");

Results in {$set:

{"user.favoriteColors.-1.name":"blue"}

which throws and error.

Workaround

bulk.Find(Query<Users>.Where(user => user.FavoriteColors.Any(f => f.Name == "green")))
.UpdateOne(Update.Set("user.favoriteColors.$.name", "blue");

NOTE: It would be nice if indexOf(-1) could be supported across ALL query builders, including legacy, current, and .net core.

References:
https://jira.mongodb.org/browse/CSHARP-531
https://github.com/mongodb/mongo-csharp-driver/blob/master/tests/MongoDB.Driver.Legacy.Tests/Operations/BulkWriteOperationTests.cs