MongoDb .NET to replace string in array using ElemMatch()

I'm using the MongoDb .NET driver (v3.7) to replace string in array using ElemMatch to match the array string element.

All examples I see regarding ElemMatch() involve matching an object in an array (or list) of objects.

Can I not use the same methods to target a string within an array?

For example, I want to replace round with triangle and tried the following code, thinking the positional operator would find/reference the string in the array that needed changing. But perhaps that is not possible with arrays of primitives?

var idFilter = Builders<Sample>.Filter.Eq(d => d.Id, "000000000000000000001002"); var elemMatchFilter = Builders<Sample>.Filter.ElemMatch(x => x.MyStringArray, "round"); var filtersAnded = Builders<Sample>.Filter.And(idFilter, elemMatchFilter); var update = Builders<Sample>.Update.Set("MyStringArray.[$]", "triangle"); UpdateResult result = collection.UpdateOne(filtersAnded, update);

This results in an exception:

JSON reader was expecting a value but found 'round'.

Data model:

internal class Sample { [BsonRepresentation(BsonType.ObjectId)] public string Id { get; set; } = ObjectId.GenerateNewId().ToString(); public int[] MyIntArray { get; set; } = []; public string[] MyStringArray { get; set; } = []; }

Code to generate 2 documents:

public const string Samples = "Samples"; IMongoDatabase? iMongoDatabase = mongoClient.GetDatabase("UpdateArraysTesting"); iMongoDatabase.CreateCollection(Samples); SamplesCollection = iMongoDatabase!.GetCollection<Sample>(Samples); Sample sample = new() { Id = "000000000000000000001001", MyIntArray = new int[] { 10, 15 }, MyStringArray = new string[] {"square","box" } }; SamplesCollection.InsertOne(sample); sample = new() { Id = "000000000000000000001002", MyIntArray = new int[] { 22, 27 }, MyStringArray = new string[] { "round", "ball" } }; SamplesCollection.InsertOne(sample);