We support HasConversion for primitive properties but although the API lets you specify it for a primitive collection we do not perform the conversion in the MongoTypeMappingSource when the property is a collection so if trying to map say a HashSet<string> on a CLR property to a List<string> in the database it will throw with the error message 

System.InvalidOperationException: 'The type 'HashSet<string>' cannot be used as a primitive collection because it is not an array and does not implement 'IList<string>'. Collections of primitive types must be arrays or ordered lists.'

Reported on the community forums https://www.mongodb.com/community/forums/t/invalidoperation-for-hashset-field-even-with-conversion-defined/322555