Our team has been working on building a collection of filter definitions to query against and we noticed in our tests that when adding a null value to the collection, we end up getting a NullReferenceException.

It appears that, while the collection is being Ensured that it's not null, the entities get no validation pass and are assumed to be correct.

public AndFilterDefinition(IEnumerable<FilterDefinition<TDocument>> filters)
{{ {}}
{{     _filters = Ensure.IsNotNull(filters, nameof(filters)).ToList();}}
{{ }}}

(https://github.com/mongodb/mongo-csharp-driver/blob/master/src/MongoDB.Driver/FilterDefinitionBuilder.cs#L1500)

If we look at the Render a little further down in the FilterDefinitionBuilder.cs, we see _filters iterated over:

foreach (var filter in _filters)
{{ {}}
{{ var renderedFilter = filter.Render(documentSerializer, serializerRegistry);}}

If _filters contains a null, this will trigger a NullReferenceException.