The aggregation $filter array expression is not currently listed in the list of array expressions:

http://mongodb.github.io/mongo-csharp-driver/2.2/reference/driver/expressions/#array-expressions

Although the test seems to have it listed:

https://github.com/mongodb/mongo-csharp-driver/blob/master/src/MongoDB.Driver.Tests/Linq/Translators/AggregateProjectTranslatorTests.cs#L966