array.length jsonpath for the selectToken method returned null instead of count about newtonsoft.json

[elgonzo]

I am only a user of Newtonsoft.Json and not otherwise associated with the project or its author/maintainer. That said, i would strongly suggest you ponder a possible migration from Newtonsoft.Json to System.Text.Json if JsonPath support is important to you. While JsonPath support in System.Text.Json itself is lacking as of now, there is the (in my opinion) excellent JsonPath.Net library (doc, github), which not only is substantially better than Newtonsoft.Json's JsonPath support (striving to support the current JsonPath specification), but it also is actively maintained. Plus, there is a handy JsonPath.Net-based online evaluator for quickly trying out and messing about with JsonPath expressions (https://json-everything.net/json-path/). However, JsonPath.Net relies on STJ and can't be used on top of Newtonsoft.Json.

If you don't want or cannot migrate to STJ, then i guess you unfortunately will be stuck with Newtonsoft.Json's quirky and outdated JsonPath implementation. Support for the length function (and other functions in JsonPath) has been asked for years ago (see this open ticket, for example: #1318), and in my frank opinion i would see it as a small miracle if Newtonsoft.Json were to receive noteworthy feature updates anymore. (Personally, i consider Newtonsoft.Json a largely feature-frozen legacy library, with System.Text.Json being the go-to for Json processing in .NET these days.)