Thanks @Kowa . btw i could not manage to use that. i'm using NAV 2018 CU9 VSIX file for extensions but i can't see "al.assemblyProbingPaths" setting. When i use the latest AL version on the market, i can see that but i can not create a package for NAV 2018. I may missing something probably. Do you have any experience for using dotnet variables on NAV 2018 extensions?
Developer Preview functionality is not backported to NAV 2018 in any way and only fully supported in the next major release, which is now the rebranded Product 'Dynamics 365 Business Central On-Premises' which was released 10 days ago.
Answers
https://docs.microsoft.com/de-de/dynamics-nav/developer/devenv-dev-faq#dotnet-types-are-not-available-in-extensions-v2-what-now
https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-get-started-call-dotnet-from-al
https://blogs.msdn.microsoft.com/nav/2018/06/25/developer-preview-june-2018-update/