When using the .net dll in X++, I faced an error "Method 'MethodName(System.Collections.Generic.List`1, System.Boolean)' is not found on type 'ClassNameInDll'.
It was because the X++ compiler cannot parse the System.Collections.Generic.List of .Net in the parameter of the method "MethodName". In order to bypass this error, in X++ try to use "System.Collections.IList" or System.Collections.IEnumerable instead of System.Collections.Generic.List
There are some posts explaining the issue:
https://cloudblogs.microsoft.com/dynamics365/no-audience/2009/04/23/dynamics-ax-and-generic-collections-of-net/
Hope it helps.