As in the labelled transform tree handling? Or a consumer/producer of tf-typed messages?
https://github.com/uml-robotics/ROS.NET has both, but I reinvented too much of the wheel with the TF tree handling code, so the inverse transform of an inverted traversal doesn't equal the transform. (Inverse of A>C doesn't equal C>A)
Other than needing to be partially rewritten, probably with more library code,
ROS.NET's TF code exists.