How can I implement an exhaustMap handler in Rx.Net?

Question

I am looking for something similar to the exhaustMap operator from rxjs, but RX.NET does not seem to have such an operator.

What I need to achieve is that, upon every element of the source stream, I need to start an async handler, and until it finishes, I would like to drop any elements from the source. As soon as the handler finishes, resume taking elements.

What I don't want is to start an async handler upon every element - while the handler runs, I want to drop source elements.

I also suspect I need to cleverly use the defer operator here?

Thank you!