Hello
You cannot use a .NET class directly from Delphi, even via Hydra. You’ll need to create a wrapper around it that will be used for interop purposes. Still it is possible to use method calls, access properties and use callback methods as event replacements.
Please take a look at this post: Need a Jumpstart with Delphi XE3 and .NET 2.0 DLL Assemblies Integration (it is being reworked into a proper tutorial article on the docs site)