There is currently no way to get a list of shared mailboxes or determine whether a User account is actually a shared mailbox. Ideally there would be a new top-level object -- eg., GET /sharedMailbox...
As you can invoke PowerShell commandlets from a c# exe file, why don't you want to create your own app that will work both with PowerShell and with GRAPH?
Here is an example of such thing: CentralOOF.