if you mean C# Client which installed on your windows to connect to your ESXi or vCenter, unfortunately is no longer available for ESXi 6.5 and later.
You can use vSphere Web Client - https://FQDN-or-IP-Address-of-VC-or-ESXi/vsphere–client
Or Host Client - https://FQDN-or-IP-Address-of-ESXi-host/UI
Best Regards