How to get cpu or hdd temperature with c#?

Probably, you can't.
In theory, you can use WMI to get them, but...
See MSDN: Win32_TemperatureProbe class[^]

"Most of the information that the Win32_TemperatureProbe WMI class provides comes from SMBIOS. Real-time readings for the CurrentReading property cannot be extracted from SMBIOS tables. For this reason, current implementations of WMI do not populate the CurrentReading property. The CurrentReading property's presence is reserved for future use."

It's all dependant on the BIOS, and WMI (et al) rely on motherboard manufacturers providing that info via their drivers - and most don't bother because it's not compulsory to do so.
So what works on one PC may not work next week on the same PC if the drivers have been updated!