Hardwarewallets offer the best security for storing cryptocurrency. That’s why I built the app Hardfolio and released it on Google Play and Windows Store. It aggregates your crypto portfolio for several Hardwarewallets such as Trezor, and KeeyKey.
Here are the C# Hardwarewallet libraries that I have built. They are all based on Usb.Net and Hid.Net.
In order to integrate Hardwarewallets with the blockchain, I have developed Cryptocurrency.Net.