How to get MAC address of client machine in c#

No.
Well - there is, it's just that it will only work in a tiny, tiny minority of the clients: they must be running IE, IE must be set to accept and run ActiveX controls, and you need to write and send an ActiveX control to your client.
But...
1) Most people don't use IE.
2) IE default setting is not to accept or run ActiveX controls
3) Most users are very wary of ActiveX because it is wide open for abuse.

So it'll work in maybe 0.01% of your visitors?

MAC addresses do not leave the local LAN segment - they are never passed to the next segment or the internet as they are already replaced by the local IP address.

And if you are thinking of using them for security, forget it: they aren't unique (only unique within a lan segment) and they are unbelievably easy to spoof.