SharpIs 
WHOIS lookups for C#
How do I use it
Just reference the Shapis.dll and add something like the following:
WhoisRecord rec = WhoisLookup.Lookup("jamiehankins.co.uk");
You can then access the returned information as an ArrayList which can be looped through like the following
foreach (var v in rec.Text) { Console.WriteLine(v); }
You can see which WHOIS server was queried by looking at the Server string of the record
Console.WriteLine(rec.Server);
How does this work?
This does a DNS query to a subdomain of whois-servers.net. The subdomain is based off the TLD of the domain being queried. Say I wanted to query google.com, The library gets the TLD and does a DNS lookup of com.whois-servers.net which returns
Name: whois.verisign-grs.com
We can then connect to this using a TCP Client and lookup the domains information which is returned as a WhoisRecord