Built-in helper to parse User.Identity.Name into Domain\Username

This is better (easier to use, no opportunity of NullReferenceExcpetion and conforms MS coding guidelines about treating empty and null string equally):

public static class Extensions
{
    public static string GetDomain(this IIdentity identity)
    {
        string s = identity.Name;
        int stop = s.IndexOf("\\");
        return (stop > -1) ?  s.Substring(0, stop) : string.Empty;
    }

    public static string GetLogin(this IIdentity identity)
    {
        string s = identity.Name;
        int stop = s.IndexOf("\\");
        return (stop > -1) ? s.Substring(stop + 1, s.Length - stop - 1) : string.Empty;
    }
}

Usage:

IIdentity id = HttpContext.Current.User.Identity;
id.GetLogin();
id.GetDomain();

This requires C# 3.0 compiler (or newer) and doesn't require 3.0 .Net for working after compilation.

System.Environment.UserDomainName gives you the domain name only

Similarly, System.Environment.UserName gives you the user name only

var components = User.Identity.Name.Split('\\');

var userName = components.Last() 

var domainName = components.Reverse().Skip(1).FirstOrDefault()

You guys might also consider parsing a string input like "[email protected]", or "user@domain".

This is what I'm currently doing:
If string contains '\' then split string at '\' and extract username and domain
Else If string contains '@' then split string at '@' and extract username and domain
Else treat string as username without a domain

I'm still hunting for a better solution in the case where the input string isn't in an easily predicted format, i.e. "domain\user@domain". I'm thinking RegEx...

Update: I stand corrected. My answer is a bit of out context, it refers to the general case of parsing username and domains out of user input, like in user login/logon prompt. Hope it still helps someone.