Search User Name in Active Directory using Domain

string Addomain = System.Configuration.ConfigurationSettings.AppSettings["Domain"];
// create your domain context
PrincipalContext domain = new PrincipalContext(ContextType.Domain, Addomain);

// find the user
UserPrincipal foundUser = UserPrincipal.FindByIdentity(domain, username);

if (foundUser != null)
{
   name = foundUser.Name;
}