IPrincipal principal = Thread.CurrentPrincipal;
IIdentity identity = principal == null ? null : principal.Identity;
string name = identity == null ? "" : identity.Name;

Thread.CurrentPrincipal = new WindowsPrincipal(WindowsIdentity.GetCurrent());

string userName = "Fred"; // todo
string[] roles = { "User", "Admin" }; // todo
Thread.CurrentPrincipal = new GenericPrincipal(new GenericIdentity(userName), roles);

string user = System.Security.Principal.WindowsIdentity.GetCurrent().Name ;