【User】How to check if a user name belongs to a domain

 using System.DirectoryServices.AccountManagement;...try{    var principalContext = new PrincipalContext(ContextType.Domain, domain);    using (        var user = UserPrincipal.FindByIdentity(            principalContext, IdentityType.SamAccountName, account))    {        if (user == null)        {            ModelState.AddModelError("UserName","UserNotExistInAD");        }    }}catch (PrincipalServerDownException){    ModelState.AddModelError("UserName", "DomainNotExistOrNotAvailable");}catch (PrincipalException){    ModelState.AddModelError("UserName", "UserNotExistInAD");}