UserPrincipal.GetAuthorizationGroups抛出异常

Question

在以下代码中

PrincipalContext* context = new PrincipalContext(ContextType::Domain, domainName);
UserPrincipal* user = UserPrincipal::FindByIdentity(context, IdentityType::SamAccountName, userName);
PrincipalSearchResult<Principal*>* groups = user->GetAuthorizationGroups();

我得到以下异常：

  

消息：对象引用未设置为对象的实例。

异常调用堆栈：

 at System.DirectoryServices.AccountManagement.Principal.CheckDisposedOrDeleted()
 at System.DirectoryServices.AccountManagement.UserPrincipal.GetAuthorizationGroupsHelper()
 at System.DirectoryServices.AccountManagement.UserPrincipal.GetAuthorizationGroups()

知道内部方法失败的原因吗？如何删除或处置用户，或者发生了什么？