Validating contact selector control programmatically

How to know if contact selector is empty?

XpathNavigator selector = this.CreateNavigator().SelectSingleNode("/my:myFields/my:contactselector", this.NamespaceManager);

if (selector.SelectChildren(XPathNodeType.Element).Count == 0)
{
this.Errors.Add(selector, "Error","Selector is empty");

}

How to raise error if user has selected more than one person in the selector?

if (selector.SelectChildren(XPathNodeType.Element).Count > 1)
{
this.Errors.Add(selector, "Error","Please select one user");

}

How to tell if user hasn't selected a valid person?

  selector = selector.SelectSingleNode("my:Person", this.NamespaceManager);

XPathNavigator displayname = this.CreateNavigator().SelectSingleNode("/my:myFields/my:contactselector/my:Person/my:DisplayName", this.NamespaceManager);
string strDisplayName = displayname.value.toString();

XPathNavigator accountid = this.CreateNavigator().SelectSingleNode("/my:myFields/my:contactselector/my:Person/my:AccountId", this.NamespaceManager);
string strAccountid = accountid.value.toString();

XPathNavigator accounttype = this.CreateNavigator().SelectSingleNode("/my:myFields/my:contactselector/my:Person/my:AccountType", this.NamespaceManager);
string strAccountType = accounttype.value.toString();

                    if (string.IsNullOrEmpty(strDisplayName)) ||
                        string.IsNullOrEmpty(strAccountId)) ||
                        string.IsNullOrEmpty(strAccountType))
                    {
                         this.Errors.Add(selector,"Error","Please select valid user");
                    }

Note: This was off the top of my head, you may find minor mistakes in XPaths or spellings.