CriteriaOperator criteria = 
    CriteriaOperator.Parse("[<Orders>][^.EmployeeID = EmployeeID].Count() > 50");
XPCollection<Employee> employees = new XPCollection<Employee>(session, criteria);


[PersistentAlias("[<First>][SomeUniqueNumber=^.TheSameUniqueNumber].Max(NameOfFirstObject)")]  
public string MyRelatedFirstObjectName { get { return Convert.ToString(EvaluateAlias("MyRelatedFirstObjectName")); } }