NHibernate error Reason: Invalid Cast (check your mapping for property type mismatches)

Many people used the nhibernate to operate a one-to-many relationship, using <set> tags, such as:
<set name= "Students",
  <key column= "CId"/
  <one-to-many class= "Nhstusmanagerapp.student"/>
</set>
for use in the corresponding class:
public class classes{
  private , ISet <Student> _students;
  Public virtual  ISet <Student> Students
  {
     get {return _students;}
     set {_students = value;}
 }
}
looks normal, but the runtime will have an error as follows: cannot type ' NHibernate.Collection.Generic.PersistentGenericSet ' 1[ Nhstusmanagerapp.student] "to cast the object to type ' System.Collections.Generic.ISet ' 1[nhstusmanagerapp.student]".

The reason for the error is obviously the type of mistake, that is, the set tag in the configuration file tells NHibernate to use the Set set property in the entity class to load the associated data (e.g. 50 students in a class), and the students attribute in class class is used to store the learner, But you should use this at this time:
Iesi.Collections.Generic ISet, the Iesi.Collections.dll assembly needs to be added.
But if it is not added, it is using the ISetunder System.Collections.Generic .
So there is an error that cannot be converted.

Solution: Use System.Collections.Generic in the class; Comment out, add reference: Using Iesi.Collections.Generic;