Cannot save entity data structure to session!! What happen?

[ABC]

class PremiumControl

{

DataTable DefaultView;

LoadData() ............

SaveData() ...................

......................

}

private class PageSessionClass

{

public PremiumControl EntityMgr = null;

public PageSessionClass()

{

// initial entitymgr

}

}

private PageSessionClass MySessionData = new PageSessionClass();

Session["MYSESSION"] = MySessionData;

When I load it from session,

MySessionData = (PageSessionClass) Session["MYSESSION"];

I found the DefaultView dataTable is null. I checked the DefaultView
datatable has records when saved. What happen?