将. net对象绑定到Crystal Report，但没有找到属性

我有一个叫做Letter的类，它有两个公共属性Letter_Header和Letter_Details。我已经成功地绑定了一个。net Linq对象到水晶报告，但是当我绑定这个对象的属性没有被检测到。我如何将Letter_Header和Letter_Details绑定到报告?

public class Letter
{
    public Letter_Header _header;
    public List<Letter_Details> _details;
    public Letter()
    {
    }
}

public class Letter_Header
{
    public Int32 ProductID;
    public String ProductName;
    public String BrokerName;

}
public class Letter_Details
{
    public Int32 ID;
    public Int32 PolicyID;
    public Int32 PersonName;
    public Decimal Amount;
    public DateTime DateTime;

}