var query = from p in list
                    group p by new
                    {
                         //？？？
//例如下拉列表中的值ddl1.SelectValue为"Purchaser"，如何写成p.Purchaser,其他同理
                          //p.Purchaser,
                         //p.Seller,
                         //p.Producer
                     }
                    into g
                    select new
                    {
                       //???
                      //如何列出购买者的姓名和购买数量，其他同理                     
                         //g.Key,
                       // Count = g.Count()                     
                    };

private static object GetPropertyValue(object obj, string property)
    {
        System.Reflection.PropertyInfo propertyInfo = obj.GetType().GetProperty(property);
        return propertyInfo.GetValue(obj, null);
    }

var result = from a in list.GroupByMany("A", "B")
             from b in a.SubGroups
             orderby b.Count descending
             select new
             {
                 Key1 = a.Key ?? "@", //当a.Key为空时，转成@
                 Key2 = b.Key ?? "$", //当b.Key为空时，转成$
                 Count = b.Count
             };