Nhibernate使用HQL返回自定义类型

摘要：使用HQL返回自定义类型的结果集。

前提：已经定义好Customer类及其Nhibernate映射文件xml。

public IList<CustomerFirstnameCounter> GetCustomerFirstnameCounts()
{
ISession session = GetSession();
return session.CreateQuery(@"select new CustomerFirstnameCounter(c.Firstname,count(c.Firstname))
from Customer c group by c.Firstname")
.List<CustomerFirstnameCounter>();
}

NHibernate: select customer0_.Firstname as col_0_0_, count(customer0_.Firstname) as col_1_0_ from Customer customer0_ group by customer0_.Firstname

CustomerFirstnameCounter中的Count属性必须定义为long类型，如果定义为int类型，则会报如下错误：

Test method DataAccessLayer.Test.NhibernateDataProviderTest.GetCustomerFirstnameCountsTest threw exception:
NHibernate.QueryException: Unable to locate appropriate constructor on class [DataTransfer.CustomerFirstnameCounter, DataTransfer] [select new CustomerFirstnameCounter(c.Firstname,count(c.Firstname))
from DataTransfer.Customer c group by c.Firstname] ---> NHibernate.InstantiationException: no constructor compatible with (System.String, System.Int64) found in class: DataTransfer.CustomerFirstnameCounter