C#Select子句返回系统异常,而不是相关的对象 [英] C# Select clause returns system exception instead of relevant object
问题描述
我试图使用SELECT子句中挑选出从数据库查询符合指定的name字段的对象如下:
的ObjectQuery =从OBJ在链表类
,其中obj.Equals(对象名)
选择OBJ;
在我查询的结果来看,我得到:
{基地System.SystemException} = {布尔等于(System.Object的)}
在哪里我应该期待像一个车
,制作
或模式
会有人请解释什么,我做错了什么?
< HR>
在讨论的方法,可以在这里看到:
//这个功能搜索数据库的表中的名与对象名
公共静态T上读<属性相匹配的单个对象; T>(字符串对象名),其中T:IEquatable< T>
{使用(ISession的会话= NHibernateHelper.OpenSession())
{
&IQueryable的LT
; T>链表类= session.Query< T>(); //拉(查询)所有从数据库中的
诠释计数= objectList.Count表的对象(); //返回在表
的对象的数目//替代:整数计数= makeList.Count&所述; T>();
&IQueryable的LT; T>的ObjectQuery = NULL; //创建(T)的对象$ B $(B T)= foundObject默认我们的可查询列表的引用; //为我们找到的对象
的对象引用,如果(计数大于0)
{
//给我说有一个匹配名称的所有对象的对象名和它们存储在链表类
'的ObjectQuery'
的ObjectQuery =从OBJ,其中obj.Equals(对象名)
选择OBJ;
//确保的ObjectQuery'只有一个对象是
试
{
foundObject =(T)objectQuery.Single();
}
抓
{
返回默认值(T);
}
//输出一些信息到控制台(屏幕输出)
Console.WriteLine(读做:+ foundObject.ToString());
}
//传给谁要求它找到对象的引用
返回foundObject;
}
}
请注意,我现在用的接口 IQuatable< T>
在我的方法描述符
我试图拉从数据库中的类的实例:
公共类品牌:IEquatable<使>
{
公共虚拟INT标识{搞定;组; }
公共虚拟字符串名称{;组; }
公共虚拟的IList<模型与GT;模型{搞定;组; }
公众制作()
{
//需要此公无参数的构造函数NHibernate的
}
公开制作(串makeName)
{
this.Name = makeName;
}
公共重写字符串的ToString()
{
返回名称;
}
// IEquatable<的执行情况; T>接口
公共虚拟BOOL等于(请MAKE)
{
如果(this.Id == make.Id)
{
返回真;
}
,否则
{
返回FALSE;
}
}
// IEquatable<的执行情况; T>接口
公共虚拟BOOL等于(字符串名称)
{
如果(this.Name.Equals(名))
{
返回真;
}
,否则
{
返回FALSE;
}
}
}
和接口被简单地描述如:
公共接口IEquatable< T>
{
布尔等于(T OBJ);
}
IQueryable的< T>
执行你对后备数据存储查询(在这种情况下,它的SQL RDBMS)。您的SQL RDBMS没有的 IEquatable<理念; T>
* ,而不能使用它的实现:查询功能必须是翻译到SQL和< 。code> obj.Equals(对象名)是不可翻译
您可以将的IQueryable< T>
到的IEnumerable< T>
并做内存中的查询,但是这将是效率太低。你应该更改签名采取表达式来; Func键< TSource,布尔>>
谓词,名称检查传递给它:
公共静态T上读< T>(表达式来; Func键< T,BOOL>>预计值){
使用(ISession的会话= NHibernateHelper.OpenSession() ){
返回session.Query< T>()的SingleOrDefault(pred的);
}
}
您现在可以使用此方法如下:
让蛇=读<使>(X => x.Name ==蛇);
* 此外,你的
IEquatable< T>
是不是在你所展示的方法应用于:等于(字符串)
方法将有资格作为一种实现的 IEquatable<字符串方式>
,但它不是在你的制作
类实现的接口列表中提及的p> I am trying to use the select clause to pick out an object which matches a specified name field from a database query as follows:
objectQuery = from obj in objectList
where obj.Equals(objectName)
select obj;
In the results view of my query, I get:
base {System.SystemException} = {"Boolean Equals(System.Object)"}
Where I should be expecting something like a Car
, Make
, or Model
Would someone please explain what I am doing wrong here?
The method in question can be seen here:
// this function searches the database's table for a single object that matches the 'Name' property with 'objectName'
public static T Read<T>(string objectName) where T : IEquatable<T>
{
using (ISession session = NHibernateHelper.OpenSession())
{
IQueryable<T> objectList = session.Query<T>(); // pull (query) all the objects from the table in the database
int count = objectList.Count(); // return the number of objects in the table
// alternative: int count = makeList.Count<T>();
IQueryable<T> objectQuery = null; // create a reference for our queryable list of objects
T foundObject = default(T); // create an object reference for our found object
if (count > 0)
{
// give me all objects that have a name that matches 'objectName' and store them in 'objectQuery'
objectQuery = from obj in objectList
where obj.Equals(objectName)
select obj;
// make sure that 'objectQuery' has only one object in it
try
{
foundObject = (T)objectQuery.Single();
}
catch
{
return default(T);
}
// output some information to the console (output screen)
Console.WriteLine("Read Make: " + foundObject.ToString());
}
// pass the reference of the found object on to whoever asked for it
return foundObject;
}
}
Note that I am using the interface "IQuatable<T>
" in my method descriptor.
An example of the classes I am trying to pull from the database is:
public class Make: IEquatable<Make>
{
public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual IList<Model> Models { get; set; }
public Make()
{
// this public no-argument constructor is required for NHibernate
}
public Make(string makeName)
{
this.Name = makeName;
}
public override string ToString()
{
return Name;
}
// Implementation of IEquatable<T> interface
public virtual bool Equals(Make make)
{
if (this.Id == make.Id)
{
return true;
}
else
{
return false;
}
}
// Implementation of IEquatable<T> interface
public virtual bool Equals(String name)
{
if (this.Name.Equals(name))
{
return true;
}
else
{
return false;
}
}
}
And the interface is described simply as:
public interface IEquatable<T>
{
bool Equals(T obj);
}
IQueryable<T>
executes your query against the backing data store (in this case, it's SQL RDBMS). Your SQL RDBMS has no idea of IEquatable<T>
*, and cannot use its implementation: the query function must be translatable to SQL, and obj.Equals(objectName)
is not translatable.
You can convert IQueryable<T>
to IEnumerable<T>
and do the query in memory, but that would be too inefficient. You should change the signature to take a Expression<Func<TSource, bool>>
predicate, and pass the name checker to it:
public static T Read<T>(Expression<Func<T,bool>> pred) {
using (ISession session = NHibernateHelper.OpenSession()) {
return session.Query<T>().SingleOrdefault(pred);
}
}
You can now use this method as follows:
Make snake = Read<Make>(x => x.Name == "snake");
* Additionally, your
IEquatable<T>
is not used in the method that you are showing: the Equals(string)
method would qualify as an implementation of IEquatable<string>
, but it is not mentioned in the list of interfaces implemented by your Make
class.
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