C# 将Linq中的int转换为字符串到实体的问题
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Problem with converting int to string in Linq to entities
提问by Shimmy Weitzhandler
var items = from c in contacts
select new ListItem
{
Value = c.ContactId, //Cannot implicitly convert type 'int' (ContactId) to 'string' (Value).
Text = c.Name
};
var items = from c in contacts
select new ListItem
{
Value = c.ContactId.ToString(), //Throws exception: ToString is not supported in linq to entities.
Text = c.Name
};
Is there anyway I can achieve this? Note, that in VB.NET there is no problem use the first snippet it works just great, VB is flexible, im unable to get used to C#'s strictness!!!
无论如何我可以做到这一点吗？请注意，在 VB.NET 中使用第一个代码段没有问题，它很好用，VB 很灵活，我无法适应 C# 的严格性！！！
采纳答案by Brian Cauthon
With EF v4 you can use SqlFunctions.StringConvert. There is no overload for int so you need to cast to a double or a decimal. Your code ends up looking like this:
使用 EF v4，您可以使用SqlFunctions.StringConvert. int 没有重载，因此您需要转换为双精度或小数。您的代码最终看起来像这样：
var items = from c in contacts
select new ListItem
{
Value = SqlFunctions.StringConvert((double)c.ContactId).Trim(),
Text = c.Name
};
回答by Tony Heupel
Can you try:
你能试一下吗：
var items = from c in contacts
select new ListItem
{
Value = Convert.ToString(c.ContactId),
Text = c.Name
};
回答by Mcbeev
My understanding is that you have to create a partial class to "extend" your model and add a property that is readonly that can utilize the rest of the class's properties.
我的理解是您必须创建一个分部类来“扩展”您的模型并添加一个只读属性，该属性可以利用该类的其余属性。
public partial class Contact{
public string ContactIdString
{
get{
return this.ContactId.ToString();
}
}
}
Then
然后
var items = from c in contacts
select new ListItem
{
Value = c.ContactIdString,
Text = c.Name
};
回答by Nestor
public static IEnumerable<SelectListItem> GetCustomerList()
{
using (SiteDataContext db = new SiteDataContext())
{
var list = from l in db.Customers.AsEnumerable()
orderby l.CompanyName
select new SelectListItem { Value = l.CustomerID.ToString(), Text = l.CompanyName };
return list.ToList();
}
}
回答by BarryC
I ran into this same problem when I was converting my MVC 2 app to MVC 3 and just to give another (clean) solution to this problem I want to post what I did...
我在将 MVC 2 应用程序转换为 MVC 3 时遇到了同样的问题，只是为了给这个问题提供另一个（干净的）解决方案，我想发布我所做的......
IEnumerable<SelectListItem> producers = new SelectList(Services.GetProducers(),
"ID", "Name", model.ProducerID);
GetProducers() simply returns an entity collection of Producers. P.S. The SqlFunctions.StringConvert didn't work for me.
GetProducers() 仅返回生产者的实体集合。PS SqlFunctions.StringConvert 对我不起作用。
回答by Walter Stabosz
SqlFunctions.StringConvert will work, but I find it cumbersome, and most of the time, I don't have a real need to perform the string conversion on the SQL side.
SqlFunctions.StringConvert 可以工作，但我觉得它很麻烦，而且大多数时候，我没有真正需要在 SQL 端执行字符串转换。
What I do if I want to do string manipulations is perform the query in linq-to-entities first, then manipulate the stings in linq-to-objects. In this example, I want to obtain a set of data containing a Contact's fullname, and ContactLocationKey, which is the string concatination of two Integer columns (ContactID and LocationID).
如果我想进行字符串操作，我要做的是首先在 linq-to-entities 中执行查询，然后在 linq-to-objects 中操作字符串。在这个例子中，我想获取一组包含联系人全名和 ContactLocationKey 的数据，它是两个整数列（ContactID 和 LocationID）的字符串连接。
// perform the linq-to-entities query, query execution is triggered by ToArray()
var data =
(from c in Context.Contacts
select new {
c.ContactID,
c.FullName,
c.LocationID
}).ToArray();
// at this point, the database has been called and we are working in
// linq-to-objects where ToString() is supported
// Key2 is an extra example that wouldn't work in linq-to-entities
var data2 =
(from c in data
select new {
c.FullName,
ContactLocationKey = c.ContactID.ToString() + "." + c.LocationID.ToString(),
Key2 = string.Join(".", c.ContactID.ToString(), c.LocationID.ToString())
}).ToArray();
Now, I grant that it does get cumbersome to have to write two anonymous selects, but I would argue that is outweighed by the convenience of which you can perform string (and other) functions not supported in L2E. Also keep in mind that there is probably a performance penalty using this method.
现在，我承认必须编写两个匿名选择确实很麻烦，但我认为这与您可以执行 L2E 不支持的字符串（和其他）函数的便利性相比更重要。还要记住，使用这种方法可能会降低性能。
回答by phil hong
var selectList = db.NewsClasses.ToList<NewsClass>().Select(a => new SelectListItem({
Text = a.ClassName,
Value = a.ClassId.ToString()
});
Firstly, convert to object, then toString() will be correct.
首先，转换为对象，然后 toString() 将是正确的。
回答by Jente Rosseel
I solved a similar problem by placing the conversion of the integer to string out of the query. This can be achieved by putting the query into an object.
我通过将整数到字符串的转换放在查询之外解决了类似的问题。这可以通过将查询放入对象来实现。
var items = from c in contacts
select new
{
Value = c.ContactId,
Text = c.Name
};
var itemList = new SelectList();
foreach (var item in items)
{
itemList.Add(new SelectListItem{ Value = item.ContactId, Text = item.Name });
}
回答by Nawaz
If your "contact" is acting as generic list, I hope the following code works well.
如果您的“联系人”充当通用列表，我希望以下代码运行良好。
var items = contact.Distinct().OrderBy(c => c.Name)
.Select( c => new ListItem
{
Value = c.ContactId.ToString(),
Text = c.Name
});
Thanks.
谢谢。
回答by Ken Blackford
var items = from c in contacts
select new ListItem
{
Value = String.Concat(c.ContactId), //This Works in Linq to Entity!
Text = c.Name
};
I found that SqlFunctions.StringConvert((double)c.Age)did not work for me either the field is of type Nullable<Int32>
我发现SqlFunctions.StringConvert((double)c.Age)该字段对我不起作用Nullable<Int32>
Took me a lot of searching over the last few days of trial and error to find this.
在过去几天的反复试验中，我花了很多时间搜索才找到这个。
I hope this helps a few coders out there.
我希望这对那里的一些编码人员有所帮助。
