将Linq转换为实体中的字符串的问题

Question

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
            };

无论如何我能做到这一点吗？注意,在VB.NET中没有问题使用它工作得很好的第一个片段,VB很灵活,我无法习惯C#的严格!

Answer 1

使用EF v4,您可以使用SqlFunctions.StringConvert.int没有重载,因此您需要强制转换为double或decimal.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
            };

Answer 2

我通过将整数转换为查询字符串来解决类似的问题.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 });
}

Answer 3

使用LinqToObject:联系人.AsEnumerable()

var items = from c in contacts.AsEnumerable()
            select new ListItem
            {
                Value = c.ContactId.ToString(),
                Text = c.Name
            };

Answer 4

SqlFunctions.StringConvert将工作,但我发现它很麻烦,而且大多数时候,我没有真正需要在SQL端执行字符串转换.

如果我想做字符串操作,我首先在linq-to-entities中执行查询,然后在linq-to-objects中操作stings.在这个例子中,我想获得一组包含Contact的全名和ContactLocationKey的数据,ContactLocationKey是两个Integer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();

现在,我承认必须编写两个匿名选项确实很麻烦,但我认为,你可以方便地执行L2E中不支持的字符串(和其他)功能.另请注意,使用此方法可能会降低性能.