Question

var result = (
    from contact in db.Contacts                             
    join user in db.Users on contact.CreatedByUserID equals user.UserID
    orderby contact.ContactID descending
    select new ContactListView
    {
        ContactID = contact.ContactID,
        FirstName = contact.FirstName,
        LastName = contact.LastName,
        Company = (
            from field in contact.XmlFields.Descendants("Company")
            select field.Value).SingleOrDefault().ToString()
    }).Take(10);

Here我描述了数据库表的外观。因此，contacts表有一个xml类型的字段。在该字段中存储公司文件名，我需要阅读它。我用这种方式试了一下：

Company = (
    from field in contact.XmlFields.Descendants("Company")
    select field.Value).SingleOrDefault().ToString()

但我收到以下错误：

成员访问'System.String Value' 'System.Xml.Linq.XElement'不合法在类型上 “System.Collections.Generic.IEnumerable`1 [System.Xml.Linq.XElement]。

对此有何解决方案？

提前致谢，
ILE

Answer 1

这里的问题是LINQ to SQL正在尝试将Descendants扩展方法和XElement.Value转换为SQL，但它当然会失败。您将不得不使用LINQ to Objects进行最后一次转换。这可行：

var temp = (
    from contact in db.Contacts                             
    join user in db.Users on contact.CreatedByUserID equals user.UserID
    orderby contact.ContactID descending
    select new
    {
        contact.ContactID, contact.FirstName, contact.LastName, contact.XmlFields
    })
    .Take(10);

var tempArray = temp.ToArray();

IEnumerable<ContactListView> result =
    from contact in tempArray
    let company =
        (from field in contact.XmlFields.Descendants("Company")
         select field.Value).SingleOrDefault()
    select new ContactListView()
    {
        ContactID = contact.ContactID,
        FirstName = contact.FirstName,
        LastName = contact.LastName,
        Company = company == null ? null : company.ToString()
    }).Take(10);

嵌套的linq-to-sql查询

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