c# - 带别名的 Linq

Question

我在 c# 中有以下行：

var name = (from x in db.authors
                    where fullName == "Jean Paul Olvera"
                    orderby x.surname
                    select new { x.id_author, fullName= String.Concat(x.name," ", x.surname) });

我的问题是我想在 where 子句中使用别名，但我不能，'fullName' 显示为未声明。

Answer 1

您可以使用let创建中间值：

var name = (from x in db.authors
            let fullName = x.name + " " + x.surname
            where fullName == "Jean Paul Olvera"
            orderby x.surname
            select new { x.id_author, fullName });

Answer 2

您需要更早地放置投影的那一部分，这很容易使用let子句：

var name = from x in db.authors
           let fullName = x.name + " " + x.surname
           where fullName == "Jean Paul Olvera"
           orderby x.surname
           select new { x.id_author, fullName };

请注意，x.name + " " + x.surname它将被编译为与 相同的代码String.Concat(x.name, " ", x.surname)，但对大多数人来说更具可读性。另请注意，由于您没有在()括号之外做任何事情，因此不需要它们。

我希望任何优秀的 SQL LINQ 提供程序都应该将此查询转换为明智且高效的 SQL 查询，但您应该自己验证这一点。另一方面，我通常建议优先查询单个字段，例如

where x.name == "Jean Paul" && x.surname == "Olvera"

Answer 3

你还没有创建它。

话虽如此，由于您事先知道名称，因此您应该能够对零件执行查询：

var name = from x in db.authors
                where name == "Jean Paul" && surname == "Olvera"
                orderby x.surname
                select new { x.id_author, fullName= String.Concat(x.name," ", x.surname) };

否则，您可以使用它let来执行此操作：

var name = from x in db.authors
           let fullName = String.Concat(x.name," ", x.surname)
           where fullName == "Jean Paul Olvera"
           orderby x.surname
           select new { x.id_author, fullName=fullName ) };

Answer 4

方法语法更容易，因为您不受操作顺序的限制：

var query = authors.OrderBy(x => x.surname)
    .Select(x => new
    {
        x.id_author,
        fullName = String.Concat(x.name, " ", x.surname)
    })
    .Where(x => x.fullName == "Jean Paul Olvera");

Answer 5

linq1DataContext ll = new linq1DataContext();
            if (comboBox1.SelectedIndex == 0)
            {

                var q = from m in ll.personals                            



                        let کد= m.id
                        let نام = m.name

                        select new { 
                                     کد,
                                     نام,
                        };
                dataGridView1.DataSource = q;
            }

Answer 6

使用let子句：

var name = (from x in db.authors
                let fullName = String.Concat(x.name," ", x.surname)
                where fullname = "Jean Paul Olvera"
                orderby x.surname
                select new { x.id_author, fullName });