带别名的Linq

本文关键字：Linq 别名 | 更新日期: 2023-09-27 18:08:30

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) });

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

带别名的Linq

您可以使用let来创建中间值:

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 });

您需要更早地放置投影的这一部分，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 };

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

我希望任何好的SQL LINQ提供程序都应该将此查询转换为合理且高效的SQL查询，但您应该自己验证这一点。另一方面，我会通常建议首选查询而不是单个字段，例如

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

您还没有创建。

话虽这么说，由于您提前知道了名称，因此您应该能够对以下部分执行查询:

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let来完成:

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 ) };

在方法语法中更容易，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");

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;
            }

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 });
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