Linq与别名

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

你需要提前放置那部分投影,这很容易用一个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将编译为相同的代码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有效的SQL查询,但您应该自己验证.另一方面,我通常建议更喜欢查询单个字段,例如

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

Answer 3

你还没有创建它.

话虽如此,因为您事先知道了名称,所以您应该能够对部件执行查询:

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let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");