C# 带有别名的 Linq
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Linq with alias
提问by Jean Paul Olvera
I have the following line in c#:
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) });
my problem is I want to use the alias in my where clause, but I can't, 'fullName' appears as not declared.
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。
采纳答案by cdhowie
You can use letto create intermediate values:
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 });
回答by Reed Copsey
You haven't created it yet.
你还没有创建它。
That being said, since you know the name in advance, you should be able to perform the query on the parts:
话虽如此，既然您事先知道名称，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) };
Otherwise, you can use letto do this:
否则，您可以使用以下let方法执行此操作：
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 ) };
回答by Jon Skeet
You need to put that part of the projection earlier, which is easy with a letclause:
您需要提前放置投影的那部分，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 };
Note that x.name + " " + x.surnamewill be compiled to the same code as String.Concat(x.name, " ", x.surname), but is more readable to most people. Also note that as you're not doing anything outside the ()parentheses, there's no need for them.
请注意，x.name + " " + x.surname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()括号外执行任何操作，因此不需要它们。
I would hopethat any good SQL LINQ provider should turn this query into a sensible and efficient SQL query, but you should validate this yourself. On the other hand, I would generallysuggest preferring querying over individual fields, e.g.
我希望任何好的 SQL LINQ 提供程序都应该把这个查询变成一个明智和高效的 SQL 查询，但你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"
回答by D Stanley
use the letclause:
使用let条款：
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 });
回答by Servy
It's easier in the method syntax, as you aren't constrained to the order of the operations:
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");
回答by hadiz7
linq1DataContext ll = new linq1DataContext();
if (comboBox1.SelectedIndex == 0)
{
var q = from m in ll.personals
let ??= m.id
let ??? = m.name
select new {
??,
???,
};
dataGridView1.DataSource = q;
}
