LINQ的别名为

Question

我在C＃中的以下行：

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) }); 

我的问题是我想使用的别名在我的where子句中，但我不能，“全名”显示为不可声明。

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

你还没有创建它。

话虽这么说，因为你事先知道名字，你应该能够对各部分进行查询：

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做到这一点：

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 3

你需要把前面投影的那部分，这是很容易与let clause：

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将被编译成相同的代码为String.Concat(x.name, " ", x.surname)，但更可读的大多数人。还要注意，因为你没有做任何事情，所以不需要它们。

我会希望任何好的SQL LINQ提供程序都应该将此查询转换为合理有效的SQL查询，但是您应该自己验证它。另一方面，我会通常建议宁愿查询单个字段，例如，

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

Answer 4

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 }); 

Answer 5

它在方法的语法更容易，因为你不会受限于操作的顺序：

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 6

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; 
      }