LINQ的別名爲

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) }); 

我的問題是我想使用的別名在我的where子句中，但我不能，「全名」顯示爲不可聲明。

Answer 1

您可以使用let創建中間值：

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

你還沒有創建它。

話雖這麼說，因爲你事先知道名字，你應該能夠對各部分進行查詢：

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) }; 

否則，您可以使用let做到這一點：

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，這是很容易與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將被編譯成相同的代碼爲String.Concat(x.name, " ", x.surname)，但更可讀的大多數人。還要注意，因爲你沒有做任何事情，所以不需要它們。

我會希望任何好的SQL LINQ提供程序都應該將此查詢轉換爲合理有效的SQL查詢，但是您應該自己驗證它。另一方面，我會通常建議寧願查詢單個字段，例如，

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

Answer 4

使用let條款：

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

它在方法的語法更容易，因爲你不會受限於操作的順序：

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; 
      }