Question

您好我正在通过MS 101 linq示例编写代码。

“JoinOperators”让我很难过，因为我试图将查询表达式重构为lambda语法，反之亦然。

无论如何，在示例105上我看到了这个查询表达式：

var supplierCusts =
    from sup in suppliers
    join cust in customers on sup.Country equals cust.Country into cs
    from c in cs.DefaultIfEmpty()  // DefaultIfEmpty preserves left-hand elements that have no matches on the right side 
    orderby sup.SupplierName
    select new
    {
        Country = sup.Country,
        CompanyName = c == null ? "(No customers)" : c.CompanyName,
        SupplierName = sup.SupplierName
    };

我尝试以这种方式将其实现为lambda：

// something is not right here because the result keeps a lot of "Join By" stuff in the output below
var supplierCusts =
    suppliers.GroupJoin(customers, s => s.Country, c => c.Country, (s, c) => new { Customers = customers, Suppliers = suppliers })
        .OrderBy(i => i.Suppliers)    // can't reference the "name" field here?
        .SelectMany(x => x.Customers.DefaultIfEmpty(), (x, p) =>    // does the DefaultIfEmpty go here?
            new
            {
                Country = p.Country,
                CompanyName = x == null ? "(No customers)" : p.CompanyName,
                SupplierName = p    // not right: JoinOperators.Program+Customer ... how do I get to supplier level?
            });

出于某种原因，我无法以这种方式访问供应商级别的信息。当我与供应商切换客户时，我无法访问客户级信息。

是否有一些SelectMany超载让我从两个对象的字段级别拉出来？

另外，我不明白为什么GroupJoin似乎返回一个包含2个集合（供应商和客户）的对象。是不是应该以某种方式加入他们？

我想我不明白GroupJoin的工作原理。

Answer 1

组连接中有错误的结果选择器，这就是问题开始的地方。这是固定查询：

var supplierCusts =
   suppliers
     .GroupJoin(customers, 
                sup => sup.Country, 
                cust => cust.Country, 
                (sup, cs) => new { sup, cs })
     .OrderBy(x => x.sup.Name)    
     .SelectMany(x => x.cs.DefaultIfEmpty(), (x, c) =>
        new
        {
            Country = x.sup.Country,
            CompanyName = c == null ? "(No customers)" : c.CompanyName,
            SupplierName = x.sup.Name   
        });

Answer 2

如果你想学习将查询表达式翻译成lambda，我建议你查看LinqPad，默认情况下可以这样做。例如，您的查询翻译如下：

    Suppliers
   .GroupJoin (
      Customers, 
      sup => sup.Country, 
      cust => cust.Country, 
      (sup, cs) => 
         new  
         {
            sup = sup, 
            cs = cs
         }
   )
   .SelectMany (
      temp0 => temp0.cs.DefaultIfEmpty (), 
      (temp0, c) => 
         new  
         {
            temp0 = temp0, 
            c = c
         }
   )
   .OrderBy (temp1 => temp1.temp0.sup.CompanyName)
   .Select (
      temp1 => 
         new  
         {
            Country = temp1.temp0.sup.Country, 
            CompanyName = (temp1.c == null) ? "(No customers)" : temp1.c.CompanyName, 
            SupplierName = temp1.temp0.sup.CompanyName
         }
   )

话虽这么说，我通常会发现使用查询语法而不是lambda语法更容易编码和维护SelectMany。

此示例中的GroupJoin用于完成左连接（通过.DefaultIfEmpty子句）。

Answer 3

试试这个：

var supplierCusts =
    suppliers.GroupJoin(customers, s => s.Country, c => c.Country, (s, c) => new { Supplier = s, Customers = c })
        .OrderBy(i => i.Supplier.SupplierName)
        .SelectMany(r => r.Customers.DefaultIfEmpty(), (r, c) => new
        {
            Country = r.Supplier.Country,
            CompanyName = c == null ? "(No customers)" : c.CompanyName,
            SupplierName = r.Supplier.SupplierName
        });

Linq SelectMany

3 个答案: