c# - Linq SelectMany

Question

您好，我正在通过 MS 101 linq 示例编写代码。

“JoinOperators”给我带来了困难，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，在例子105我看到这个查询表达式:

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，我无法通过这种方式访问​​供应商级别的信息。当我将 customers 切换到 suppliers 时，我无法访问客户级别的信息。

是否有一些 SelectMany() 的重载让我从两个对象的字段级中提取？

另外，我不明白为什么 GroupJoin() 似乎返回一个包含 2 个集合(suppliers 和 customers)的对象。难道不应该以某种方式加入他们吗？

我想我不明白 GroupJoin() 是如何工作的。

Answer 1

您在群组加入中使用了错误的结果选择器，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   
        });