将此 LINQ 表达式转换为 Lambda

Question

伙计们，我很难将下面的 linq 表达式（左连接实现）转换为 lambda 表达式（用于学习）。

var result = from g in grocery
       join f in fruit on g.fruitId equals f.fruitId into tempFruit
       join v in veggie on g.vegid equals v.vegid into tempVegg
       from joinedFruit in tempFruit.DefaultIfEmpty()
       from joinedVegg in tempVegg.DefaultIfEmpty()
       select new { g.fruitId, g.vegid, fname = ((joinedFruit == null) ? string.Empty :     joinedFruit.fname), vname = ((joinedVegg == null) ? string.Empty : joinedVegg.vname) };

有人可以建议我如何做到这一点。

如果有人给我“C# Lambdas & Linqs”的优秀教程链接，我真的很感激

Answer 1

要将 Linq 查询转换为其等效的 Lambda：

1. 下载Linqpad 并运行您的 询问。
2. 在结果窗口中，单击 在工具栏中的“λ”按钮上。 它就在结果窗口的正上方
3. 您的查询将转换为 等效的 Lambda 表达式！

Answer 2

你可以看看101 LINQ Samples和C# 3.0 QUERY EXPRESSION TRANSLATION CHEAT SHEET

Answer 3

这是我遵循的启发式方法：

当你有连接时，优先使用 LINQ 表达式而不是 lambda。

我认为带有连接的 lambdas 看起来很乱而且难以阅读。

Answer 4

我通常使用 ReSharper 帮助我将事物转换为方法链和 lambda，这可以帮助我相当轻松地来回转换。

    var result = from g in grocery
                 join f in fruit on g.fruitId equals f.fruitId into tempFruit
                 join v in veggie on g.vegid equals v.vegid into tempVegg
                 from joinedFruit in tempFruit.DefaultIfEmpty()
                 from joinedVegg in tempVegg.DefaultIfEmpty()
                 select new { g.fruitId, g.vegid, fname = ((joinedFruit == null) ? string.Empty : joinedFruit.fname), vname = ((joinedVegg == null) ? string.Empty : joinedVegg.vname) };

然后使用 ReSharper 的将 LINQ 转换为方法链的选项等于以下内容：

        var result =grocery .GroupJoin(fruit, g => g.fruitId, f => f.fruitId, (g, tempFruit) => new {g, tempFruit})
                            .GroupJoin(veggie, @t => @t.g.vegid, v => v.vegid, (@t, tempVegg) => new {@t, tempVegg})
                            .SelectMany(@t => @t.@t.tempFruit.DefaultIfEmpty(), (@t, joinedFruit) => new {@t, joinedFruit})
                            .SelectMany(@t => @t.@t.tempVegg.DefaultIfEmpty(),(@t, joinedVegg) =>
                                new
                                    {
                                        @t.@t.@t.g.fruitId,
                                        @t.@t.@t.g.vegid,
                                        fname = ((@t.joinedFruit == null) ? string.Empty : @t.joinedFruit.fname),
                                        vname = ((joinedVegg == null) ? string.Empty : joinedVegg.vname)
                                    });

虽然输出并不理想，但它至少有助于从某个地方开始理解语法。

Answer 5

以下是在 lambda 中编写此查询的方法：

var cus­tomers = new List {
new Cus­tomer { Com­pa­nyId = “AC”, Cus­tomerId = “Customer1” },
new Cus­tomer { Com­pa­nyId = “not-AC”, Cus­tomerId = “Customer2” },
};

var user­Cus­tomers = new List {
new User­Cus­tomer { Com­pa­nyId = “AC”, Cus­tomerId = “Customer1”, User = “not-admin” },
new User­Cus­tomer { Com­pa­nyId = “AC”, Cus­tomerId = “Customer1”, User = “admin” },
new User­Cus­tomer { Com­pa­nyId = “AC”, Cus­tomerId = “Customer2”, User = “not-admin” },
new User­Cus­tomer { Com­pa­nyId = “AC”, Cus­tomerId = “Customer2”, User = “admin” },
new User­Cus­tomer { Com­pa­nyId = “not-AC”, Cus­tomerId = “Customer1”, User = “not-admin”     },
new User­Cus­tomer { Com­pa­nyId = “not-AC”, Cus­tomerId = “Customer1”, User = “admin” },
new User­Cus­tomer { Com­pa­nyId = “not-AC”, Cus­tomerId = “Customer2”, User = “not-admin” },
new User­Cus­tomer { Com­pa­nyId = “not-AC”, Cus­tomerId = “Customer2”, User = “admin” }
};

使用查询表达式

var query =
from c in cus­tomers
join uc in user­Cus­tomers on
new { c.CompanyId, c.CustomerId } equals new { uc.CompanyId, uc.CustomerId }
where c.CompanyId == “AC” && uc.User == “admin“
select c;

使用 lambda 表达式

var lambda =  cus­tomers.Where(c => c.CompanyId == “AC”) // inner sequence
.Join(userCustomers.Where(uc => uc.User == “admin”), // outer sequence
c => new { c.CompanyId, c.CustomerId }, // inner key selec­tor
uc => new { uc.CompanyId, uc.CustomerId }, // outer key selec­tor
(c, uc) => c);

两种方法产生相同的结果（客户的公司 ID 为“AC”，客户 ID 为“Customer1”），但如您所见，lambda 表达式更难编写和阅读！

希望这会有所帮助！

Answer 6

下载LINQPad;它带有用于学习 LINQ 的内置示例。

Answer 7

使用 Reflector .NET :)