C# 中 lambda 变量的范围是什么? [英] What is the scope of a lambda variable in C#?

问题描述

我对 lambda 变量的范围感到困惑，例如以下内容

I'm confused about the scope of the lambda variable, take for instance the following

var query = 
    from customer in clist
    from order in olist
    .Where(o => o.CustomerID == customer.CustomerID && o.OrderDate ==  // line 1
        olist.Where(o1 => o1.CustomerID == customer.CustomerID)        // line 2
             .Max(o1 => o1.OrderDate)                                  // line 3
    )
    select new {
        customer.CustomerID,
        customer.Name,
        customer.Address,
        order.Product,
        order.OrderDate
    };

在第 1 行中，我声明了一个 lambda 变量o"，这意味着我无法在第 2 行中再次声明它(或者至少编译器会在我尝试时抱怨)但是即使 'o1' 已经存在，它也不会抱怨第 3 行??

In line 1 I have declare a lambda variable 'o' which means I cannot declare it again in line 2 (or at least the compiler complains if I try to) But it doesn't complain about line 3 even though 'o1' already exists??

lambda 变量的作用域是什么?

What is the scope of a lambda variable?

推荐答案

括号给出了线索​​ - lambda 变量在其声明的范围内被捕获:

The brackets give the clue - the lambda variable is captured in the scope of where it's declared:

.Where(o => ... olist.Where(o1 => ...).Max(o1 => ...))
  //  |----------------------------------------------| scope of o
  //                       |---------|                 scope of first o1
  //                                      |---------|  scope of second o1

请注意，两个 o1 变量没有重叠，但它们都重叠(或隐藏)了 o 变量，因此不能使用相同的名称.

Note that there's no overlap for the two o1 variables, but they both overlap (or shadow) the o variable and hence can't use the same name.

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