为什么表达式树不能包含命名参数规范？ [英] Why can't an expression tree contain a named argument specification?

问题描述

使用AutoMapper，我找到了一个适合命名参数的地方：

Using AutoMapper, I hit a place where a named argument would've fit very nicely:

.ForMember(s => s.MyProperty, opt => opt.MapFrom(s => BuildMyProperty(s, isAdvanced: false)))

但是编译器对我大喊：

表达式树可能不包含命名参数规范

An expression tree may not contain a named argument specification

所以我不得不恢复为：

.ForMember(s => s.MyProperty, opt => opt.MapFrom(s => BuildMyProperty(s, false)))

有人知道为什么在这种情况下编译器不允许命名参数吗？

Does anyone know why the compiler disallows named arguments in this situation?

推荐答案

考虑一下以下：

static int M() { Console.Write("M"); return 1; }
static int N() { Console.Write("N"); return 2; }
static int Q(int m, int n) { return m + n; }
...
Func<int> f = ()=>Q(n : N(), m: M());
Expression<Func<int>> x = ()=>Q(n : N(), m: M());
Func<int> fx = x.Compile();
Console.WriteLine(f());
Console.WriteLine(fx());

您同意我希望最后两行必须做完全相同的事情，对吗？现在要打印 NM3 。

You agree I hope that the last two lines must do exactly the same thing, right? Which is to print NM3.

现在，表达式树库调用什么，希望表达式树转换生成确保这一点？没有了！因此，我们面临以下选择：

Now, what expression tree library calls would you like the expression tree conversion to generate that ensure this? There are none! We are therefore faced with the following choices:

1. 在表达式树库中实现该功能。在表达式树降低引擎中添加一个转换，以保留命名参数的执行顺序。在 Compile 方法中实现将执行顺序考虑在内的代码。


2. 使 x =（）=> Q（n：N（），m：M（））; 实际上被实现为 x =（）=> Q（M（），N（））; ，并且与非表达式树版本不兼容。


3. 禁止在表达式树中使用命名参数。为此，请执行一条错误消息。

1. Implement the feature in the expression tree library. Add a transformation in the expression tree lowering engine that preserves the order of execution of the named arguments. Implement code in the Compile method that takes the execution order into account.
2. Make x = ()=>Q(n : N(), m: M()); actually be implemented as x = ()=>Q(M(), N()); and be incompatible with the non-expression-tree version.
3. Disallow named arguments in expression trees. Implement an error message to that effect.

（1）很好，但是很昂贵。 （2）是非入门者；我们不能凭良心介绍这种陷阱。 （3）价格便宜但令人讨厌。

(1) is nice, but expensive. (2) is a non-starter; we can't in good conscience introduce this kind of "gotcha". (3) is cheap but irritating.

我们选择了（3）。

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