当我声明与Lambda中的变量同名的变量时，C#会发疯 [英] C# going nuts when I declare variables with the same name as the ones in a lambda

问题描述

我有以下代码(给定a，b和c生成二次函数)
Func<double, double, double, Func<double, double>> funcGenerator = (a, b, c) => f => f * f * a + b * f + c;
到目前为止，很可爱.
但是，如果我尝试声明一个名为a，b，c或f的变量，Visual Studio会弹出一个"A local variable named 'f' could not be declared at this scope because it would give a different meaning to 'f' which is used in a child scope."
基本上，这失败了，我也不知道为什么，因为子作用域甚至没有任何意义.

I have the following code (generates a quadratic function given the a, b, and c)
Func<double, double, double, Func<double, double>> funcGenerator = (a, b, c) => f => f * f * a + b * f + c;
Up until now, lovely.
But then, if i try to declare a variable named a, b, c or f, visual studio pops a "A local variable named 'f' could not be declared at this scope because it would give a different meaning to 'f' which is used in a child scope."
Basically, this fails, and I have no idea why, because a child scope doesn't even make any sense.

Func<double, double, double, Func<double, double>> funcGenerator =
    (a, b, c) => f => f * f * a + b * f + c;  
var f = 3; // Fails  
var d = 3; // Fine

这是怎么回事?

推荐答案

我认为您会误解的是，关于作用域规则，声明的顺序对C#编译器无关紧要.

I think what you're misunderstanding is that the order of declarations does not matter to the C# compiler with respect to scoping rules.

此:

Func<double, double, double, Func<double, double>> funcGenerator =
    (a, b, c) => f => f * f * a + b * f + c;  
var f = 3;
var d = 3;

与此完全相同:

var f = 3;
Func<double, double, double, Func<double, double>> funcGenerator =
    (a, b, c) => f => f * f * a + b * f + c;  
var d = 3;

范围不是顺序敏感的.您有一个名为f的局部变量，并且试图在lambda中声明另一个名为f的变量.根据C#规范，这是非法的.

Scopes aren't order-sensitive. You have a local variable named f, and you are trying to declare another variable named f inside the lambda. This is illegal according to the C# spec.

具体来说，它将与lambda执行变量捕获的能力冲突.例如，此代码是合法的:

Specifically, it would conflict with the ability of lambdas to do variable capturing. For example, this code is legal:

int x = 3;
Func<int> func = () => x + 1;

这是完全合法的，执行func()将返回4.这就是为什么您不能在lambda中声明另一个变量x的原因-因为lambda实际上需要能够捕获外部的x.

This is completely legal and executing func() will return 4. That's why you can't declare another variable x here inside the lambda - because the lambda actually needs to be capable of capturing the outer x.

只需更改f变量之一的名称即可.

Just change the name of one of the f variables.

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