为什么 C# 编译器允许嵌套范围内的重复变量？

Question

Historically, when developing in .Net I could not duplicate the name of variable in nested scope. However, after recently updating Visual Studio 2019 to version 16.4.2 I have noticed that variable names can be duplicated in nested scope.

For example:

var test = "hello";
Console.WriteLine(test);
var things = new []{"one", "two", "three"};
things.Select(test => // <- test is duplicated here, normally this breaks compilation
{
    Console.WriteLine(test);
    return test;
}).ToList();

// output:
// hello
// one
// two
// three

https://dotnetfiddle.net/h85BK4

Why is this suddenly allowed?

Follow up question: If this is a new language "feature", is there a way to configure Visual Studio to continue to break when a variable is duplicated in nested scope?

Answer 1

这是 C# 8.0 中的一项新功能，本地函数和 lambda 参数可以隐藏外部名称。

Answer 2

my guess is that the select function is being compiled as static since it has no connections to the host method body. it just prints and returns the item in question. this does not require any access to the method and as such, it can for optimization reasons be moved out of the method body and put on its own within the global scope.

and in this case, that test variable is its own variable and has no connections to the above test variable.

as for preventing this, you can't tell vs to throw an error when it occurs, you will just have manually change the variable name.