C＃7：下划线（_）&星号（*）输入Out变量 [英] C#7: Underscore ( _ ) & Star ( * ) in Out variable
问题描述
我正在阅读有关C＃7 此处。我有两个问题：
I was reading about new out variable features in C#7 here. I have two questions:
-
上面写着
It says
我们也允许丢弃作为输出参数，以
_的形式，让您忽略不需要的参数：
We allow "discards" as out parameters as well, in the form of a
_, to let you ignore out parameters you don’t care about:
p.GetCoordinates(out var x, out _); // I only care about x
Q：我想这只是C＃7的一个信息而不是一项新功能，因为我们也可以在C＃7.0之前的版本中这样做：
Q: I guess this is just an info and not a new feature of C#7 because we can do so in pre C#7.0 too:
var _;
if (Int.TryParse(str, out _))
...
还是我在这里丢失了某些东西？
or am I missing something here?
当我按照同一博客所述进行操作时，我的代码给出了错误：
My code gives an error when I do as mentioned in same blog:
~Person() => names.TryRemove(id, out *);
* 不是有效的标识符。我猜是Mads Torgersen的疏忽吗？
* is not a valid identifier. An oversight by Mads Torgersen I guess?
推荐答案
Discards 在C＃7中可以在声明变量的任何地方使用，以-作为名称建议-丢弃结果。因此，可以将out与变量一起使用：
Discards, in C#7 can be used wherever a variable is declared, to - as the name suggests - discard the result. So a discard can be used with out variables:
p.GetCoordinates(out var x, out _);
它可用于丢弃表达式结果：
and it can be used to discard an expression result:
_ = 42;
在示例中，
p.GetCoordinates(out var x, out _);
_ = 42;
没有变量 _ 被介绍。仅有两种情况使用了丢弃。
There is no variable, _, being introduced. There are just two cases of a discard being used.
但是，如果范围中存在标识符 _ ，则不能使用丢弃：
If however, an identifier _ exists in the scope, then discards cannot be used:
var _ = 42;
_ = "hello"; // error - a string cannot explicitly convert from string to int
code> _ 变量用作out变量。在这种情况下，编译器将忽略类型或 var 并将其视为丢弃：
The exception to this is when a _ variable is used as an out variable. In this case, the compiler ignores the type or var and treats it as a discard:
if (p.GetCoordinates(out double x, out double _))
{
_ = "hello"; // works fine.
Console.WriteLine(_); // error: _ doesn't exist in this context.
}
请注意，在这种情况下，仅当 out var _ 或 out double _ 。只需使用 out _ ，然后将其视为对现有变量 _ 的引用，如果它在范围内，例如：
Note that this only occurs if, in this case, out var _ or out double _ is used. Just use out _ and then it's treated as a reference to an existing variable, _, if it's in scope, eg:
string _;
int.TryParse("1", out _); // complains _ is of the wrong type
最后， * 表示法是在有关丢弃物的讨论的早期提出的，但为了<$而放弃了。 c $ c> _ ，因为后者是其他语言中更常用的符号。
Finally, the * notation was proposed early in the discussions around discards, but was abandoned in favour of _ due to the latter being a more commonly used notation in other languages.
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