还是，我想知道我的原始语法出了什么问题.

Still, I would like to know what is wrong with my original syntax.

您的原始语法(从根本上正确)使用

Your original syntax (fundamentally correctly) uses argument mode, in which arguments are, loosely speaking, evaluated as follows:

• 不以 $ ，(或 @ 开头的参数被视为字符串，即使未加引号；值得注意的是，在这些特殊字符中， [不是 .

• An argument that doesn't start with either $, (, or @ is treated as a string, even if not quoted; notably, [ is not among these special characters.

因此， [System.Text.Encoding] :: UTF8 被解释为字符串文字 ，而不是被解释为返回 System.Text.Encoding 实例，找不到第三个参数为 string 的 System.IO.StreamWriter 构造函数.

Therefore, [System.Text.Encoding]::UTF8 is interpreted as a string literal rather than as an expression that returns an System.Text.Encoding instance, and no System.IO.StreamWriter constructor whose 3rd argument is a string can be found.

• 不幸的是，错误消息仅提及参数的 count ，而没有指出错误的 type 可能是原因.这是一个已知的问题-请参见此GitHub问题.

• Unfortunately, the error message only mentions the count of arguments, without indicating that an incorrect type might be the cause; this is a known problem - see this GitHub issue.

如 PetSerAl 的评论中所述，正确的解决方案是封闭 [System.text.Encoding] :: UTF8 放在(...)中，以便在 expression 模式中强制其求值，从而产生理想的结果.

The correct solution, as mentioned in a comment by PetSerAl, is to enclose [System.Text.Encoding]::UTF8 in (...) so as to force its evaluation in expression mode, where it yields the desired result.

请注意，以上内容还暗示 a.txt 周围的"..." (双引号)不是必需的(但无害)，因此我们得到:

Note that the above also implies that the "..." (double quotes) around a.txt aren't necessary (but do no harm), so we get:

^{注意:为简洁起见，在以下示例命令的完整类型中，我省略了初始的 System 组件.例如， IO.StreamWriter 是指 System.IO.StreamWriter .在大多数情况下，在PowerShell中指定 System.部分是可选.}

^{Note: For brevity, I've omitted the initial System. components from the full types in the following sample commands; e.g., IO.StreamWriter refers to System.IO.StreamWriter. Specifying the System. part is optional in PowerShell in most contexts.}

$f = New-Object IO.StreamWriter a.txt, $false, ([Text.Encoding]::UTF8)

请注意，是各个构造函数参数之间的 ，导致它们以 array 的形式传递-即，单个参数-到 新对象 ，它(在位置上)绑定到数组类型的 -ArgumentList ( -Args )参数.
顺便说一句:将个别参数按位置传递给 separate 参数更为常见，并且需要空格来分隔它们；例如 Select-String foo t.txt 被解析为
选择字符串-模式foo-路径t.txt .

Note that it is the , between the individual constructor arguments that causes them to be passed as an array - i.e., a single argument - to New-Object, where it is (positionally) bound to the array-typed -ArgumentList (-Args) parameter.
As an aside: passing individual arguments positionally to separate parameters is more common, and requires spaces to separate them; e.g., Select-String foo t.txt is parsed as
Select-String -Pattern foo -Path t.txt.

您自己的答案(已删除)使用了 伪方法语法，该语法***避免，并且仅发生工作:

Your own answer (since deleted) uses pseudo method syntax that is best avoided and only happens to work:

# AVOID: pseudo method syntax.
$f = New-Object IO.StreamWriter("a.txt", $false, [Text.Encoding]::UTF8)

即使此看起来像一个方法调用(构造函数调用)，它也不是，并且实际上按以下方式进行了解析:

Even though this looks like a method call (constructor call), it isn't, and is actually parsed as follows:

$f = New-Object IO.StreamWriter -ArgumentList ("a.txt", $false, [Text.Encoding]::UTF8)

也就是说，您已经将原始参数数组包含在(...)中，这导致其元素以 expression 模式进行解析，其中包括解决您问题的 [Text.Encoding] :: UTF8 .

That is, you've enclosed the original argument array in (...), which causes its elements to be parsed in expression mode, including [Text.Encoding]::UTF8, which happened to solve your problem.

请注意-与参数模式不同，必须将字符串 a.txt 括在"..." (或'...')在表达式模式下.

Note that - unlike in argument mode - string a.txt does have to be enclosed in "..." (or '...') in expression mode.

请注意， PSv5 + 确实确实通过静态 new()提供了一种基于方法的构建对象的方式.方法在类型信息对象上公开 ，在这种情况下，所有参数都以 expression 模式进行解析:>

Note that PSv5+ does offer a method-based way to construct objects, via the static new() method exposed on type-information objects, in which case all arguments are parsed in expression mode:

# PowerShell version 5 and above; you can use the ::new() method on types.
$f = [IO.StreamWriter]::new("a.txt", $false, [Text.Encoding]::UTF8)

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关于何时需要 [System.Text.Encoding] :: UTF8 的注释:

---

A note on when [System.Text.Encoding]::UTF8 is needed:

与Window PowerShell不同，.NET 默认为 UTF-8(PowerShell [Core](v6 +)现在也是如此).

Unlike Window PowerShell, .NET defaults to UTF-8 (which PowerShell [Core] (v6+) now does too).

• 读取数据时，因此通常不需要显式请求UTF-8编码 .

• When you read data, you therefore normally don't need to request UTF-8 encoding explicitly.

写入数据时，传递 [System.Text.Encoding] :: UTF8 会导致UTF-8文件 BOM ，而依赖于默认 UTF-8编码会创建一个没有 BOM的文件(这对于跨平台互操作性更好)；要显式请求无BOM编码，请使用 [System.Text.Utf8Encoding] :: new().

When you write data, passing [System.Text.Encoding]::UTF8 results in a UTF-8 file with a BOM, whereas relying on the default UTF-8 encoding creates a file without a BOM (which is better for cross-platform interoperabiliy); to request a BOM-less encoding explicitly, use [System.Text.Utf8Encoding]::new().