[xml]$xmlfile = Get-Content -Path $xmlpath
$subbooks = Select-Xml -Path $xmlpath -XPath '//Subbook'
$subbooks | ForEach-Object {
    $MDC = $_.Node.SelectNodes('variable[@Name="Metadata_Code"]/@Value')
}

($xmlfile = [xml]::new()).Load($xmlpath)
$xmlfile.Book.Subbook.variable |
    Where-Object Name -EQ 'Metadata_Code' |
    ForEach-Object Value

[Santiago的有益答案](https://stackoverflow.com/a/76432968/45375)向你展示了如何使用PowerShell的点符号适配`[xml]` DOM来解决你的问题。
至于你基于XPath的尝试：
根据[你自己的答案](https://stackoverflow.com/a/76433080/45375)进行了更新：
**简而言之**：**你的代码是有效的**，但一个**显示错误**阻止了你意识到这一点。详情如下。
- `$MDC`实际上**并不是空的**，但是你试图通过[`System.Xml.XmlNodeList`](https://learn.microsoft.com/en-US/dotnet/api/System.Xml.XmlNodeList)实例来进行可视化（由[`.SelectNodes()`](https://learn.microsoft.com/en-US/dotnet/api/System.Xml.XmlDocument.SelectNodes)返回），也就是通过[`Write-Host`](https://learn.microsoft.com/en-us/powershell/module/Microsoft.PowerShell.Utility/Write-Host)，结果是**没有显示输出**。
  - 虽然这应该被视为一个**错误** - 请参见[GitHub问题＃19769](https://github.com/PowerShell/PowerShell/issues/19769) - 但使用`Write-Host`来显示**复杂对象**通常是没有帮助的。
  - 实际上，[通常使用`Write-Host`是错误的工具](http://www.jsnover.com/blog/2013/12/07/write-host-considered-harmful/)，除非意图是仅仅写入**显示**，绕过成功输出流，以及将输出发送到其他命令，将其捕获到一个变量中，或者将其重定向到文件中的能力。要输出一个值，请使用它**单独**，例如`$MDC`，而不是`Write-Host $MDC`（或使用`Write-Output $MDC`）；参见[这个答案](https://stackoverflow.com/a/60534138/45375)。要明确地仅仅打印到显示器**但具有丰富的格式**，请使用[`Out-Host`](https://learn.microsoft.com/en-us/powershell/module/Microsoft.PowerShell.Core/Out-Host)。
- 此外 - 正如下面的代码中也显示的那样 - 你需要访问`$MDC`中`System.Xml.XmlAttribute`实例的`.Value`属性，以便只获取属性的**值**（文本）。
- 顺便说一句：如果你只是使用一个**标识属性**来查找**单个节点**，请使用[`.SelectSingleNode()`](https://learn.microsoft.com/en-US/dotnet/api/System.Xml.XmlDocument.SelectSingleNode)而不是`.SelectNodes()`
除此之外，你的代码可以**简化**。以下是一个包含你的样本XML的独立示例：
```powershell
# 创建样本XML文件
@'
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="tmp.xslt"?>
<Book>
<Subbook id="1848">
<variable Name="Metadata_Code" Value="A-"/>
<variable Name="Metadata_Installation" Value="pod - A"/>
</Subbook>
<Subbook id="1849">
<variable Name="Metadata_Code" Value="B-"/>
<variable Name="Metadata_Installation" Value="pod - B"/>
</Subbook>
</Book>
'@ > file.xml
# 让Select-Xml直接解析文件，并使用单个XPath查询提取目标节点。
Get-Item file.xml | 
  Select-Xml '//Subbook/variable[@Name="Metadata_Code"]/@Value' | 
  ForEach-Object { $_.Node.Value }

A-
B-

<details>
<summary>英文:</summary>
&lt;!-- language-all: sh --&gt;
[Santiago&#39;s helpful answer](https://stackoverflow.com/a/76432968/45375) shows you how to solve your problem using [PowerShell&#39;s dot-notation-based adaptation of the `[xml]` DOM](https://stackoverflow.com/a/49213568/45375).
As for your **XPath-based** attempt:
_Update_, based on [your own answer](https://stackoverflow.com/a/76433080/45375):
**tl;dr**: **Your code *works***, but a **display bug** prevented you from realizing that. Details below.
* **`$MDC` wasn&#39;t actually _empty_**, but the way you tried to _visualize_ the [`System.Xml.XmlNodeList`](https://learn.microsoft.com/en-US/dotnet/api/System.Xml.XmlNodeList) instance it contained (returned by [`.SelectNodes()`](https://learn.microsoft.com/en-US/dotnet/api/System.Xml.XmlDocument.SelectNodes)), namely via [`Write-Host`](https://learn.microsoft.com/en-us/powershell/module/Microsoft.PowerShell.Utility/Write-Host) resulted in _no display output_.
  * While this should be considered a _bug_ - see [GitHub issue #19769](https://github.com/PowerShell/PowerShell/issues/19769) - it is rarely helpful to use `Write-Host` to display _complex objects_.
  * In fact, [`Write-Host` is typically the wrong tool to use](http://www.jsnover.com/blog/2013/12/07/write-host-considered-harmful/), unless the intent is to write _to the display only_, bypassing the success output stream and with it the ability to send output to other commands, capture it in a variable, or redirect it to a file. To output a value, use it _by itself_; e.g, `$MDC`, instead of `Write-Host $MDC` (or use `Write-Output $MDC`); see [this answer](https://stackoverflow.com/a/60534138/45375). To explicitly print only to the display _but with rich formatting_, use [`Out-Host`](https://learn.microsoft.com/en-us/powershell/module/Microsoft.PowerShell.Core/Out-Host).
* Additionally - as also shown in the code below - you needed to access the `.Value` property of the instance [`System.Xml.XmlAttribute`](https://learn.microsoft.com/en-US/dotnet/api/System.Xml.XmlAttribute) in `$MDC` in order to get only the attribute&#39;s _value_ (text).
* As an aside: If you&#39;re looking for just a _single_ node using an _identifying attribute_, use [`.SelectSingleNode()`](https://learn.microsoft.com/en-US/dotnet/api/System.Xml.XmlDocument.SelectSingleNode) instead of `.SelectNodes()`
Aside from that, your **code can be simplified**.
Here&#39;s a self-contained example with your sample XML:

Output (prepend something like `$MDC = ` to the pipeline above to capture the output strings in a variable):
```none
A-
B-

$MDC = $_.Node.SelectNodes('variable[@Name="Metadata_Code"]/@Value')

$MDC = $_.Node.SelectNodes('variable[@Name="Metadata_Code"]/@Value').Value