编写 PowerShell cmdlet 时 WriteObject(x, true) 和多个 writeobjects 有什么区别?

问题描述

我想编写一个 cmdlet，从数据库中读取多条记录并将它们放入管道中.

I want to write a cmdlet that reads multiple records from a database and puts them onto the pipeline.

我想我可以执行单个 WriteObject(Enumerable, true) 或者我可以循环自己并多次调用 WriteObject.

I think I can do either a single WriteObject(Enumerable<rec>, true) or I can loop myself and call WriteObject multiple times.

这两者有什么区别?

推荐答案

这里是文档:Cmdlet.WriteObject 方法(对象，布尔值)

这是一个例子:

# Writes objects one by one
function Test1
{
    [CmdletBinding()]param()
    $data | %{ $PSCmdlet.WriteObject($_) }
}

# Writes the collection and allows the core to enumerate it one level.
# Numbers of written objects is the collection item count.
function Test2
{
    [CmdletBinding()]param()
    $PSCmdlet.WriteObject($data, $true)
}

# Writes the collection as a single object.
# Numbers of written objects is 1.
function Test3
{
    [CmdletBinding()]param()
    $PSCmdlet.WriteObject($data, $false)
}

function Test
{
    (Test1).GetType().Name
    (Test2).GetType().Name
    (Test3).GetType().Name
}

$data = New-Object System.Collections.ArrayList

Write-Host "1 item"
$null = $data.Add('hello')
Test

Write-Host "2+ items"
$null = $data.Add('world')
Test

输出:

1 item
String
String
ArrayList
2+ items
Object[]
Object[]
ArrayList

因此，为集合中的每个项目调用WriteObject(item)与WriteObject(items, true)基本相同；在这两种情况下，集合本身都消失了.

Thus, calling WriteObject(item) for each item in a collection is basically the same as WriteObject(items, true); in both cases the collection itself has gone.

WriteObject(items, false) 不同；它返回对集合的引用，调用者可以根据场景有效地使用它.例如，如果集合是 DataTable 对象(不是展开的 DataRow 项集)，则调用者可以对返回的 DataTable 成员进行操作对象.

WriteObject(items, false) is different; it returns a reference to the collection and the caller can use that effectively depending on a scenario. For example, if a collection is a DataTable object (not unrolled set of DataRow items) then a caller can operate on DataTable members of the returned object.

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