当选择失败时，为什么foreach在这里工作？

Question

I've stored a list of attachments from my RavenDB server in the $Response variable.

[{"Size":3040,"Key":"attachments/deployments-7928/output-log","Metadata":{"ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0D0"},{"Size":2524,"Key":"attachments/deployments-7927/output-log","Metadata":{
"ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0CF"},{"Size":530,"Key":"attachments/tasks-7842/output-log","Metadata":{"ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0AA"}]

I want to get the Key value so I can fetch the attachments.

Why can't I select the Key value like this?

$Response | ConvertFrom-Json | Select Key

All the keys look empty when I do that.

Bizarrely, it works when I use Foreach instead.

$Response | ConvertFrom-Json | Foreach { $_.Key }

I see this:

attachments/deployments-7928/output-log
attachments/deployments-7927/output-log
attachments/tasks-7842/output-log

What's the difference?

Why doesn't Select work here?

Answer 1

It works if you assign the results to a variable first, and then pass that into Select-Object .

$response = @'
    [{"Size":3040,"Key":"attachments/deployments-7928/output-log","Metadata":{"ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0D0"},{"Size":2524,"Key":"attachments/deployments-7927/output-log","Metadata":{
    "ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0CF"},{"Size":530,"Key":"attachments/tasks-7842/output-log","Metadata":{"ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0AA"}]
'@

$ObjectList = $response | ConvertFrom-Json;

$ObjectList | Select-Object -Property Key;

Answer 2

Because convertfrom-json does not return array of PSObjects, it returns an array system.object[]. The array does not have a 'Key' member.

 Write-Output '[{"Size":3040,"Key":"attachments/deployments-7928/output-log","Metadata":{"ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0D0"},{"Size":2524,"Key":"attachments/deployments-7927/output-log","Metadata":{
"ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0CF"},{"Size":530,"Key":"attachments/tasks-7842/output-log","Metadata":{"ContentType":"text/xml"},"Etag":"02000000-0000-0014-0000-00000000A0AA"}]' `
| convertfrom-json | get-member

Output

   TypeName: System.Object[]
   ...

See, it only has the following members :

Name

Count
Add
Address
Clear
Clone
CompareTo
Contains
CopyTo
Equals
Get
GetEnumerator
GetHashCode
GetLength
GetLongLength
GetLowerBound
GetType
GetUpperBound
GetValue
IndexOf
Initialize
Insert
Remove
RemoveAt
Set
SetValue
ToString
Item
IsFixedSize
IsReadOnly
IsSynchronized
Length
LongLength
Rank
SyncRoot

Assigning to a variable converts to TypeName: System.Management.Automation.PSCustomObject and this has a 'key' member. Powershell's built-in enumeration awareness expands this properly.

Answer 3

如果要避免中间变量赋值：

{$response | ConvertFrom-Json}.invokereturnasis() | select key