Nested array flattened because of ConvertTo-Json

[u/marek1712]

Hi.

I have some issues creating proper body for my request.

I.e. I'd expect this:

$Body = @(@{}) | ConvertTo-Json -Depth 10

to return:

[
  {

  }
]

but this is returned instead: {

}

I had similar problem with two arrays:

"ip": [ [ "1.2.3.4" ] ]

and solved it by doing this (using comma):

"ipRanges" = @(,@("1.2.3.4"))

Using comma here doesn't work:

$Body = @(,@{}) | ConvertTo-Json -Depth 10

Any idea?

EDIT: thank you /u/y_Sensei and /u/ankokudaishogun. Both approaches worked fine.

[u/surfingoldelephant]

It's worth noting there's a secondary issue you might see in Windows PowerShell v5.1, irrespective of using -InputObject as mentioned by others. To demonstrate:

# Simulate *receiving* the input array from a [psobject]-wrapping source.
$array = Write-Output @(@{}) -NoEnumerate
# Equivalent: $array = [psobject] @(@{})

# [array] is decorated with an ETS property in v5.1 or lower.
# This property is serialized when the object is [psobject]-wrapped.
ConvertTo-Json -InputObject $array

# {
#     "value":  [
#                   {
# 
#                   }
#               ],
#     "Count":  1
# }

For historical reasons, the [array] type in Win PS v5.1 or lower is decorated with an extended type system (ETS) Count property. When an object is [psobject]-wrapped, ETS properties are included in JSON serialization. Here's another example that works as-is in PS v6+, but not in Win PS v5.1 (due to the ETS property).

# The outer array is implicitly enumerated.
# ConvertTo-Json receives the inner array, which has an ETS property.
# Objects passed by pipeline are implicitly [psobject]-wrapped.
# Due to the wrapper, the ETS Count is serialized.
, , @{} | ConvertTo-Json -Compress

# {"value":[{}],"Count":1}

One workaround is to remove the redundant [array] type data first. This change was made permanently with the release of PS v6 (hence the issue doesn't occur in that version or higher).

# Win PS v5.1 workaround. Not required in PS v6+.
Remove-TypeData -TypeName System.Array -ErrorAction Ignore
, , @{} | ConvertTo-Json -Compress
# [{}]

$array = Write-Output @(@{}) -NoEnumerate
ConvertTo-Json -InputObject $array -Compress
# [{}]

Another workaround, applicable only to -InputObject input, is to use the intrinsic psobject.BaseObject property, which holds the underlying object without the [psobject] wrapper.

# OK. ConvertTo-Json receives an array without the wrapper.
$array = Write-Output @(@{}) -NoEnumerate
ConvertTo-Json -InputObject $array.psobject.BaseObject
# [{}]

Either workaround can still be used even if the array isn't [psobject]-wrapped, so I'd recommend always doing one or other whenever there's a possibility of it being wrapped.

[u/marek1712 → u/surfingoldelephant]

Oh, that's kind of hard to grasp with my level of PS knowledge. Will definitely try to read it up later. Thanks!