Prerequisites

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• Refer to Differences between Windows PowerShell 5.1 and PowerShell.

There is asymmetry between the serialization of properties with either AutomationNull ([Management.Automation.Internal.AutomationNull]::Value) or $null values, that affects the following formats:

• CSV
• CLIXML
• XML

This issue focuses on CSV. I will address (CLI)XML in a separate issue as the behavior differs.

Consider the following contrived example:

[pscustomobject] @{
    Prop = if ($false) { 'Foo' } else { $null }
} | ConvertTo-Csv

# "Prop"

[pscustomobject] @{
    # Statement assignments have pipeline semantics, so Prop receives AutomationNull.
    Prop = if ($false) { 'Foo' }
} | ConvertTo-Csv

# "Prop"
# ""

A typical PS user who is unaware of AutomationNull (and arguably should not have to be) would reasonably expect equivalent CSV serialization. Yet, the result differs because AutomationNull as a property value serializes as a quoted ("") empty field whereas $null serializes as unquoted.

Notes:

• Due to The last cell of an empty column read by ConvertFrom-Csv is inconsistently $Null #17702, this issue is especially problematic when the resultant CSV data is deserialized with ConvertFrom-Csv/Import-Csv .
• This issue is similar to a problem affecting JSON/ConvertTo-Json in Windows PowerShell, but has since been addressed. In the latest PS version, AutomationNull and $null both serialize as null in JSON.

Steps to reproduce

[pscustomobject] @{ P1 = & {}; P2 = $null } | ConvertTo-Csv

Expected behavior

CSV serialization of AutomationNull and $null property values should be identical.

The result should be either:

# "P1","P2"
# "",""

# Or...

# "P1","P2"
# ,

Depending on whether AutomationNull/$null should both be a quoted or unquoted empty field. Note that this has deserialization implications, as noted in the aforementioned #17702 issue.

Actual behavior

Serialization of AutomationNull and $null property values differs.

• AutomationNull: "" (quoted)
• $null: Nothing (unquoted)

"P1","P2"
"",

Environment data

Name                           Value
----                           -----
PSVersion                      7.5.0-rc.1
PSEdition                      Core
GitCommitId                    7.5.0-rc.1
OS                             Microsoft Windows 10.0.19045
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0