Null member access operators ?. and ?[]

[adityapatwardhan]

PR Summary

PR for ?. and ?[] operators.
Braces need around variable name. Like ${x}?.Name and ${x}?.Get()
Implementation on RFC - PowerShell/PowerShell-RFC#223

PR Context

The null conditional member access operators allow to check the item for being null before a property is accessed or an index is used or a method is invoked. This helps reduced the null checking logic from scripts.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed: Add documentation for ?? ??= ?. and ?[] operators MicrosoftDocs/PowerShell-Docs#4925
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
  • OR
  • I have considered the user experience from a tooling perspective and enumerated concerns in the summary. This may include:
    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).
      • Filed issue: Possible update to syntax highlighting for ?? ??= ?. ?[] operators EditorSyntax#185

[adityapatwardhan]

@PoshChan please retry Windows

[PoshChan]

@adityapatwardhan, successfully started retry of PowerShell-CI-Windows

[daxian-dbw]

Remove trailing spaces here.

[adityapatwardhan]

Fixed

[daxian-dbw]

Not fixed yet.

[daxian-dbw]

In C#, there is no point to support ?. for member access to static members, but in PowerShell, you can do $a = [string]; $a::Equals(...), which makes it debatable whether we want to support it.

[adityapatwardhan]

Though it is technically possible to implement something like: $a = [string]; ${a}?::Equals(...), i doubt there is any usefulness to this.

[daxian-dbw]

Yeah, it's technically possible. I vote for postponing it until we have a ask. @SteveL-MSFT any thoughts?

[SeeminglyScience]

Though it is technically possible to implement something like: $a = [string]; ${a}?::Equals(...), i doubt there is any usefulness to this.

Where I think it would be most useful is situations where you want to invoke a static method only if an assembly is already loaded. e.g.

('MyCustomType' -as [type])?::Initialize()

[vexx32]

That certainly beats my current method! (and has better utility, probably, as well)

switch ($null) {
    ('typename' -as [type]) { . Type.ps1 }
    ('type2name' -as [type]) { . Type2.ps1 }
}

[SteveL-MSFT]

Since it's additive we can postpone

[adityapatwardhan]

@daxian-dbw - Please re-review.

[daxian-dbw]

Not fixed yet.

[daxian-dbw]

@rjmholt and @JamesWTruher Can you please review this PR as well? Thanks!

[PoshChan]

@adityapatwardhan, your last commit had 1 failures in PowerShell-CI-windows
Enter-PSHostProcess tests.By Process Id.Can enter using NamedPipeConnectionInfo

Exception calling "Invoke" with "0" argument(s): "The runspace state is not valid for this operation."
at <ScriptBlock>, D:\a\1\s\test\powershell\Modules\Microsoft.PowerShell.Core\Enter-PSHostProcess.Tests.ps1: line 136
136:                 $ps.AddScript('$pid').Invoke() | Should -Be $pwshId

[PoshChan]

@adityapatwardhan, your last commit had 1 failures in PowerShell-CI-windows
NullConditionalMemberAccess.?. operator tests.Use ?. on a dynamic method name that does not exist

Expected an exception, with FullyQualifiedErrorId 'Argument' to be thrown, but the FullyQualifiedErrorId was 'MethodNotFound'. from D:\a\1\s\test\powershell\Language\Operators\NullConditional.Tests.ps1:343 char:15
    +             { (Get-Date '11/11/2019')?.$methodName() } | Should -Thro ?
    +               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
at <ScriptBlock>, D:\a\1\s\test\powershell\Language\Operators\NullConditional.Tests.ps1: line 343
343:             { (Get-Date '11/11/2019')?.$methodName() } | Should -Throw -ErrorId 'Argument'

[PoshChan]

@adityapatwardhan, your last commit had 1 failures in PowerShell-CI-linux
NullConditionalMemberAccess.?. operator tests.Use ?. on a dynamic method name that does not exist

Expected an exception, with FullyQualifiedErrorId 'Argument' to be thrown, but the FullyQualifiedErrorId was 'MethodNotFound'. from /home/vsts/work/1/s/test/powershell/Language/Operators/NullConditional.Tests.ps1:343 char:15
    +             { (Get-Date '11/11/2019')?.$methodName() } | Should -Thro ?
    +               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
at <ScriptBlock>, /home/vsts/work/1/s/test/powershell/Language/Operators/NullConditional.Tests.ps1: line 343
343:             { (Get-Date '11/11/2019')?.$methodName() } | Should -Throw -ErrorId 'Argument'

[PoshChan]

@adityapatwardhan, your last commit had 1 failures in PowerShell-CI-macos
NullConditionalMemberAccess.?. operator tests.Use ?. on a dynamic method name that does not exist

Expected an exception, with FullyQualifiedErrorId 'Argument' to be thrown, but the FullyQualifiedErrorId was 'MethodNotFound'. from /Users/runner/runners/2.160.0/work/1/s/test/powershell/Language/Operators/NullConditional.Tests.ps1:343 char:15
    +             { (Get-Date '11/11/2019')?.$methodName() } | Should -Thro ?
    +               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
at <ScriptBlock>, /Users/runner/runners/2.160.0/work/1/s/test/powershell/Language/Operators/NullConditional.Tests.ps1: line 343
343:             { (Get-Date '11/11/2019')?.$methodName() } | Should -Throw -ErrorId 'Argument'

[PoshChan]

@adityapatwardhan, your last commit had 20 failures in PowerShell-CI-static-analysis
(These are 5 of the failures)

Verify Markdown Links.Verify links in /home/vsts/work/1/s/README.md.https://docs.microsoft.com/powershell/scripting/setup/installing-powershell-core-on-windows?view=powershell-6 should work

retry of URL failed with error: Response status code does not indicate success: 404 (Not Found).
at <ScriptBlock>, /home/vsts/work/1/s/test/common/markdown/markdown-link.tests.ps1: line 117
117:                                     throw "retry of URL failed with error: $($_.Exception.Message)"

Verify Markdown Links.Verify links in /home/vsts/work/1/s/README.md.https://docs.microsoft.com/powershell/scripting/setup/installing-powershell-core-on-linux?view=powershell-6#ubuntu-1804 should work

retry of URL failed with error: Response status code does not indicate success: 404 (Not Found).
at <ScriptBlock>, /home/vsts/work/1/s/test/common/markdown/markdown-link.tests.ps1: line 117
117:                                     throw "retry of URL failed with error: $($_.Exception.Message)"

Verify Markdown Links.Verify links in /home/vsts/work/1/s/README.md.https://docs.microsoft.com/powershell/scripting/setup/installing-powershell-core-on-linux?view=powershell-6#ubuntu-1604 should work

retry of URL failed with error: Response status code does not indicate success: 404 (Not Found).
at <ScriptBlock>, /home/vsts/work/1/s/test/common/markdown/markdown-link.tests.ps1: line 117
117:                                     throw "retry of URL failed with error: $($_.Exception.Message)"

Verify Markdown Links.Verify links in /home/vsts/work/1/s/README.md.https://docs.microsoft.com/powershell/scripting/setup/installing-powershell-core-on-linux?view=powershell-6#debian-9 should work

retry of URL failed with error: Response status code does not indicate success: 404 (Not Found).
at <ScriptBlock>, /home/vsts/work/1/s/test/common/markdown/markdown-link.tests.ps1: line 117
117:                                     throw "retry of URL failed with error: $($_.Exception.Message)"

Verify Markdown Links.Verify links in /home/vsts/work/1/s/README.md.https://docs.microsoft.com/powershell/scripting/setup/installing-powershell-core-on-linux?view=powershell-6#centos-7 should work

retry of URL failed with error: Response status code does not indicate success: 404 (Not Found).
at <ScriptBlock>, /home/vsts/work/1/s/test/common/markdown/markdown-link.tests.ps1: line 117
117:                                     throw "retry of URL failed with error: $($_.Exception.Message)"

[daxian-dbw]

One open issue: [System.DBNull]::Value?.GetTypeCode() and [NullString]::Value?.GetType() will return nothing, which might raise confusion. We need to discuss and decide if it's fine to have this semantics for the null-conditional operator.
Talked with @adityapatwardhan offline, and we will discuss and address this in the RC release.
/cc @JamesWTruher @SteveL-MSFT @rjmholt

[adityapatwardhan]

Filed issue: #11084

[adityapatwardhan]

@PoshChan please retry static

[PoshChan]

@adityapatwardhan, successfully started retry of PowerShell-CI-static-analysis

[adityapatwardhan]

@daxian-dbw Ready to merge?

[ghost]

🎉v7.0.0-preview.6 has been released which incorporates this pull request.:tada:

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