Implement Null Coalescing and Null Coalescing assignment operators by adityapatwardhan · Pull Request #10636 · PowerShell/PowerShell · GitHub

PR Summary

Implement the Null Coalescing ?? and Null Coalescing Assignment ??= operators.

PR Context

The operators are discussed in the issue #3240
This PR addresses part of RFC PowerShell/PowerShell-RFC#223

The PR is marked as a Breaking Change due to changes in the TokenFlags enum. The changes were made to include the new TokenFlag - BinaryPrecedenceCoalesce. While making this change, it was also decided to create more space in the BinaryPrecedence section of the enum for future binary operators. The BinaryPrecedenceMask was also changed from 0x07 to 0x0f. The order of precedence is not changed. We expected it can cause a breaking change for binary modules as C# treats enums as constants. Though, it is a breaking change we expect the impact to be pretty low as the usage of precedence token flags should be fairly low.

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).
    - Issue opened Possible update to syntax highlighting for ?? ??= ?. ?[] operators EditorSyntax#185

FYI this will clash with Posh-Git that implements an alias of ?? as below

CommandType     Name                                               Version    Source
-----------     ----                                               -------    ------
Alias           ?? -> Invoke-NullCoalescing                        0.7.3      Posh-git

We've already moved the use of ?? to internal-only in v1 and will happily get rid of it altogether for PS v7.

Really nice that this didn't require any changes to the parser and generally quite contained elsewhere -- feels like a vindication of good code for assignment processing.

Left a few comments.

Since enum's are treated like constants, this would be a breaking change for compiled projects right?

Yes, this will be a breaking change. We are expecting the impact to very small though. I have updated the PR Context in description with more details.

This issue has changes that could potentially change syntax. Please consider adding this feature to EditorSyntax which is used for syntax highlighting in GitHub, Visual Studio Code, Atom, Sublime Text, and many more locations.

Consistent syntax highlighting is very important for the language and a feature isn't "complete" until syntax highlighting is what is expected.

If you can't contribute to EditorSyntax, at least open an issue to track the work - however, please note, that no one is actively working on the repo and so the work will likely not get done in a timely manner.

We hope that you consider contributing to EditorSyntax.

(note this is copy/pasted text for any change that looks like it could impact EditorSyntax - and will be a bot in the future)

(thanks for opening the issue already!)

LGTM! Now waiting on the breaking change review from the committee.

It just occurred to me that none of the existing ICustomAstVisitor2.VisitBinaryExpression implementation is changed.
Aditya Patwardhan (@adityapatwardhan) did you review all existing VisitBinaryExpression and VisitAssignmentStatement to see if anything needs to be changed? For example, IsConstantValueVisitor might need to be updated for the ?? binary operation, as this operation can be potentially constant value.

IsConstantValueVisitor might need to be updated for the ?? binary operation, as this operation can be potentially constant value.

It looks like that's just a question of the token flag:

PowerShell/src/System.Management.Automation/engine/parser/ConstantValues.cs

Lines 196 to 201 in beb8b44

    
           public object VisitBinaryExpression(BinaryExpressionAst binaryExpressionAst) 
        
           { 
        
               return binaryExpressionAst.Operator.HasTrait(TokenFlags.CanConstantFold) && 
        
                   (bool)binaryExpressionAst.Left.Accept(this) && (bool)binaryExpressionAst.Right.Accept(this) 
        
                   && !IsNullDivisor(binaryExpressionAst.Right); 
        
           }

Dongbo Wang (@daxian-dbw) I will review all the existing VisitBinaryExpression and VisitAssignmentStatement to see if anything needs to change. I will submit those changes as part of a different PR.

@PowerShell/powershell-committee reviewed the changes to TokenFlags and agreed to accept this breaking change. We can revisit this decision if there is new feedback coming in during the use of 7.0.0-preview.5 and 7.0.0-preview.6 of pwsh.

🎉v7.0.0-preview.5 has been released which incorporates this pull request.:tada:

Handy links:

Release Notes

…owerShell#10636)

Aditya Patwardhan (adityapatwardhan) added 8 commits September 25, 2019 15:07

Implement null condtional assigment operator

ecb94dd

Implement ?? operator

15119cf

Add QuestionDot token

0cb1592

Fix issues after merge and test fix

cfd73f9

Change Null coalescing assigned operator to ??=

137a37b

Make feature experimental

94ad01f

Add logic for skipping tests if experimental feature is disabled

dee8092

Fix parsing tests

9826e69

Aditya Patwardhan (adityapatwardhan) requested a review from Dongbo Wang (daxian-dbw) September 27, 2019 00:31

PoshChan-Bot (PoshChan) reviewed Sep 27, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

PoshChan-Bot (PoshChan) reviewed Sep 27, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

PoshChan-Bot (PoshChan) reviewed Sep 27, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

Ilya (iSazonov) reviewed Sep 27, 2019

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Rain Sallow (/u/ta11ow) (vexx32) reviewed Sep 28, 2019

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Comment thread test/powershell/Language/Operators/NullConditional.Tests.ps1 Outdated

Comment thread test/powershell/Language/Operators/NullConditional.Tests.ps1

Patrick Meinecke (SeeminglyScience) reviewed Sep 29, 2019

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Comment thread src/System.Management.Automation/engine/parser/Compiler.cs Outdated

Address code review feedback

9e599c4

PoshChan-Bot (PoshChan) reviewed Oct 1, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

PoshChan-Bot (PoshChan) reviewed Oct 1, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

PoshChan-Bot (PoshChan) reviewed Oct 1, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

Remove parsing test as it interfers with ternary operator

8fd9572

Ilya (iSazonov) reviewed Oct 2, 2019

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Comment thread src/System.Management.Automation/engine/parser/Compiler.cs

Add few more tests

02dc105

PoshChan-Bot (PoshChan) reviewed Oct 2, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

Aditya Patwardhan (adityapatwardhan) marked this pull request as ready for review October 2, 2019 21:48

Aditya Patwardhan (adityapatwardhan) assigned Dongbo Wang (daxian-dbw) Oct 2, 2019

Rob Holt (rjmholt) reviewed Oct 3, 2019

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Address Rob's feedback

4e8019e

PoshChan-Bot (PoshChan) reviewed Oct 4, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

Updated to move Coalesce code out of Binder

8f39bbb

Patrick Meinecke (SeeminglyScience) reviewed Oct 12, 2019

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Update TokenFlag name

dea5793

PoshChan-Bot (PoshChan) reviewed Oct 14, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

PoshChan-Bot (PoshChan) reviewed Oct 14, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

Dongbo Wang (daxian-dbw) reviewed Oct 14, 2019

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Comment thread src/System.Management.Automation/engine/parser/Compiler.cs Outdated

Comment thread src/System.Management.Automation/engine/parser/Compiler.cs Outdated

Dongbo Wang (daxian-dbw) added the Breaking-Change breaking change that may affect users label Oct 14, 2019

Address feedback

4cf6ba4

Dongbo Wang (daxian-dbw) approved these changes Oct 15, 2019

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Dongbo Wang (daxian-dbw) added the Review - Committee The PR/Issue needs a review from the PowerShell Committee label Oct 15, 2019

Ilya (iSazonov) reviewed Oct 15, 2019

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Comment thread src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs Outdated

Comment thread src/System.Management.Automation/engine/parser/Compiler.cs Outdated

Comment thread test/powershell/Language/Operators/NullConditional.Tests.ps1

Address Ilya's feedback

268a609

Ilya (iSazonov) added Committee-Reviewed PS-Committee has reviewed this and made a decision and removed Review - Committee The PR/Issue needs a review from the PowerShell Committee labels Oct 17, 2019

Dongbo Wang (daxian-dbw) merged commit 425bc36 into PowerShell:master Oct 17, 2019

Steve Lee (SteveL-MSFT) added this to the 7.0.0-preview.5 milestone Oct 17, 2019

Dongbo Wang (daxian-dbw) added the CL-Engine Indicates that a PR should be marked as an engine change in the Change Log label Oct 17, 2019

Dongbo Wang (daxian-dbw) mentioned this pull request Nov 1, 2019

Update all existing AST visitor types in PowerShell as necessary for them to work correctly with null coalescing operators #10970

Closed

Ryan Yates (kilasuit) pushed a commit to kilasuit/PowerShell that referenced this pull request Nov 9, 2019

Implement Null Coalescing and Null Coalescing assignment operators (P…

9bd8a27

…owerShell#10636)

Kalle Olavi Niemitalo (KalleOlaviNiemitalo) mentioned this pull request Aug 11, 2022

Error running UpdateTemplate.ps1: "Unexpected token '??' in expression or statement." dotnet/docfx#8130

Closed

coderabbitai Bot mentioned this pull request Jul 20, 2026

Feat/winusb phase2a usbdk Troughie/MacroDeck#1

Merged

Thatgfsj (Thatgfsj) pushed a commit to Thatgfsj/PowerShell that referenced this pull request Aug 6, 2026

Implement Null Coalescing and Null Coalescing assignment operators (P…

56e922a

…owerShell#10636)

                       /// and all of the case sensitive variants of these operators, if they exists.
                       /// </summary>
-                      BinaryPrecedenceComparison = 3,
+                      BinaryPrecedenceComparison = 0x5,

Implement Null Coalescing and Null Coalescing assignment operators by adityapatwardhan · Pull Request #10636 · PowerShell/PowerShell · GitHub

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Implement Null Coalescing and Null Coalescing assignment operators - #10636

Implement Null Coalescing and Null Coalescing assignment operators#10636Dongbo Wang (daxian-dbw) merged 24 commits intoPowerShell:masterPowerShell/PowerShell:masterfrom adityapatwardhan:NullAssignmentadityapatwardhan/PowerShell:NullAssignmentCopy head branch name to clipboard

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Implement Null Coalescing and Null Coalescing assignment operators#10636
Dongbo Wang (daxian-dbw) merged 24 commits into
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Aditya Patwardhan (adityapatwardhan) commented Sep 27, 2019 •
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