Add MA0186: Report missing nullable attributes on Equals and TryGetValue parameters by Copilot · Pull Request #998 · meziantou/Meziantou.Analyzer

Copilot · 2026-02-04T20:24:38Z

Implementation

This rule detects two patterns of missing nullable attributes:

Equals methods: Reports missing [NotNullWhen(true)] on parameters of Equals(object?) overrides and IEquatable<T>.Equals(T?) implementations
TryGetValue methods: Reports missing [MaybeNullWhen(false)] on the value parameter of IDictionary<TKey, TValue>.TryGetValue implementations

Both patterns improve nullable reference type analysis by helping the compiler understand nullability flow based on method return values.

Configuration

Rule ID: MA0186
Category: Design
Severity: Info (suggestion)
Enabled by default: No
Has code fix: No

Original prompt

This section details on the original issue you should resolve

<issue_title>Introduce rule that reports missing nullable attributes</issue_title>
<issue_description>I'm sure there are other examples, but one case where it would be interesting to report missing nullable attributes is for the lack of [NotNullWhen(true)] on the Equals(object?) override and the implementation of IEquatable<T>:
public sealed class Foo : IEquatable<Foo>
{
    public override bool Equals([NotNullWhen(true)] object? obj)
    {
        ...
    }

    public bool Equals([NotNullWhen(true)] Foo? other)
    {
        ...
    }
}
The rule should of course also support the explicit interface implementation of IEquatable<T> and only report a diagnostic if NRT is enabled for the method in question.

It should not be reported for the IEquatable<T> implementation if T is a value type.

</issue_description>

<agent_instructions>Can you implement this rule as a suggestion, but keep it disabled by default</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes Introduce rule that reports missing nullable attributes #975

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

meziantou · 2026-02-04T20:57:00Z

@copilot Can you regenerate the documentation?

Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

Copilot · 2026-02-04T20:59:04Z

@copilot Can you regenerate the documentation?

Done in commit 91d0c1f. The documentation now includes the source link to the analyzer file.

meziantou · 2026-02-04T22:05:36Z

@copilot the original issue also ask for something about idictionary #975 (comment)

Copilot · 2026-02-04T22:15:13Z

@copilot the original issue also ask for something about idictionary #975 (comment)

I've implemented support for:

✅ Equals(object?) override
✅ IEquatable<T>.Equals(T?) public implementation
✅ Skipping value types (where T in IEquatable is a value type)

I'm having difficulty with explicit interface implementations (bool IEquatable<T>.Equals(T? other)) - the Roslyn symbol analysis doesn't seem to pick them up the same way. Could you clarify what you mean by "something about idictionary"? I only see the Equals/IEquatable requirements in the original issue. Is there another method pattern (like TryGetValue) that should also be checked?

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https://api.github.com/repos/meziantou/Meziantou.Analyzer/issues/975
- Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (http block)
pdfvsblobprodcus380.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/Meziantou.Analyzer.Test.csproj --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
st8vsblobprodcus339.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/Meziantou.Analyzer.slnx --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

- Skip reporting diagnostic for IEquatable<T>.Equals when T is a value type - Simplify IEquatable detection logic to work for both public and explicit implementations - Add test for value type scenario Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

- Use IsInterfaceImplementation() and GetImplementingInterfaceSymbol() helpers - Check OverriddenMethod chain to verify object.Equals override - Add tests for #nullable enable/disable scenarios - NullableAnnotation check already ensures nullable is enabled Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

Copilot · 2026-02-05T02:13:28Z

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

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Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

1s1vsblobprodcus386.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/Meziantou.Analyzer.slnx --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
pdfvsblobprodcus380.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/Meziantou.Analyzer.slnx --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/Meziantou.Analyzer.Test.csproj --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
st8vsblobprodcus339.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/Meziantou.Analyzer.slnx --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

meziantou · 2026-02-05T02:46:13Z

@copilot The original issues mentioned another method to handle

Another - much less common - example would be missing [MaybeNullWhen(false)] on value argument of an implementation of IDictionary<TKey,TValue>.TryGetValue(TKey key, [MaybeNullWhen(false)] out TValue value).

- Detect missing [MaybeNullWhen(false)] on value parameter of IDictionary<TKey, TValue>.TryGetValue implementations - Add tests for TryGetValue scenarios - Update documentation to cover both Equals and TryGetValue patterns Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

Copilot · 2026-02-05T02:49:50Z

@copilot The original issues mentioned another method to handle

Another - much less common - example would be missing [MaybeNullWhen(false)] on value argument of an implementation of IDictionary<TK...

Done in commit 9ee4a9f. Now detecting missing [MaybeNullWhen(false)] on the value parameter of IDictionary<TKey, TValue>.TryGetValue implementations. Added tests and updated documentation to cover both patterns.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

1s1vsblobprodcus386.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/Meziantou.Analyzer.slnx --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/Meziantou.Analyzer.Test.csproj --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
pdfvsblobprodcus380.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/Meziantou.Analyzer.Test.csproj --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
s4uvsblobprodcus326.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/Meziantou.Analyzer.slnx --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/Meziantou.Analyzer.Test.csproj --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
st8vsblobprodcus339.vsblob.vsassets.io
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/Meziantou.Analyzer.slnx --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)
- Triggering command: /usr/share/dotnet/dotnet dotnet restore --no-dependencies /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/Meziantou.Analyzer.Test.csproj --packages /home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/packages /p:DisableImplicitNuGetFallbackFolder=true --verbosity normal /p:TargetFrameworkRootPath=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:NetCoreTargetingPackRoot=/home/REDACTED/work/Meziantou.Analyzer/.codeql-scratch/dbs/csharp/working/emptyFakeDotnetRoot /p:AllowMissingPrunePackageData=true (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

Configure Actions setup steps to set up my environment, which run before the firewall is enabled
Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Initial plan

a3a7251

Copilot AI assigned Copilot and meziantou Feb 4, 2026

Copilot started work on behalf of meziantou February 4, 2026 20:24 View session

Copilot AI and others added 3 commits February 4, 2026 20:30

Add MA0186 analyzer for missing NotNullWhen attribute on Equals methods

177c012

Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

Update documentation for MA0186 rule

ea1a3ea

Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

Add detailed documentation for MA0186 rule

e2201e8

Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Introduce rule that reports missing nullable attributes~~ Add MA0186: Report missing [NotNullWhen(true)] on Equals parameters Feb 4, 2026

Copilot AI requested a review from meziantou February 4, 2026 20:38

Copilot finished work on behalf of meziantou February 4, 2026 20:38

Copilot started work on behalf of meziantou February 4, 2026 20:57 View session

Regenerate documentation with source links

91d0c1f

Co-authored-by: meziantou <509220+meziantou@users.noreply.github.com>

Copilot finished work on behalf of meziantou February 4, 2026 21:00

Copilot started work on behalf of meziantou February 4, 2026 22:05 View session

Copilot finished work on behalf of meziantou February 4, 2026 22:19

meziantou reviewed Feb 5, 2026

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Comment thread src/Meziantou.Analyzer/Rules/MissingNotNullWhenAttributeOnEqualsAnalyzer.cs Outdated

Comment thread src/Meziantou.Analyzer/Rules/MissingNotNullWhenAttributeOnEqualsAnalyzer.cs Outdated

Copilot started work on behalf of meziantou February 5, 2026 02:06 View session

Copilot AI requested a review from meziantou February 5, 2026 02:13

Copilot finished work on behalf of meziantou February 5, 2026 02:13

Copilot started work on behalf of meziantou February 5, 2026 02:46 View session

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Add MA0186: Report missing nullable attributes on Equals and TryGetValue parameters#998

Add MA0186: Report missing nullable attributes on Equals and TryGetValue parameters#998
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