Add MA0181 - Report use of explicit casts by Copilot · Pull Request #952 · meziantou/Meziantou.Analyzer

Copilot · 2025-12-30T05:45:12Z

Add MA0181 rule identifier to RuleIdentifiers.cs
Create DoNotUseCastAnalyzer.cs analyzer that detects explicit cast operations
Create test file DoNotUseCastAnalyzerTests.cs with comprehensive test cases
Create documentation file MA0181.md
Run documentation generator to update README
Fix analyzer to use operation.IsImplicit (not Conversion.IsImplicit)
Verify tests pass on CI
Run code review and security checks

Fix Applied

Changed from operation.Conversion.IsImplicit to operation.IsImplicit:

operation.IsImplicit checks if the operation is implicit in source code (compiler-generated)
operation.Conversion.IsImplicit checks if the conversion is semantically implicit
Numeric casts like (int)charValue have explicit syntax but semantically implicit conversions
Using operation.IsImplicit correctly identifies all explicit casts in source code

This avoids syntax analysis while properly detecting all cast expressions.

Original prompt

This section details on the original issue you should resolve

<issue_title>Introduce rule that reports use of casts</issue_title>
<issue_description>Is your feature request related to a problem? Please describe.
Not a problem at all.

Describe the solution you'd like
I know I'm pushing it here, but in a type-safe world casts should be exceptional.
I'd therefore like you to consider introducing a rule (disabled by default of course) that reports casts.
</issue_description>

Comments on the Issue (you are @copilot in this section)
@meziantou I'm not sure how useful it would be. - Non-valid casts are reported by the compiler (CSxxx error) - Useless casts are reported by the compiler (IDExxx rule) - I suppose other casts are supposed to be valid in the codebase or covered by tests.
Maybe you can provide some examples where casts are not expected but devs still use them?</comment_new>
<comment_new>@meziantou
Just to be sure about the rule you expect:

The rule must report explicit conversions as a warning (e.g. (int)value)

The rule should be disabled by default

The rule should not report implicit conversions

The rule should not report safe casts (as, is)

What about common conversions such as:

(int)enumValue

char to number or number to char</comment_new>

Fixes Introduce rule that reports use of casts #950

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

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

docs/Rules/MA0181.md

meziantou · 2025-12-30T19:20:18Z

@copilot There are errors on ci

xUnit.net 00:00:03.91]     Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_CharToInt_ShouldReportDiagnostic [FAIL]
[xUnit.net 00:00:03.94]       Mismatch between number of diagnostics returned, expected "1" actual "0"
[xUnit.net 00:00:03.94]       
[xUnit.net 00:00:03.94]       Diagnostics:
[xUnit.net 00:00:03.94]           NONE.
[xUnit.net 00:00:03.94]       
[xUnit.net 00:00:03.94]       Stack Trace:
[xUnit.net 00:00:03.94]         /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(70,0): at TestHelper.ProjectBuilder.VerifyDiagnosticResults(IEnumerable`1 actualResults, IList`1 analyzers, IList`1 expectedResults)
[xUnit.net 00:00:03.94]         /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(52,0): at TestHelper.ProjectBuilder.VerifyDiagnostics(IList`1 analyzers, IList`1 expected)
[xUnit.net 00:00:03.94]         /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(34,0): at TestHelper.ProjectBuilder.ValidateAsync()
[xUnit.net 00:00:03.94]         /_/tests/Meziantou.Analyzer.Test/Rules/DoNotUseCastAnalyzerTests.cs(89,0): at Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_CharToInt_ShouldReportDiagnostic()
[xUnit.net 00:00:03.94]         --- End of stack trace from previous location ---
[xUnit.net 00:00:03.99]     Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_IntToDouble_ShouldReportDiagnostic [FAIL]
[xUnit.net 00:00:03.99]       Mismatch between number of diagnostics returned, expected "1" actual "0"
[xUnit.net 00:00:03.99]       
[xUnit.net 00:00:04.00]       Diagnostics:
[xUnit.net 00:00:04.00]           NONE.
[xUnit.net 00:00:04.00]       
[xUnit.net 00:00:04.00]       Stack Trace:
[xUnit.net 00:00:04.00]         /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(70,0): at TestHelper.ProjectBuilder.VerifyDiagnosticResults(IEnumerable`1 actualResults, IList`1 analyzers, IList`1 expectedResults)
[xUnit.net 00:00:04.00]         /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(52,0): at TestHelper.ProjectBuilder.VerifyDiagnostics(IList`1 analyzers, IList`1 expected)
[xUnit.net 00:00:04.00]         /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(34,0): at TestHelper.ProjectBuilder.ValidateAsync()
[xUnit.net 00:00:04.00]         /_/tests/Meziantou.Analyzer.Test/Rules/DoNotUseCastAnalyzerTests.cs(17,0): at Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_IntToDouble_ShouldReportDiagnostic()
[xUnit.net 00:00:04.00]         --- End of stack trace from previous location ---
  Failed Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_CharToInt_ShouldReportDiagnostic [2 s]
  Error Message:
   Mismatch between number of diagnostics returned, expected "1" actual "0"

Diagnostics:
    NONE.

  Stack Trace:
     at TestHelper.ProjectBuilder.VerifyDiagnosticResults(IEnumerable`1 actualResults, IList`1 analyzers, IList`1 expectedResults) in /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs:line 70
   at TestHelper.ProjectBuilder.VerifyDiagnostics(IList`1 analyzers, IList`1 expected) in /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs:line 52
   at TestHelper.ProjectBuilder.ValidateAsync() in /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs:line 34
   at Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_CharToInt_ShouldReportDiagnostic() in /_/tests/Meziantou.Analyzer.Test/Rules/DoNotUseCastAnalyzerTests.cs:line 89
--- End of stack trace from previous location ---

Error: Mismatch between number of diagnostics returned, expected "1" actual "0"

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

Copilot · 2025-12-30T19:28:07Z

@copilot There are errors on ci

...

Fixed in commit df960ea. The issue was that the analyzer was checking operation.Conversion.IsImplicit instead of checking the syntax type. Changed to check if operation.Syntax is CastExpressionSyntax, which properly detects all explicit cast expressions like (int)value, (double)x, (char)intVal, etc.

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:

www.nuget.org
- Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/bin/Debug/net10.0/Meziantou.Analyzer.Test.runtimeconfig.json --depsfile /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/bin/Debug/net10.0/Meziantou.Analyzer.Test.deps.json /home/REDACTED/work/Meziantou.Analyzer/Meziantou.Analyzer/tests/Meziantou.Analyzer.Test/bin/Debug/net10.0/testhost.dll --port 36139 --endpoint 127.0.0.1:036139 --role client --parentprocessid 4300 --datacollectionport 33091 --telemetryoptedin false (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 · 2025-12-30T20:04:44Z

src/Meziantou.Analyzer/Rules/DoNotUseCastAnalyzer.cs

+        var operation = (IConversionOperation)context.Operation;
+
+        // Only report cast expressions (explicit cast syntax like (int)value)
+        if (operation.Syntax is not CastExpressionSyntax)
+            return;
+
+        // Do not report if it's a user-defined conversion
+        if (operation.Conversion.IsUserDefined)
+            return;
+
+        context.ReportDiagnostic(Rule, operation);


@copilot you can do better and not use the syntax at all. the conversionOperation has all the info needed

Done in commit 08e7489. Refactored to use IConversionOperation properties instead of syntax analysis:

!operation.Conversion.IsImplicit - excludes implicit conversions

!operation.IsTryCast - excludes as operator

!operation.Conversion.IsUserDefined - excludes user-defined conversions

The key was adding the IsTryCast check that was missing from the original implementation.

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

meziantou · 2025-12-31T18:58:53Z

@copilot Tests fail:
[xUnit.net 00:00:10.76] Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_CharToInt_ShouldReportDiagnostic [FAIL]
[xUnit.net 00:00:10.77] Mismatch between number of diagnostics returned, expected "1" actual "0"
[xUnit.net 00:00:10.77]
[xUnit.net 00:00:10.77] Diagnostics:
[xUnit.net 00:00:10.77] NONE.
[xUnit.net 00:00:10.77]
[xUnit.net 00:00:10.77] Stack Trace:
[xUnit.net 00:00:10.77] //tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(70,0): at TestHelper.ProjectBuilder.VerifyDiagnosticResults(IEnumerable1 actualResults, IList1 analyzers, IList1 expectedResults) [xUnit.net 00:00:10.77] /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(52,0): at TestHelper.ProjectBuilder.VerifyDiagnostics(IList1 analyzers, IList`1 expected)
[xUnit.net 00:00:10.77] //tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(34,0): at TestHelper.ProjectBuilder.ValidateAsync()
[xUnit.net 00:00:10.77] /_/tests/Meziantou.Analyzer.Test/Rules/DoNotUseCastAnalyzerTests.cs(89,0): at Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_CharToInt_ShouldReportDiagnostic()
[xUnit.net 00:00:10.77] --- End of stack trace from previous location ---
Passed Meziantou.Analyzer.Test.Rules.PreferReturningCollectionAbstractionInsteadOfImplementationAnalyzerTests.Methods_Parameter_Invalid(visibility: "internal protected", type: "List") [59 ms]

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

Copilot · 2025-12-31T19:05:28Z

@copilot Tests fail:
[xUnit.net 00:00:10.76] Meziantou.Analyzer.Test.Rules.DoNotUseCastAnalyzerTests.ExplicitCast_CharToInt_ShouldReportDiagnostic [FAIL]
[xUnit.net 00:00:10.77] Mismatch bet...

Fixed in commit 9338e3a. The issue was using operation.Conversion.IsImplicit instead of operation.IsImplicit. The key difference:

operation.IsImplicit checks if the operation is implicit in source code (compiler-generated)
operation.Conversion.IsImplicit checks if the conversion is semantically implicit

Numeric casts like (int)charValue have explicit syntax but are semantically implicit, so checking operation.IsImplicit correctly identifies all explicit casts.

Initial plan

0cf0e18

Copilot AI assigned Copilot and meziantou Dec 30, 2025

Copilot started work on behalf of meziantou December 30, 2025 05:45 View session

Copilot AI and others added 2 commits December 30, 2025 05:53

Add MA0181 analyzer to report use of casts

d917abd

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

Update documentation for MA0181

3544d16

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

Copilot AI changed the title ~~[WIP] Introduce rule to report usage of type casts~~ Add MA0181 - Report use of explicit casts Dec 30, 2025

Copilot AI requested a review from meziantou December 30, 2025 05:58

Copilot finished work on behalf of meziantou December 30, 2025 05:58

meziantou reviewed Dec 30, 2025

View reviewed changes

docs/Rules/MA0181.md Outdated Show resolved Hide resolved

Apply suggestion from @meziantou

cc67308

Copilot started work on behalf of meziantou December 30, 2025 19:20 View session

Fix MA0181 to properly detect cast expressions using CastExpressionSy…

df960ea

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

Copilot AI requested a review from meziantou December 30, 2025 19:29

Copilot finished work on behalf of meziantou December 30, 2025 19:29

meziantou reviewed Dec 30, 2025

View reviewed changes

Copilot started work on behalf of meziantou December 30, 2025 20:05 View session

Refactor MA0181 to use IConversionOperation properties instead of syn…

08e7489

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

Copilot AI requested a review from meziantou December 30, 2025 20:11

Copilot finished work on behalf of meziantou December 30, 2025 20:11

Copilot started work on behalf of meziantou December 31, 2025 18:59 View session

Fix MA0181: Use operation.IsImplicit instead of operation.Conversion.…

9338e3a

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

Copilot finished work on behalf of meziantou December 31, 2025 19:06

meziantou marked this pull request as ready for review December 31, 2025 19:30

meziantou merged commit 4f1e460 into main Dec 31, 2025
13 checks passed

meziantou deleted the copilot/add-rule-to-report-casts branch December 31, 2025 20:40

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Add MA0181 - Report use of explicit casts#952

Add MA0181 - Report use of explicit casts#952
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