Fix MA0185 false-positive for nullable value types

[Copilot]

Fix MA0185 False-positive with int?

• Analyze the issue and understand the root cause
• Add test case reproducing the false-positive with int?
• Fix the analyzer correctly by using UnwrapNullableOfT option
• Verify the fix with tests (13 MA0185 tests pass)
• Run broader tests to ensure no regressions (122 culture-sensitive tests, 76 MA0011 tests pass)
• Update documentation (no changes needed)
• Run code review and security checks

Root Cause

The MA0185 analyzer was using CultureSensitiveOptions.None which doesn't unwrap nullable types. This caused int? to be checked as Nullable<int> which doesn't implement IFormattable, leading to the false positive.

Changes Made

• Modified SimplifyStringCreateWhenAllParametersAreCultureInvariantAnalyzer to use CultureSensitiveOptions.UnwrapNullableOfT instead of CultureSensitiveOptions.None
• This ensures nullable types like int? are checked by their underlying type (int) which correctly identifies them as culture-sensitive
• Reverted the change to CultureSensitiveFormattingContext.IsFormattableType() which was breaking MA0011's consider_nullable_types configuration option
• Added test cases for nullable types:
  • StringCreateWithInvariantCulture_WithNullableInteger_NoDiagnostic - verifies fix for int?
  • StringCreateWithInvariantCulture_WithNullableDouble_NoDiagnostic - verifies fix for double?

Testing

• All 13 MA0185 analyzer-specific tests pass
• All 76 MA0011 analyzer tests pass (including the previously failing NullableInt32ToStringWithoutCultureInfo_DisabledConfig)
• All 122 culture-sensitive related tests pass

Why This Approach

The previous approach of always unwrapping in IsFormattableType broke MA0011's behavior when consider_nullable_types is false. The correct fix is to make MA0185 explicitly unwrap nullable types by using the UnwrapNullableOfT option, which respects the semantics of checking underlying types for string.Create with InvariantCulture.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>MA0185 False-positive with int?</issue_title>
<issue_description>### Version of the Meziantou.Analyzer NuGet package

2.0.295

Rule Identifier

MA0185

Target Framework

net10.0

C# Language version

C# 14

Description

I'm getting false positive error

0>Program.cs(10,14): Error MA0185 : Simplify string.Create when all parameters are culture invariant (https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0185.md)

Changing to int n (non-nullable) doesn't trigger the rule.

If I change to interpolated string, I'm getting a different warning

MA0076 - Do not use implicit culture-sensitive ToString in interpolated strings

Reproduction Steps

Minimal code:

class Program
{
	static void Main(string[] args)
	{
		int? n = 42;
		var s = string.Create(CultureInfo.InvariantCulture, $@"/v{n}");
		Console.WriteLine(s);
	}
}

Other information

No response</issue_description>

<agent_instructions>It seems MA0185 consider the expression to be culture invariant, when it is not</agent_instructions>

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

• Fixes MA0185 False-positive with int? #994

---

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[meziantou]

@copilot Test fails

[xUnit.net 00:01:30.36] Meziantou.Analyzer.Test.Rules.UseIFormatProviderAnalyzerTests.NullableInt32ToStringWithoutCultureInfo_DisabledConfig [FAIL]
[xUnit.net 00:01:30.37] Mismatch between number of diagnostics returned, expected "0" actual "1"
[xUnit.net 00:01:30.37]
[xUnit.net 00:01:30.37] Diagnostics:
[xUnit.net 00:01:30.37] // Test0.cs(1,1): warning MA0011: Use an overload of 'ToString' that has a 'System.IFormatProvider' parameter
[xUnit.net 00:01:30.37] GetCSharpResultAt(1, 1, List1.MA0011) [xUnit.net 00:01:30.37] [xUnit.net 00:01:30.37] [xUnit.net 00:01:30.37] Source: [xUnit.net 00:01:30.37] ((int?)1).ToString();^^^^^^^^^^^^^^^^^^^^ MA0011: Use an overload of 'ToString' that has a 'System.IFormatProvider' parameter [xUnit.net 00:01:30.37] [xUnit.net 00:01:30.37] Stack Trace: [xUnit.net 00:01:30.37] /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(79,0): at TestHelper.ProjectBuilder.VerifyDiagnosticResults(IEnumerable1 actualResults, IList1 analyzers, IList1 expectedResults)
[xUnit.net 00:01:30.37] //tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(53,0): at TestHelper.ProjectBuilder.VerifyDiagnostics(IList1 analyzers, IList1 expected)
[xUnit.net 00:01:30.37] //tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs(35,0): at TestHelper.ProjectBuilder.ValidateAsync()
[xUnit.net 00:01:30.37] /_/tests/Meziantou.Analyzer.Test/Rules/UseIFormatProviderAnalyzerTests.cs(242,0): at Meziantou.Analyzer.Test.Rules.UseIFormatProviderAnalyzerTests.NullableInt32ToStringWithoutCultureInfo_DisabledConfig()
[xUnit.net 00:01:30.37] --- End of stack trace from previous location ---
Passed Meziantou.Analyzer.Test.Rules.DoNotUseZeroToInitializeAnEnumValueTests.EnumBaseType_NonZero(baseType: "uint", value: "(int)1") [83 ms]
Passed Meziantou.Analyzer.Test.Rules.UseIFormatProviderAnalyzerTests.Int32ToStringWithoutCultureInfo_ShouldReportDiagnostic [124 ms]
Passed Meziantou.Analyzer.Test.Rules.AvoidUnusedInternalTypesAnalyzerTests.InternalClassUsedInTypeGenericConstraint_NoDiagnostic [85 ms]
Passed Meziantou.Analyzer.Test.Rules.DoNotUseZeroToInitializeAnEnumValueTests.EnumBaseType_NonZero(baseType: "uint", value: "1L") [97 ms]
Passed Meziantou.Analyzer.Test.Rules.AvoidUnusedInternalTypesAnalyzerTests.UnusedInternalDelegate_Diagnostic [93 ms]
Failed Meziantou.Analyzer.Test.Rules.UseIFormatProviderAnalyzerTests.NullableInt32ToStringWithoutCultureInfo_DisabledConfig [127 ms]
Error Message:
Mismatch between number of diagnostics returned, expected "0" actual "1"

Diagnostics:
// Test0.cs(1,1): warning MA0011: Use an overload of 'ToString' that has a 'System.IFormatProvider' parameter
GetCSharpResultAt(1, 1, List`1.MA0011)

Source:
((int?)1).ToString();^^^^^^^^^^^^^^^^^^^^ MA0011: Use an overload of 'ToString' that has a 'System.IFormatProvider' parameter

Stack Trace:
at TestHelper.ProjectBuilder.VerifyDiagnosticResults(IEnumerable1 actualResults, IList1 analyzers, IList1 expectedResults) in /_/tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs:line 79 at TestHelper.ProjectBuilder.VerifyDiagnostics(IList1 analyzers, IList`1 expected) in //tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs:line 53
at TestHelper.ProjectBuilder.ValidateAsync() in //tests/Meziantou.Analyzer.Test/Helpers/ProjectBuilder.Validation.cs:line 35
at Meziantou.Analyzer.Test.Rules.UseIFormatProviderAnalyzerTests.NullableInt32ToStringWithoutCultureInfo_DisabledConfig() in /_/tests/Meziantou.Analyzer.Test/Rules/UseIFormatProviderAnalyzerTests.cs:line 242
--- End of stack trace from previous location ---

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

[Copilot]

@copilot Test fails

[xUnit.net 00:01:30.36] Meziantou.Analyzer.Test.Rules.UseIFormatProviderAnalyzerTests.NullableInt32ToStringWithoutCultureInfo_DisabledConfig [FAIL]
...

Fixed in e2c59fe. The issue was that I modified IsFormattableType to always unwrap nullable types, which broke MA0011's consider_nullable_types=false configuration. The correct fix is to make MA0185 use CultureSensitiveOptions.UnwrapNullableOfT instead of None, which respects the semantic that for string.Create with InvariantCulture we should check the underlying type of nullable values. All tests now pass.

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