feat: Add PH2164 analyzer to enforce StringComparer for string-keyed collections

Documentation/Diagnostics/PH2164.md

@@ -0,0 +1,73 @@
+# PH2164: String-keyed collections should specify a StringComparer
+
+| Property | Value  |
+|--|--|
+| Package | [Philips.CodeAnalysis.MaintainabilityAnalyzers](https://www.nuget.org/packages/Philips.CodeAnalysis.MaintainabilityAnalyzers) |
+| Diagnostic ID | PH2164 |
+| Category  | [Maintainability](../Maintainability.md) |
+| Analyzer | [StringDictionaryNeedsComparerAnalyzer](https://github.com/philips-software/roslyn-analyzers/blob/main/Philips.CodeAnalysis.MaintainabilityAnalyzers/Maintainability/StringDictionaryNeedsComparerAnalyzer.cs)
+| CodeFix  | No |
+| Severity | Error |
+| Enabled By Default | Yes |
+
+## Introduction
+
+String-keyed collections should explicitly specify a StringComparer (or IComparer<string> for sorted collections) to avoid culture-dependent or case-sensitivity surprises. Without an explicit comparer, collections may use culture-sensitive string comparisons that can lead to unexpected behavior in different locales.
+
+## How to solve
+
+Explicitly specify a StringComparer when creating string-keyed collections:
+
+- For equality-based collections: Use `StringComparer.Ordinal`, `StringComparer.OrdinalIgnoreCase`, or other appropriate StringComparer options
+- For sorted collections: Use `StringComparer.Ordinal` (cast to IComparer<string>) or other appropriate comparer
+
+## Example
+
+Code that triggers a diagnostic:
+``` cs
+class BadExample
+{
+    public void BadMethod()
+    {
+        // Missing StringComparer - uses default culture-sensitive comparison
+        var dict = new Dictionary<string, int>();
+        var set = new HashSet<string>();
+
+        // Also problematic for sorted collections
+        var sortedDict = new SortedDictionary<string, int>();
+        var sortedSet = new SortedSet<string>();
+    }
+}
+```
+
+And the improved replacement:
+``` cs
+class GoodExample
+{
+    public void GoodMethod()
+    {
+        // Explicit StringComparer for predictable behavior
+        var dict = new Dictionary<string, int>(StringComparer.Ordinal);
+        var set = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+
+        // Explicit comparer for sorted collections
+        var sortedDict = new SortedDictionary<string, int>(StringComparer.Ordinal);
+        var sortedSet = new SortedSet<string>(StringComparer.OrdinalIgnoreCase);
+    }
+}
+```
+
+## Supported Collections
+
+The analyzer detects the following string-keyed collection types:
+
+- `Dictionary<string, TValue>`
+- `HashSet<string>`
+- `ConcurrentDictionary<string, TValue>` (planned)
+- `SortedDictionary<string, TValue>` (planned)
+- `SortedSet<string>` (planned)
+- `ImmutableDictionary<string, TValue>` (planned)
+
+## Configuration
+
+This analyzer is enabled by default. The general ways of [suppressing](https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/suppress-warnings) diagnostics apply.

Philips.CodeAnalysis.Common/DiagnosticId.cs

@@ -145,5 +145,6 @@ public enum DiagnosticId
 		AvoidUnlicensedPackages = 2155,
 		AvoidPkcsPaddingWithRsaEncryption = 2158,
 		AvoidUnnecessaryAttributeParentheses = 2159,
+		StringDictionaryNeedsComparer = 2164,
 	}
 }

Philips.CodeAnalysis.MaintainabilityAnalyzers/Maintainability/StringDictionaryNeedsComparerAnalyzer.cs

@@ -0,0 +1,236 @@
+// © 2025 Koninklijke Philips N.V. See License.md in the project root for license information.
+
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Operations;
+using Philips.CodeAnalysis.Common;
+
+namespace Philips.CodeAnalysis.MaintainabilityAnalyzers.Maintainability
+{
+	[DiagnosticAnalyzer(LanguageNames.CSharp)]
+	public class StringDictionaryNeedsComparerAnalyzer : DiagnosticAnalyzerBase
+	{
+		private const string Title = @"String-keyed collections should specify a StringComparer";
+		public const string MessageFormat = @"{0} with a string key should explicitly specify a {1}";
+		private const string Description = @"String-keyed collections should explicitly specify a StringComparer (or IComparer<string> for sorted collections) to avoid culture-dependent or case-sensitivity surprises.";
+		private const string Category = Categories.Maintainability;
+
+		private static readonly DiagnosticDescriptor Rule = new(DiagnosticId.StringDictionaryNeedsComparer.ToId(), Title, MessageFormat, Category, DiagnosticSeverity.Error, isEnabledByDefault: true, description: Description);
+
+		public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get { return ImmutableArray.Create(Rule); } }
+
+		protected override void InitializeCompilation(CompilationStartAnalysisContext context)
+		{
+			var wellKnownTypes = new WellKnownTypes(context.Compilation);
+
+			// Debug: Only proceed if we have the basic types
+			if (wellKnownTypes.String == null || wellKnownTypes.Dictionary_T2 == null)
+			{
+				return;
+			}
+
+			context.RegisterOperationAction(ctx => AnalyzeObjectCreation(ctx, wellKnownTypes), OperationKind.ObjectCreation);
+			context.RegisterOperationAction(ctx => AnalyzeInvocation(ctx, wellKnownTypes), OperationKind.Invocation);
+		}
+
+		private static void AnalyzeObjectCreation(OperationAnalysisContext context, WellKnownTypes types)
+		{
+			var creation = (IObjectCreationOperation)context.Operation;
+			if (creation.Type is not INamedTypeSymbol constructed)
+			{
+				return;
+			}
+
+			// Determine if this is one of the supported collection types with string key
+			if (!TryGetCollectionInfo(constructed, types, out _, out RequiredComparer requiresComparerType))
+			{
+				return;
+			}
+
+			// If a comparer argument is already provided, nothing to do
+			IMethodSymbol ctor = creation.Constructor;
+			if (ctor == null)
+			{
+				return;
+			}
+
+			if (HasRequiredComparerParameter(ctor, requiresComparerType, types))
+			{
+				return; // explicit comparer provided
+			}
+
+			// No comparer in the selected overload → report
+			var collectionDisplay = constructed.ToDisplayString(SymbolDisplayFormat.MinimallyQualifiedFormat);
+			var comparerDisplay = requiresComparerType == RequiredComparer.IEqualityComparer
+				? "StringComparer/IEqualityComparer<string>"
+				: "StringComparer/IComparer<string>";
+
+			context.ReportDiagnostic(Diagnostic.Create(
+				Rule,
+				creation.Syntax.GetLocation(),
+				collectionDisplay,
+				comparerDisplay));
+		}
+
+		private static void AnalyzeInvocation(OperationAnalysisContext context, WellKnownTypes types)
+		{
+			var invocation = (IInvocationOperation)context.Operation;
+			IMethodSymbol targetMethod = invocation.TargetMethod;
+
+			// ImmutableDictionary.Create<TKey, TValue>(...) without comparer
+			if (types.ImmutableDictionary != null && targetMethod.ContainingType.Equals(types.ImmutableDictionary, SymbolEqualityComparer.Default) &&
+				targetMethod.Name is "Create" or "CreateRange")
+			{
+				ImmutableArray<ITypeSymbol> typeArgs = targetMethod.TypeArguments;
+				if (typeArgs.Length == 2 && SymbolEqualityComparer.Default.Equals(typeArgs[0], types.String) &&
+					!HasRequiredComparerParameter(targetMethod, RequiredComparer.IEqualityComparer, types))
+				{
+					var collectionDisplay = $"ImmutableDictionary<{typeArgs[0].ToDisplayString(SymbolDisplayFormat.MinimallyQualifiedFormat)}, {typeArgs[1].ToDisplayString(SymbolDisplayFormat.MinimallyQualifiedFormat)}>";
+					context.ReportDiagnostic(Diagnostic.Create(
+						Rule,
+						invocation.Syntax.GetLocation(),
+						collectionDisplay,
+						"StringComparer/IEqualityComparer<string>"));
+				}
+			}
+		}
+
+
+		private enum RequiredComparer { IEqualityComparer, IComparer }
+
+		private static bool TryGetCollectionInfo(INamedTypeSymbol constructed, WellKnownTypes types, out string kind, out RequiredComparer requires)
+		{
+			kind = default;
+			requires = default;
+
+			// HashSet<string>
+			if (types.HashSet_T != null && constructed.ConstructedFrom.Equals(types.HashSet_T, SymbolEqualityComparer.Default))
+			{
+				if (constructed.TypeArguments.Length == 1 && SymbolEqualityComparer.Default.Equals(constructed.TypeArguments[0], types.String))
+				{
+					kind = "HashSet";
+					requires = RequiredComparer.IEqualityComparer;
+					return true;
+				}
+				return false;
+			}
+
+			// Dictionary<string, TValue>
+			if (types.Dictionary_T2 != null && constructed.ConstructedFrom.Equals(types.Dictionary_T2, SymbolEqualityComparer.Default))
+			{
+				if (constructed.TypeArguments.Length == 2 && SymbolEqualityComparer.Default.Equals(constructed.TypeArguments[0], types.String))
+				{
+					kind = "Dictionary";
+					requires = RequiredComparer.IEqualityComparer;
+					return true;
+				}
+				return false;
+			}
+
+			// ConcurrentDictionary<string, TValue>
+			if (types.ConcurrentDictionary_T2 != null && constructed.ConstructedFrom.Equals(types.ConcurrentDictionary_T2, SymbolEqualityComparer.Default))
+			{
+				if (constructed.TypeArguments.Length == 2 && SymbolEqualityComparer.Default.Equals(constructed.TypeArguments[0], types.String))
+				{
+					kind = "ConcurrentDictionary";
+					requires = RequiredComparer.IEqualityComparer;
+					return true;
+				}
+				return false;
+			}
+
+			// SortedDictionary<string, TValue>
+			if (types.SortedDictionary_T2 != null && constructed.ConstructedFrom.Equals(types.SortedDictionary_T2, SymbolEqualityComparer.Default))
+			{
+				if (constructed.TypeArguments.Length == 2 && SymbolEqualityComparer.Default.Equals(constructed.TypeArguments[0], types.String))
+				{
+					kind = "SortedDictionary";
+					requires = RequiredComparer.IComparer;
+					return true;
+				}
+				return false;
+			}
+
+			// SortedSet<string>
+			if (types.SortedSet_T != null && constructed.ConstructedFrom.Equals(types.SortedSet_T, SymbolEqualityComparer.Default))
+			{
+				if (constructed.TypeArguments.Length == 1 && SymbolEqualityComparer.Default.Equals(constructed.TypeArguments[0], types.String))
+				{
+					kind = "SortedSet";
+					requires = RequiredComparer.IComparer;
+					return true;
+				}
+				return false;
+			}
+
+			// ImmutableDictionary<string, TValue>
+			if (types.ImmutableDictionary_T2 != null && constructed.ConstructedFrom.Equals(types.ImmutableDictionary_T2, SymbolEqualityComparer.Default))
+			{
+				if (constructed.TypeArguments.Length == 2 && SymbolEqualityComparer.Default.Equals(constructed.TypeArguments[0], types.String))
+				{
+					kind = "ImmutableDictionary";
+					requires = RequiredComparer.IEqualityComparer;
+					return true;
+				}
+				return false;
+			}
+
+			return false;
+		}
+
+		private static bool HasRequiredComparerParameter(IMethodSymbol method, RequiredComparer requires, WellKnownTypes types)
+		{
+			foreach (IParameterSymbol param in method.Parameters)
+			{
+				if (requires == RequiredComparer.IEqualityComparer &&
+					types.IEqualityComparer_T != null &&
+					param.Type is INamedTypeSymbol p &&
+					p.OriginalDefinition.Equals(types.IEqualityComparer_T, SymbolEqualityComparer.Default) &&
+					p.TypeArguments.Length == 1 && SymbolEqualityComparer.Default.Equals(p.TypeArguments[0], types.String))
+				{
+					return true;
+				}
+
+				if (requires == RequiredComparer.IComparer &&
+					types.IComparer_T != null &&
+					param.Type is INamedTypeSymbol p2 &&
+					p2.OriginalDefinition.Equals(types.IComparer_T, SymbolEqualityComparer.Default) &&
+					p2.TypeArguments.Length == 1 && SymbolEqualityComparer.Default.Equals(p2.TypeArguments[0], types.String))
+				{
+					return true;
+				}
+			}
+
+			return false;
+		}
+
+		private sealed class WellKnownTypes
+		{
+			public INamedTypeSymbol String { get; }
+			public INamedTypeSymbol IEqualityComparer_T { get; }
+			public INamedTypeSymbol IComparer_T { get; }
+			public INamedTypeSymbol Dictionary_T2 { get; }
+			public INamedTypeSymbol HashSet_T { get; }
+			public INamedTypeSymbol ConcurrentDictionary_T2 { get; }
+			public INamedTypeSymbol SortedDictionary_T2 { get; }
+			public INamedTypeSymbol SortedSet_T { get; }
+			public INamedTypeSymbol ImmutableDictionary_T2 { get; }
+			public INamedTypeSymbol ImmutableDictionary { get; }
+
+			public WellKnownTypes(Compilation compilation)
+			{
+				String = compilation.GetSpecialType(SpecialType.System_String);
+				IEqualityComparer_T = compilation.GetTypeByMetadataName("System.Collections.Generic.IEqualityComparer`1");
+				IComparer_T = compilation.GetTypeByMetadataName("System.Collections.Generic.IComparer`1");
+				Dictionary_T2 = compilation.GetTypeByMetadataName("System.Collections.Generic.Dictionary`2");
+				HashSet_T = compilation.GetTypeByMetadataName("System.Collections.Generic.HashSet`1");
+				ConcurrentDictionary_T2 = compilation.GetTypeByMetadataName("System.Collections.Concurrent.ConcurrentDictionary`2");
+				SortedDictionary_T2 = compilation.GetTypeByMetadataName("System.Collections.Generic.SortedDictionary`2");
+				SortedSet_T = compilation.GetTypeByMetadataName("System.Collections.Generic.SortedSet`1");
+				ImmutableDictionary_T2 = compilation.GetTypeByMetadataName("System.Collections.Immutable.ImmutableDictionary`2");
+				ImmutableDictionary = compilation.GetTypeByMetadataName("System.Collections.Immutable.ImmutableDictionary");
+			}
+		}
+	}
+}