New rules: MA0141 and MA0142 to convert null check to pattern matching

README.md

@@ -156,6 +156,8 @@ If you are already using other analyzers, you can check [which rules are duplica
 |[MA0138](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0138.md)|Design|Do not use 'Async' suffix when a method does not return an awaitable type|⚠️|❌|❌|
 |[MA0139](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0139.md)|Design|Log Parameter type is not valid|⚠️|✔️|❌|
 |[MA0140](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0140.md)|Design|Both if and else branch have identical code|⚠️|✔️|❌|
+|[MA0141](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0141.md)|Usage|Use pattern matching instead of inequality operators|ℹ️|❌|✔️|
+|[MA0142](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0142.md)|Usage|Use pattern matching instead of equality operators|ℹ️|❌|✔️|
 
 <!-- rules -->
 

docs/README.md

@@ -140,6 +140,8 @@
 |[MA0138](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0138.md)|Design|Do not use 'Async' suffix when a method does not return an awaitable type|<span title='Warning'>⚠️</span>|❌|❌|
 |[MA0139](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0139.md)|Design|Log Parameter type is not valid|<span title='Warning'>⚠️</span>|✔️|❌|
 |[MA0140](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0140.md)|Design|Both if and else branch have identical code|<span title='Warning'>⚠️</span>|✔️|❌|
+|[MA0141](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0141.md)|Usage|Use pattern matching instead of inequality operators|<span title='Info'>ℹ️</span>|❌|✔️|
+|[MA0142](https://github.com/meziantou/Meziantou.Analyzer/blob/main/docs/Rules/MA0142.md)|Usage|Use pattern matching instead of equality operators|<span title='Info'>ℹ️</span>|❌|✔️|
 
 |Id|Suppressed rule|Justification|
 |--|---------------|-------------|
@@ -566,6 +568,12 @@ dotnet_diagnostic.MA0139.severity = warning
 
 # MA0140: Both if and else branch have identical code
 dotnet_diagnostic.MA0140.severity = warning
+
+# MA0141: Use pattern matching instead of inequality operators
+dotnet_diagnostic.MA0141.severity = none
+
+# MA0142: Use pattern matching instead of equality operators
+dotnet_diagnostic.MA0142.severity = none
 ```
 
 # .editorconfig - all rules disabled
@@ -987,4 +995,10 @@ dotnet_diagnostic.MA0139.severity = none
 
 # MA0140: Both if and else branch have identical code
 dotnet_diagnostic.MA0140.severity = none
+
+# MA0141: Use pattern matching instead of inequality operators
+dotnet_diagnostic.MA0141.severity = none
+
+# MA0142: Use pattern matching instead of equality operators
+dotnet_diagnostic.MA0142.severity = none
 ```

docs/Rules/MA0141.md

@@ -0,0 +1,7 @@
+# MA0141 - Use pattern matching instead of inequality operators
+
+````c#
+value != null; // not compliant
+
+value is not null; // ok
+````

docs/Rules/MA0142.md

@@ -0,0 +1,7 @@
+# MA0142 - Use pattern matching instead of equality operators
+
+````c#
+value == null; // not compliant
+
+value is null; // ok
+````

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
     "version": "7.0.400",
-    "rollForward": "feature"
+    "rollForward": "latestMajor"
   }
 }

src/Meziantou.Analyzer.CodeFixers/Rules/UsePatternMatchingForNullCheckFixer.cs

@@ -0,0 +1,61 @@
+using System.Collections.Immutable;
+using System.Composition;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CodeActions;
+using Microsoft.CodeAnalysis.CodeFixes;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Editing;
+using Microsoft.CodeAnalysis.Operations;
+using static Microsoft.CodeAnalysis.CSharp.SyntaxFactory;
+
+namespace Meziantou.Analyzer.Rules;
+
+[ExportCodeFixProvider(LanguageNames.CSharp), Shared]
+public sealed class UsePatternMatchingForNullCheckFixer : CodeFixProvider
+{
+    public override ImmutableArray<string> FixableDiagnosticIds => ImmutableArray.Create(RuleIdentifiers.UsePatternMatchingForNullCheck, RuleIdentifiers.UsePatternMatchingForNullEquality);
+
+    public override FixAllProvider GetFixAllProvider() => WellKnownFixAllProviders.BatchFixer;
+
+    public override async Task RegisterCodeFixesAsync(CodeFixContext context)
+    {
+        var root = await context.Document.GetSyntaxRootAsync(context.CancellationToken).ConfigureAwait(false);
+        var nodeToFix = root?.FindNode(context.Span, getInnermostNodeForTie: true);
+        if (nodeToFix is not BinaryExpressionSyntax invocation)
+            return;
+
+        context.RegisterCodeFix(
+            CodeAction.Create(
+                "Use pattern matching",
+                ct => Update(context.Document, invocation, ct),
+                equivalenceKey: "Use pattern matching"),
+            context.Diagnostics);
+    }
+
+    private static async Task<Document> Update(Document document, BinaryExpressionSyntax node, CancellationToken cancellationToken)
+    {
+        var editor = await DocumentEditor.CreateAsync(document, cancellationToken).ConfigureAwait(false);
+        if (editor.SemanticModel.GetOperation(node, cancellationToken) is not IBinaryOperation operation)
+            return document;
+
+        var valueSyntax = IsNull(operation.LeftOperand) ? operation.RightOperand.Syntax : operation.LeftOperand.Syntax;
+        if (valueSyntax is not ExpressionSyntax expression)
+            return document;
+
+        PatternSyntax constantExpression = ConstantPattern(LiteralExpression(SyntaxKind.NullLiteralExpression));
+        if (operation.OperatorKind is BinaryOperatorKind.NotEquals)
+        {
+            constantExpression = UnaryPattern(constantExpression);
+        }
+
+        var newSyntax = IsPatternExpression(expression, constantExpression);
+        editor.ReplaceNode(node, newSyntax);
+        return editor.GetChangedDocument();
+    }
+
+    private static bool IsNull(IOperation operation)
+        => operation.UnwrapConversionOperations() is ILiteralOperation { ConstantValue: { HasValue: true, Value: null } };
+}

src/Meziantou.Analyzer/RuleIdentifiers.cs

@@ -143,6 +143,8 @@ internal static class RuleIdentifiers
     public const string MethodsNotReturningAnAwaitableTypeMustNotHaveTheAsyncSuffix = "MA0138";
     public const string LoggerParameterType_Serilog = "MA0139";
     public const string IfElseBranchesAreIdentical = "MA0140";
+    public const string UsePatternMatchingForNullCheck = "MA0141";
+    public const string UsePatternMatchingForNullEquality = "MA0142";
 
     public static string GetHelpUri(string identifier)
     {

src/Meziantou.Analyzer/Rules/UsePatternMatchingForNullCheckAnalyzer.cs

@@ -0,0 +1,56 @@
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Operations;
+
+namespace Meziantou.Analyzer.Rules;
+
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class UsePatternMatchingForNullCheckAnalyzer : DiagnosticAnalyzer
+{
+    private static readonly DiagnosticDescriptor s_ruleEqual = new(
+        RuleIdentifiers.UsePatternMatchingForNullEquality,
+        title: "Use pattern matching instead of equality operators",
+        messageFormat: "Use pattern matching instead of equality operators",
+        RuleCategories.Usage,
+        DiagnosticSeverity.Info,
+        isEnabledByDefault: false,
+        description: "",
+        helpLinkUri: RuleIdentifiers.GetHelpUri(RuleIdentifiers.UsePatternMatchingForNullEquality));
+
+    private static readonly DiagnosticDescriptor s_ruleNotEqual = new(
+        RuleIdentifiers.UsePatternMatchingForNullCheck,
+        title: "Use pattern matching instead of inequality operators",
+        messageFormat: "Use pattern matching instead of inequality operators",
+        RuleCategories.Usage,
+        DiagnosticSeverity.Info,
+        isEnabledByDefault: false,
+        description: "",
+        helpLinkUri: RuleIdentifiers.GetHelpUri(RuleIdentifiers.UsePatternMatchingForNullCheck));
+
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(s_ruleEqual, s_ruleNotEqual);
+
+    public override void Initialize(AnalysisContext context)
+    {
+        context.EnableConcurrentExecution();
+        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.Analyze | GeneratedCodeAnalysisFlags.ReportDiagnostics);
+        context.RegisterOperationAction(AnalyzeBinary, OperationKind.Binary);
+    }
+
+    private void AnalyzeBinary(OperationAnalysisContext context)
+    {
+        var operation = (IBinaryOperation)context.Operation;
+        if (operation is { OperatorKind: BinaryOperatorKind.Equals or BinaryOperatorKind.NotEquals, OperatorMethod: null })
+        {
+            var leftIfNull = IsNull(operation.LeftOperand);
+            var rightIfNull = IsNull(operation.RightOperand);
+            if (leftIfNull ^ rightIfNull)
+            {
+                context.ReportDiagnostic(operation.OperatorKind is BinaryOperatorKind.Equals ? s_ruleEqual : s_ruleNotEqual, operation);
+            }
+        }
+    }
+
+    private static bool IsNull(IOperation operation)
+        => operation.UnwrapConversionOperations() is ILiteralOperation { ConstantValue: { HasValue: true, Value: null } };
+}

tests/Meziantou.Analyzer.Test/Rules/UsePatternMatchingForNullCheckAnalyzerTests.cs

@@ -0,0 +1,104 @@
+using System.Threading.Tasks;
+using Meziantou.Analyzer.Rules;
+using TestHelper;
+using Xunit;
+
+namespace Meziantou.Analyzer.Test.Rules;
+
+public sealed class UsePatternMatchingForNullCheckAnalyzerTests
+{
+    private static ProjectBuilder CreateProjectBuilder()
+    {
+        return new ProjectBuilder()
+            .WithOutputKind(Microsoft.CodeAnalysis.OutputKind.ConsoleApplication)
+            .WithAnalyzer<UsePatternMatchingForNullCheckAnalyzer>()
+            .WithCodeFixProvider<UsePatternMatchingForNullCheckFixer>();
+    }
+
+    [Fact]
+    public async Task NullCheckForNullableOfT()
+    {
+        await CreateProjectBuilder()
+              .WithSourceCode("_ = [|(int?)0 == null|];")
+              .ShouldFixCodeWith("_ = (int?)0 is null;")
+              .ValidateAsync();
+    }
+
+    [Fact]
+    public async Task NullCheckForNullableOfT_NotNull()
+    {
+        await CreateProjectBuilder()
+              .WithSourceCode("_ = [|(int?)0 != null|];")
+              .ShouldFixCodeWith("_ = (int?)0 is not null;")
+              .ValidateAsync();
+    }
+
+    [Fact]
+    public async Task NullCheckForObject()
+    {
+        await CreateProjectBuilder()
+              .WithSourceCode("_ = [|new object() == null|];")
+              .ShouldFixCodeWith("_ = new object() is null;")
+              .ValidateAsync();
+    }
+
+    [Fact]
+    public async Task NullCheckForObject_NullFirst()
+    {
+        await CreateProjectBuilder()
+              .WithSourceCode("_ = [|null == new object()|];")
+              .ShouldFixCodeWith("_ = new object() is null;")
+              .ValidateAsync();
+    }
+
+    [Fact]
+    public async Task NullCheckForObject_NotNull_NullFirst()
+    {
+        await CreateProjectBuilder()
+              .WithSourceCode("_ = [|null != new object()|];")
+              .ShouldFixCodeWith("_ = new object() is not null;")
+              .ValidateAsync();
+    }
+
+    [Fact]
+    public async Task NullEqualsNull()
+    {
+        // no report as "null is null" is not valid
+        await CreateProjectBuilder()
+              .WithSourceCode("_ = null == null;")
+              .ValidateAsync();
+    }
+
+    [Fact]
+    public async Task NotNullCheck()
+    {
+        // no report as "null is null" is not valid
+        await CreateProjectBuilder()
+              .WithSourceCode("_ = new object() == new object();")
+              .ValidateAsync();
+    }
+
+    [Fact]
+    public async Task NullCheckForObjectWithCustomOperator()
+    {
+        await CreateProjectBuilder()
+              .WithSourceCode("""
+                _ = new Sample() == null;
+
+                class Sample
+                {
+                    public static bool operator ==(Sample left, Sample right) => false;
+                    public static bool operator !=(Sample left, Sample right) => false;
+                }                
+                """)
+              .ValidateAsync();
+    }
+
+    [Fact]
+    public async Task NullCheckForNullableOfT_IsNull()
+    {
+        await CreateProjectBuilder()
+              .WithSourceCode(@"_ = (int?)0 is null;")
+              .ValidateAsync();
+    }
+}