Parameter null-checking analyzer and fixer

src/Analyzers/CSharp/Analyzers/CSharpAnalyzers.projitems

@@ -103,6 +103,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)UseIsNullCheck\CSharpUseNullCheckOverTypeCheckDiagnosticAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)UseIsNullCheck\CSharpUseIsNullCheckForCastAndEqualityOperatorDiagnosticAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)UseIsNullCheck\CSharpUseIsNullCheckForReferenceEqualsDiagnosticAnalyzer.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)UseParameterNullChecking\CSharpUseParameterNullCheckingDiagnosticAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)UseTupleSwap\CSharpUseTupleSwapDiagnosticAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)UseLocalFunction\CSharpUseLocalFunctionDiagnosticAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)UseNullPropagation\CSharpUseNullPropagationDiagnosticAnalyzer.cs" />

src/Analyzers/CSharp/Analyzers/CSharpAnalyzersResources.resx

@@ -340,4 +340,7 @@
   <data name="Use_tuple_to_swap_values" xml:space="preserve">
     <value>Use tuple to swap values</value>
   </data>
+  <data name="Use_parameter_null_checking" xml:space="preserve">
+    <value>Use parameter null checking</value>
+  </data>
 </root>

src/Analyzers/CSharp/Analyzers/UseParameterNullChecking/CSharpUseParameterNullCheckingDiagnosticAnalyzer.cs

@@ -0,0 +1,294 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Linq;
+using Microsoft.CodeAnalysis.CodeStyle;
+using Microsoft.CodeAnalysis.CSharp.CodeStyle;
+using Microsoft.CodeAnalysis.CSharp.Shared.Extensions;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Roslyn.Utilities;
+
+namespace Microsoft.CodeAnalysis.CSharp.UseParameterNullChecking
+{
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    internal sealed class CSharpUseParameterNullCheckingDiagnosticAnalyzer
+        : AbstractBuiltInCodeStyleDiagnosticAnalyzer
+    {
+        private const string ArgumentNullExceptionName = $"{nameof(System)}.{nameof(ArgumentNullException)}";
+
+        public CSharpUseParameterNullCheckingDiagnosticAnalyzer()
+            : base(IDEDiagnosticIds.UseParameterNullCheckingId,
+                   EnforceOnBuildValues.UseParameterNullChecking,
+                   CSharpCodeStyleOptions.PreferParameterNullChecking,
+                   CSharpAnalyzersResources.Use_parameter_null_checking,
+                   new LocalizableResourceString(nameof(AnalyzersResources.Null_check_can_be_simplified), AnalyzersResources.ResourceManager, typeof(AnalyzersResources)))
+        {
+        }
+
+        public override DiagnosticAnalyzerCategory GetAnalyzerCategory()
+            => DiagnosticAnalyzerCategory.SemanticSpanAnalysis;
+
+        protected override void InitializeWorker(AnalysisContext context)
+            => context.RegisterCompilationStartAction(context =>
+            {
+                var compilation = (CSharpCompilation)context.Compilation;
+                if (compilation.LanguageVersion < LanguageVersionExtensions.CSharpNext)
+                {
+                    return;
+                }
+
+                var argumentNullException = compilation.GetTypeByMetadataName(ArgumentNullExceptionName);
+                if (argumentNullException is null)
+                {
+                    return;
+                }
+
+                IMethodSymbol? argumentNullExceptionConstructor = null;
+                IMethodSymbol? argumentNullExceptionStringConstructor = null;
+                foreach (var constructor in argumentNullException.InstanceConstructors)
+                {
+                    if (argumentNullExceptionConstructor is not null && argumentNullExceptionStringConstructor is not null)
+                    {
+                        break;
+                    }
+
+                    switch (constructor)
+                    {
+                        case { DeclaredAccessibility: Accessibility.Public, Parameters.Length: 0 }:
+                            argumentNullExceptionConstructor = constructor;
+                            break;
+                        case { DeclaredAccessibility: Accessibility.Public, Parameters.Length: 1 }
+                            when constructor.Parameters[0].Type.SpecialType == SpecialType.System_String:
+
+                            argumentNullExceptionStringConstructor = constructor;
+                            break;
+                    }
+                }
+
+                if (argumentNullExceptionConstructor is null || argumentNullExceptionStringConstructor is null)
+                {
+                    return;
+                }
+
+                var objectType = compilation.GetSpecialType(SpecialType.System_Object);
+                var referenceEqualsMethod = (IMethodSymbol?)objectType
+                    .GetMembers(nameof(ReferenceEquals))
+                    .FirstOrDefault(m => m is IMethodSymbol { DeclaredAccessibility: Accessibility.Public, Parameters.Length: 2 });
+
+                // We are potentially interested in any declaration that has parameters.
+                // However, we avoid indexers specifically because of the complexity of locating and deleting equivalent null checks across multiple accessors.
+                context.RegisterSyntaxNodeAction(
+                    context => AnalyzeSyntax(context, argumentNullExceptionConstructor, argumentNullExceptionStringConstructor, referenceEqualsMethod),
+                    SyntaxKind.ConstructorDeclaration,
+                    SyntaxKind.MethodDeclaration,
+                    SyntaxKind.LocalFunctionStatement,
+                    SyntaxKind.SimpleLambdaExpression,
+                    SyntaxKind.ParenthesizedLambdaExpression,
+                    SyntaxKind.AnonymousMethodExpression,
+                    SyntaxKind.OperatorDeclaration,
+                    SyntaxKind.ConversionOperatorDeclaration);
+            });
+
+        private void AnalyzeSyntax(
+            SyntaxNodeAnalysisContext context,
+            IMethodSymbol argumentNullExceptionConstructor,
+            IMethodSymbol argumentNullExceptionStringConstructor,
+            IMethodSymbol? referenceEqualsMethod)
+        {
+            var cancellationToken = context.CancellationToken;
+
+            var semanticModel = context.SemanticModel;
+            var syntaxTree = semanticModel.SyntaxTree;
+
+            var option = context.Options.GetOption(CSharpCodeStyleOptions.PreferParameterNullChecking, syntaxTree, cancellationToken);
+            if (!option.Value)
+            {
+                return;
+            }
+
+            var node = context.Node;
+            var block = node switch
+            {
+                MethodDeclarationSyntax methodDecl => methodDecl.Body,
+                ConstructorDeclarationSyntax constructorDecl => constructorDecl.Body,
+                LocalFunctionStatementSyntax localFunctionStatement => localFunctionStatement.Body,
+                AnonymousFunctionExpressionSyntax anonymousFunction => anonymousFunction.Block,
+                OperatorDeclarationSyntax operatorDecl => operatorDecl.Body,
+                ConversionOperatorDeclarationSyntax conversionDecl => conversionDecl.Body,
+                _ => throw ExceptionUtilities.UnexpectedValue(node)
+            };
+
+            // More scenarios should be supported eventually: https://github.com/dotnet/roslyn/issues/58699
+            if (block is null)
+            {
+                return;
+            }
+
+            var methodSymbol = node is AnonymousFunctionExpressionSyntax
+                ? (IMethodSymbol?)semanticModel.GetSymbolInfo(node, cancellationToken).Symbol
+                : (IMethodSymbol?)semanticModel.GetDeclaredSymbol(node, cancellationToken);
+            if (methodSymbol is null || methodSymbol.Parameters.IsEmpty)
+            {
+                return;
+            }
+
+            foreach (var statement in block.Statements)
+            {
+                var parameter = TryGetParameterNullCheckedByStatement(statement);
+                if (parameter is not null
+                    && (!parameter.Type.IsValueType
+                        || parameter.Type.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T
+                        || parameter.Type.TypeKind is TypeKind.Pointer or TypeKind.FunctionPointer)
+                    && parameter.RefKind == RefKind.None
+                    && parameter.DeclaringSyntaxReferences.FirstOrDefault() is SyntaxReference reference
+                    && reference.SyntaxTree.Equals(statement.SyntaxTree)
+                    && reference.GetSyntax() is ParameterSyntax parameterSyntax)
+                {
+                    context.ReportDiagnostic(DiagnosticHelper.Create(
+                        Descriptor,
+                        statement.GetLocation(),
+                        option.Notification.Severity,
+                        additionalLocations: new[] { parameterSyntax.GetLocation() },
+                        properties: null));
+                }
+            }
+
+            return;
+
+            IParameterSymbol? TryGetParameterNullCheckedByStatement(StatementSyntax statement)
+            {
+                switch (statement)
+                {
+                    // if (param == null) { throw new ArgumentNullException(nameof(param)); }
+                    // if (param is null) { throw new ArgumentNullException(nameof(param)); }
+                    // if (object.ReferenceEquals(param, null)) { throw new ArgumentNullException(nameof(param)); }
+                    case IfStatementSyntax ifStatement:
+                        ExpressionSyntax left, right;
+                        switch (ifStatement)
+                        {
+                            case { Condition: BinaryExpressionSyntax { OperatorToken.RawKind: (int)SyntaxKind.EqualsEqualsToken } binary }
+                                // Only suggest the fix on built-in `==` operators where we know we won't change behavior
+                                when semanticModel.GetSymbolInfo(binary).Symbol is IMethodSymbol { MethodKind: MethodKind.BuiltinOperator }:
+
+                                left = binary.Left;
+                                right = binary.Right;
+                                break;
+                            case { Condition: IsPatternExpressionSyntax { Expression: var patternInput, Pattern: ConstantPatternSyntax { Expression: var patternExpression } } }:
+                                left = patternInput;
+                                right = patternExpression;
+                                break;
+                            case { Condition: InvocationExpressionSyntax { Expression: var receiver, ArgumentList.Arguments: { Count: 2 } arguments } }
+                                when referenceEqualsMethod != null && referenceEqualsMethod.Equals(semanticModel.GetSymbolInfo(receiver, cancellationToken).Symbol):
+
+                                left = arguments[0].Expression;
+                                right = arguments[1].Expression;
+                                break;
+
+                            default:
+                                return null;
+                        }
+
+                        var parameterInBinary = left.IsKind(SyntaxKind.NullLiteralExpression) ? TryGetParameter(right)
+                            : right.IsKind(SyntaxKind.NullLiteralExpression) ? TryGetParameter(left)
+                            : null;
+                        if (parameterInBinary is null)
+                        {
+                            return null;
+                        }
+
+                        var throwStatement = ifStatement.Statement switch
+                        {
+                            ThrowStatementSyntax @throw => @throw,
+                            BlockSyntax { Statements: { Count: 1 } statements } => statements[0] as ThrowStatementSyntax,
+                            _ => null
+                        };
+
+                        if (throwStatement?.Expression is not ObjectCreationExpressionSyntax thrownInIf
+                            || !IsConstructorApplicable(thrownInIf, parameterInBinary))
+                        {
+                            return null;
+                        }
+
+                        return parameterInBinary;
+
+                    // this.field = param ?? throw new ArgumentNullException(nameof(param));
+                    case ExpressionStatementSyntax
+                    {
+                        Expression: AssignmentExpressionSyntax
+                        {
+                            Right: BinaryExpressionSyntax
+                            {
+                                OperatorToken.RawKind: (int)SyntaxKind.QuestionQuestionToken,
+                                Left: ExpressionSyntax maybeParameter,
+                                Right: ThrowExpressionSyntax { Expression: ObjectCreationExpressionSyntax thrownInNullCoalescing }
+                            }
+                        }
+                    }:
+                        var coalescedParameter = TryGetParameter(maybeParameter);
+                        if (coalescedParameter is null || !IsConstructorApplicable(thrownInNullCoalescing, coalescedParameter))
+                        {
+                            return null;
+                        }
+
+                        return coalescedParameter;
+
+                    default:
+                        return null;
+                }
+            }
+
+            IParameterSymbol? TryGetParameter(ExpressionSyntax maybeParameter)
+            {
+                // `(object)x == null` is often used to ensure reference equality is used.
+                // therefore, we specially unwrap casts when the cast is to `object`.
+                if (maybeParameter is CastExpressionSyntax { Type: var type, Expression: var operand })
+                {
+                    if (semanticModel.GetTypeInfo(type).Type?.SpecialType != SpecialType.System_Object)
+                    {
+                        return null;
+                    }
+
+                    maybeParameter = operand;
+                }
+
+                if (semanticModel.GetSymbolInfo(maybeParameter).Symbol is not IParameterSymbol { ContainingSymbol: { } containingSymbol } parameterSymbol || !containingSymbol.Equals(methodSymbol))
+                {
+                    return null;
+                }
+
+                return parameterSymbol;
+            }
+
+            bool IsConstructorApplicable(ObjectCreationExpressionSyntax exceptionCreation, IParameterSymbol parameterSymbol)
+            {
+                if (exceptionCreation.ArgumentList?.Arguments is not { } arguments)
+                {
+                    return false;
+                }
+
+                if (arguments.Count == 0)
+                {
+                    // 'new ArgumentNullException()'
+                    return argumentNullExceptionConstructor.Equals(semanticModel.GetSymbolInfo(exceptionCreation, cancellationToken).Symbol);
+                }
+
+                if (arguments.Count != 1)
+                {
+                    return false;
+                }
+
+                // 'new ArgumentNullException(nameof(param))' (or equivalent)
+                var constantValue = semanticModel.GetConstantValue(arguments[0].Expression, cancellationToken);
+                if (constantValue.Value is not string constantString || !string.Equals(constantString, parameterSymbol.Name, StringComparison.Ordinal))
+                {
+                    return false;
+                }
+
+                return argumentNullExceptionStringConstructor.Equals(semanticModel.GetSymbolInfo(exceptionCreation, cancellationToken).Symbol);
+            }
+        }
+    }
+}