REFL024 Prefer null over empty array. Fix #18.

Author: JohanLarsson

README.md

@@ -88,5 +88,9 @@ Analyzers checking System.Reflection
     <td><a href="https://github.com/DotNetAnalyzers/ReflectionAnalyzers/tree/master/documentation/REFL023.md">REFL023</a></td>
     <td>The type does not implement the interface.</td>
   </tr>
+  <tr>
+    <td><a href="https://github.com/DotNetAnalyzers/ReflectionAnalyzers/tree/master/documentation/REFL024.md">REFL024</a></td>
+    <td>Prefer null over empty array.</td>
+  </tr>
 <table>
 <!-- end generated table -->

ReflectionAnalyzers.Tests/REFL022UseFullyQualifiedNameTests/Diagnostics.cs renamed to ReflectionAnalyzers.Tests/REFL022UseFullyQualifiedNameTests/CodeFix.cs

@@ -0,0 +1,54 @@
+namespace ReflectionAnalyzers.Tests.REFL024PreferNullOverEmptyArrayTests
+{
+    using Gu.Roslyn.Asserts;
+    using Microsoft.CodeAnalysis.CodeFixes;
+    using Microsoft.CodeAnalysis.Diagnostics;
+    using NUnit.Framework;
+    using ReflectionAnalyzers.Codefixes;
+
+    public class CodeFix
+    {
+        private static readonly DiagnosticAnalyzer Analyzer = new InvokeAnalyzer();
+        private static readonly CodeFixProvider Fix = new PreferNullFix();
+        private static readonly ExpectedDiagnostic ExpectedDiagnostic = ExpectedDiagnostic.Create(REFL024PreferNullOverEmptyArray.Descriptor);
+
+        [TestCase("Array.Empty<object>()")]
+        [TestCase("new object[0]")]
+        [TestCase("new object[0] { }")]
+        [TestCase("new object[] { }")]
+        public void MemberInfoInvoke(string emptyArray)
+        {
+            var code = @"
+namespace RoslynSandbox
+{
+    using System;
+    using System.Reflection;
+
+    public class Foo
+    {
+        public Foo(MethodInfo member)
+        {
+            _ = member.Invoke(null, Array.Empty<object>());
+        }
+    }
+}".AssertReplace("Array.Empty<object>()", emptyArray);
+
+            var fixedCode = @"
+namespace RoslynSandbox
+{
+    using System;
+    using System.Reflection;
+
+    public class Foo
+    {
+        public Foo(MethodInfo member)
+        {
+            _ = member.Invoke(null, null);
+        }
+    }
+}";
+
+            AnalyzerAssert.CodeFix(Analyzer, Fix, ExpectedDiagnostic, code, fixedCode);
+        }
+    }
+}

ReflectionAnalyzers.Tests/REFL024PreferNullOverEmptyArrayTests/CodeFix.cs

@@ -52,6 +52,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = ".docs", ".docs", "{1C271AF2
 		documentation\REFL020.md = documentation\REFL020.md
 		documentation\REFL022.md = documentation\REFL022.md
 		documentation\REFL023.md = documentation\REFL023.md
+		documentation\REFL024.md = documentation\REFL024.md
 		RELEASE_NOTES.md = RELEASE_NOTES.md
 	EndProjectSection
 EndProject

ReflectionAnalyzers.sln

@@ -0,0 +1,38 @@
+namespace ReflectionAnalyzers.Codefixes
+{
+    using System.Collections.Immutable;
+    using System.Composition;
+    using System.Threading.Tasks;
+    using Gu.Roslyn.CodeFixExtensions;
+    using Microsoft.CodeAnalysis;
+    using Microsoft.CodeAnalysis.CodeFixes;
+    using Microsoft.CodeAnalysis.CSharp;
+    using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+    [ExportCodeFixProvider(LanguageNames.CSharp, Name = nameof(PreferNullFix))]
+    [Shared]
+    internal class PreferNullFix : DocumentEditorCodeFixProvider
+    {
+        public override ImmutableArray<string> FixableDiagnosticIds { get; } = ImmutableArray.Create(
+            REFL024PreferNullOverEmptyArray.DiagnosticId);
+
+        protected override async Task RegisterCodeFixesAsync(DocumentEditorCodeFixContext context)
+        {
+            var syntaxRoot = await context.Document.GetSyntaxRootAsync(context.CancellationToken)
+                                          .ConfigureAwait(false);
+            foreach (var diagnostic in context.Diagnostics)
+            {
+                if (syntaxRoot.TryFindNode(diagnostic, out ArgumentSyntax argument))
+                {
+                    context.RegisterCodeFix(
+                        "Prefer null.",
+                        (editor, _) => editor.ReplaceNode(
+                            argument.Expression,
+                            x => SyntaxFactory.LiteralExpression(SyntaxKind.NullLiteralExpression)),
+                        nameof(PreferNullFix),
+                        diagnostic);
+                }
+            }
+        }
+    }
+}

ReflectionAnalyzers/Codefixes/PreferNullFix.cs

@@ -12,7 +12,8 @@ internal class InvokeAnalyzer : DiagnosticAnalyzer
     {
         /// <inheritdoc/>
         public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(
-            REFL002InvokeDiscardReturnValue.Descriptor);
+            REFL002InvokeDiscardReturnValue.Descriptor,
+            REFL024PreferNullOverEmptyArray.Descriptor);
 
         /// <inheritdoc/>
         public override void Initialize(AnalysisContext context)
@@ -28,8 +29,48 @@ private static void Handle(SyntaxNodeAnalysisContext context)
                 context.Node is InvocationExpressionSyntax invocation &&
                 invocation.TryGetMethodName(out var name) &&
                 name == "Invoke" &&
-                context.SemanticModel.TryGetSymbol(invocation, context.CancellationToken, out var target))
+                context.SemanticModel.TryGetSymbol(invocation, context.CancellationToken, out var invoke) &&
+                invoke.ContainingType.IsAssignableTo(KnownSymbol.MemberInfo, context.Compilation))
             {
+                if (invoke.TryFindParameter("parameters", out var parameter) &&
+                    invocation.TryFindArgument(parameter, out var paramsArg) &&
+                    IsEmptyArray(paramsArg, context))
+                {
+                    context.ReportDiagnostic(Diagnostic.Create(REFL024PreferNullOverEmptyArray.Descriptor, paramsArg.GetLocation()));
+                }
+            }
+        }
+
+        private static bool IsEmptyArray(ArgumentSyntax argument, SyntaxNodeAnalysisContext context)
+        {
+            switch (argument.Expression)
+            {
+                case InvocationExpressionSyntax invocation when context.SemanticModel.TryGetSymbol(invocation, context.CancellationToken, out var symbol) &&
+                                                                symbol == KnownSymbol.Array.Empty:
+                    return true;
+                case ArrayCreationExpressionSyntax arrayCreation:
+                    if (arrayCreation.Type is ArrayTypeSyntax arrayType)
+                    {
+                        foreach (var rankSpecifier in arrayType.RankSpecifiers)
+                        {
+                            foreach (var size in rankSpecifier.Sizes)
+                            {
+                                if (size is LiteralExpressionSyntax literal &&
+                                    literal.Token.ValueText != "0")
+                                {
+                                    return false;
+                                }
+                            }
+                        }
+
+                        var initializer = arrayCreation.Initializer;
+                        return initializer == null ||
+                               initializer.Expressions.Count == 0;
+                    }
+
+                    return false;
+                default:
+                    return false;
             }
         }
     }

ReflectionAnalyzers/NodeAnalzers/InvokeAnalyzer.cs

@@ -16,4 +16,4 @@ internal static class REFL023TypeDoesNotImplementInterface
             description: "The type does not implement the interface.",
             helpLinkUri: HelpLink.ForId(DiagnosticId));
     }
-}
+}

ReflectionAnalyzers/REFL023TypeDoesNotImplementInterface.cs

@@ -0,0 +1,19 @@
+namespace ReflectionAnalyzers
+{
+    using Microsoft.CodeAnalysis;
+
+    internal static class REFL024PreferNullOverEmptyArray
+    {
+        public const string DiagnosticId = "REFL024";
+
+        internal static readonly DiagnosticDescriptor Descriptor = new DiagnosticDescriptor(
+            id: DiagnosticId,
+            title: "Prefer null over empty array.",
+            messageFormat: "Prefer null over empty array.",
+            category: AnalyzerCategory.SystemReflection,
+            defaultSeverity: DiagnosticSeverity.Warning,
+            isEnabledByDefault: true,
+            description: "Prefer null over empty array.",
+            helpLinkUri: HelpLink.ForId(DiagnosticId));
+    }
+}

ReflectionAnalyzers/REFL024PreferNullOverEmptyArray.cs

@@ -0,0 +1,67 @@
+# REFL024
+## Prefer null over empty array.
+
+<!-- start generated table -->
+<table>
+  <tr>
+    <td>CheckId</td>
+    <td>REFL024</td>
+  </tr>
+  <tr>
+    <td>Severity</td>
+    <td>Warning</td>
+  </tr>
+  <tr>
+    <td>Enabled</td>
+    <td>true</td>
+  </tr>
+  <tr>
+    <td>Category</td>
+    <td>ReflectionAnalyzers.SystemReflection</td>
+  </tr>
+  <tr>
+    <td>Code</td>
+    <td><a href="https://github.com/DotNetAnalyzers/ReflectionAnalyzers/blob/master/ReflectionAnalyzers/NodeAnalzers/InvokeAnalyzer.cs">InvokeAnalyzer</a></td>
+  </tr>
+</table>
+<!-- end generated table -->
+
+## Description
+
+Prefer null over empty array.
+
+## Motivation
+
+ADD MOTIVATION HERE
+
+## How to fix violations
+
+ADD HOW TO FIX VIOLATIONS HERE
+
+<!-- start generated config severity -->
+## Configure severity
+
+### Via ruleset file.
+
+Configure the severity per project, for more info see [MSDN](https://msdn.microsoft.com/en-us/library/dd264949.aspx).
+
+### Via #pragma directive.
+```C#
+#pragma warning disable REFL024 // Prefer null over empty array.
+Code violating the rule here
+#pragma warning restore REFL024 // Prefer null over empty array.
+```
+
+Or put this at the top of the file to disable all instances.
+```C#
+#pragma warning disable REFL024 // Prefer null over empty array.
+```
+
+### Via attribute `[SuppressMessage]`.
+
+```C#
+[System.Diagnostics.CodeAnalysis.SuppressMessage("ReflectionAnalyzers.SystemReflection", 
+    "REFL024:Prefer null over empty array.", 
+    Justification = "Reason...")]
+```
+<!-- end generated config severity -->