feat: Introduce analyzer to warn against DateTime.Now and DateTime.UtcNow in favor of DateTimeOffset.UtcNow, including tests.

Author: aalmada

src/ApiService/BookStore.ApiService.Analyzers.Tests/Analyzers/UseDateTimeOffsetUtcNowAnalyzerTests.cs

@@ -0,0 +1,32 @@
+using BookStore.ApiService.Analyzers.Analyzers;
+using Microsoft.CodeAnalysis.Testing;
+using TUnit.Core;
+
+namespace BookStore.ApiService.Analyzers.Tests.Analyzers;
+
+public class UseDateTimeOffsetUtcNowAnalyzerTests
+{
+    [Test]
+    [Arguments("TestData/BS1007/NoDiagnostic/ValidDateTimeUsage.cs")]
+    public async Task Verify_NoDiagnostics(string path)
+    {
+        var source = await File.ReadAllTextAsync(path);
+        await CSharpAnalyzerVerifier<UseDateTimeOffsetUtcNowAnalyzer>.VerifyAnalyzerAsync(source);
+    }
+
+    [Test]
+    [Arguments("TestData/BS1007/Diagnostic/InvalidDateTimeNow.cs", 10, 19, "DateTime.Now")]
+    [Arguments("TestData/BS1007/Diagnostic/InvalidDateTimeUtcNow.cs", 10, 19, "DateTime.UtcNow")]
+    [Arguments("TestData/BS1007/Diagnostic/InvalidDateTimeInField.cs", 8, 35, "DateTime.Now")]
+    public async Task Verify_Diagnostics(string path, int line, int column, string dateTimeUsage)
+    {
+        var source = await File.ReadAllTextAsync(path);
+        
+        var expected = CSharpAnalyzerVerifier<UseDateTimeOffsetUtcNowAnalyzer>
+            .Diagnostic(DiagnosticIds.UseDateTimeOffsetUtcNow)
+            .WithLocation(line, column)
+            .WithArguments(dateTimeUsage);
+
+        await CSharpAnalyzerVerifier<UseDateTimeOffsetUtcNowAnalyzer>.VerifyAnalyzerAsync(source, expected);
+    }
+}

src/ApiService/BookStore.ApiService.Analyzers.Tests/TestData/BS1007/Diagnostic/InvalidDateTimeInField.cs

@@ -0,0 +1,9 @@
+using System;
+
+namespace BookStore.ApiService.Tests;
+
+// Invalid: Using DateTime.Now in field initializer
+public class InvalidDateTimeInField
+{
+    private DateTime _timestamp = DateTime.Now;
+}

src/ApiService/BookStore.ApiService.Analyzers.Tests/TestData/BS1007/Diagnostic/InvalidDateTimeNow.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace BookStore.ApiService.Tests;
+
+// Invalid: Using DateTime.Now
+public class InvalidDateTimeNow
+{
+    public void CreateEvent()
+    {
+        var now = DateTime.Now;
+    }
+}

src/ApiService/BookStore.ApiService.Analyzers.Tests/TestData/BS1007/Diagnostic/InvalidDateTimeUtcNow.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace BookStore.ApiService.Tests;
+
+// Invalid: Using DateTime.UtcNow
+public class InvalidDateTimeUtcNow
+{
+    public void CreateEvent()
+    {
+        var now = DateTime.UtcNow;
+    }
+}

src/ApiService/BookStore.ApiService.Analyzers.Tests/TestData/BS1007/NoDiagnostic/ValidDateTimeUsage.cs

@@ -0,0 +1,21 @@
+using System;
+
+namespace BookStore.ApiService.Tests;
+
+// Valid: Using DateTimeOffset.UtcNow
+public class ValidDateTimeUsage
+{
+    public DateTimeOffset Timestamp { get; init; } = DateTimeOffset.UtcNow;
+
+    public void CreateEvent()
+    {
+        var now = DateTimeOffset.UtcNow;
+        var parsed = DateTimeOffset.Parse("2025-01-01T00:00:00Z");
+        var min = DateTimeOffset.MinValue;
+    }
+}
+
+public record ValidEvent(DateTimeOffset Timestamp)
+{
+    public static ValidEvent Create() => new(DateTimeOffset.UtcNow);
+}

src/ApiService/BookStore.ApiService.Analyzers/Analyzers/UseDateTimeOffsetUtcNowAnalyzer.cs

@@ -0,0 +1,62 @@
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace BookStore.ApiService.Analyzers.Analyzers;
+
+/// <summary>
+/// Analyzer to enforce the use of DateTimeOffset.UtcNow instead of DateTime.Now (BS1007)
+/// </summary>
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public class UseDateTimeOffsetUtcNowAnalyzer : DiagnosticAnalyzer
+{
+    static readonly DiagnosticDescriptor Rule = new(
+        id: DiagnosticIds.UseDateTimeOffsetUtcNow,
+        title: "Use DateTimeOffset.UtcNow instead of DateTime.Now",
+        messageFormat: "Use 'DateTimeOffset.UtcNow' instead of '{0}' for timezone-aware timestamps",
+        category: DiagnosticCategories.BestPractices,
+        defaultSeverity: DiagnosticSeverity.Warning,
+        isEnabledByDefault: true,
+        description: "DateTimeOffset.UtcNow provides timezone-aware timestamps and is preferred over DateTime.Now or DateTime.UtcNow for distributed systems and event sourcing.");
+
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => [Rule];
+
+    public override void Initialize(AnalysisContext context)
+    {
+        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+        context.EnableConcurrentExecution();
+        context.RegisterSyntaxNodeAction(AnalyzeMemberAccess, SyntaxKind.SimpleMemberAccessExpression);
+    }
+
+    static void AnalyzeMemberAccess(SyntaxNodeAnalysisContext context)
+    {
+        var memberAccess = (MemberAccessExpressionSyntax)context.Node;
+
+        // Check if this is accessing Now or UtcNow
+        var memberName = memberAccess.Name.Identifier.Text;
+        if (memberName != "Now" && memberName != "UtcNow")
+        {
+            return;
+        }
+
+        // Get the symbol information
+        var symbolInfo = context.SemanticModel.GetSymbolInfo(memberAccess, context.CancellationToken);
+        if (symbolInfo.Symbol is not IPropertySymbol propertySymbol)
+        {
+            return;
+        }
+
+        // Check if this is DateTime.Now or DateTime.UtcNow
+        var containingType = propertySymbol.ContainingType?.ToDisplayString();
+        if (containingType == "System.DateTime")
+        {
+            var diagnostic = Diagnostic.Create(
+                Rule,
+                memberAccess.GetLocation(),
+                $"DateTime.{memberName}");
+            context.ReportDiagnostic(diagnostic);
+        }
+    }
+}

src/ApiService/BookStore.ApiService.Analyzers/DiagnosticIds.cs

@@ -14,6 +14,7 @@ public static class DiagnosticIds
 
     // Best Practices (BS1xxx continued)
     public const string UseCreateVersion7 = "BS1006";
+    public const string UseDateTimeOffsetUtcNow = "BS1007";
 
     // Command Rules (BS2xxx)
     public const string CommandMustBeRecord = "BS2001";