Merge pull request #17 from jonathanalgar/best-practices-statelessness

Beyond the rules: info-level guidance for best practices > statelessness checks

Author: jonathanalgar

src/CustomCode-Analyzer/Analyzer.cs

@@ -41,6 +41,7 @@ public static class DiagnosticIds
             public const string MissingStructureDecoration = "MissingStructureDecoration";
             public const string UnsupportedParameterType = "UnsupportedParameterType";
             public const string UnsupportedDefaultValue = "UnsupportedDefaultValue";
+            public const string PotentialStatefulImplementation = "PotentialStatefulImplementation";
         }
 
         /// <summary>
@@ -292,6 +293,18 @@ public static class Categories
 
         // https://www.outsystems.com/tk/redirect?g=OS-ELG-MODL-05029 - not implementing
 
+        // ----------------------------------------------- BEST PRACTICES
+
+        private static readonly DiagnosticDescriptor PotentialStatefulImplementationRule = new(
+            DiagnosticIds.PotentialStatefulImplementation,
+            title: "Possible stateful behavior",
+            messageFormat: "The class '{0}' contains static members ({1}) which could persist state between calls. External libraries should be designed to be stateless.",
+            category: Categories.Design,
+            defaultSeverity: DiagnosticSeverity.Info,
+            isEnabledByDefault: true,
+            description: "External libraries should be designed to be stateless. Consider passing state information as method parameters instead of storing it in fields.",
+            helpLinkUri: "https://success.outsystems.com/documentation/outsystems_developer_cloud/building_apps/extend_your_apps_with_custom_code/external_libraries_sdk_readme/#architecture");
+
         /// <summary>
         /// Returns the full set of DiagnosticDescriptors that this analyzer is capable of producing.
         /// </summary>
@@ -318,7 +331,8 @@ public static class Categories
                 UnsupportedTypeMappingRule,
                 MissingStructureDecorationRule,
                 UnsupportedParameterTypeRule,
-                UnsupportedDefaultValueRule);
+                UnsupportedDefaultValueRule,
+                PotentialStatefulImplementationRule);
 
         /// <summary>
         /// Entry point for the analyzer. Initializes analysis by setting up compilation-level
@@ -777,6 +791,27 @@ private static void AnalyzeClass(SymbolAnalysisContext context, INamedTypeSymbol
                                 ctorDecl.Identifier.GetLocation(),
                                 typeSymbol.Name));
                     }
+
+                    // Check for static members that could indicate attempts to maintain state
+                    var staticMembers = typeSymbol.GetMembers()
+                        .Where(member =>
+                            member.IsStatic &&
+                            !member.IsImplicitlyDeclared && // Skip compiler-generated members
+                            member is IFieldSymbol { IsConst: false } or IPropertySymbol)
+                        .Select(m => m.Name)
+                        .OrderBy(name => name)
+                        .ToList();
+
+                    if (staticMembers.Any() &&
+                        typeSymbol.DeclaringSyntaxReferences.FirstOrDefault()?.GetSyntax() is ClassDeclarationSyntax stateDecl)
+                    {
+                        context.ReportDiagnostic(
+                            Diagnostic.Create(
+                                PotentialStatefulImplementationRule,
+                                stateDecl.Identifier.GetLocation(),
+                                typeSymbol.Name,
+                                string.Join(", ", staticMembers)));
+                    }
                 }
             }
         }

tests/CustomCode-Analyzer.Tests/AnalyzerTests.cs

@@ -1338,6 +1338,51 @@ public void TestMethod(string text1 = Constants.DefaultText,
 
         // https://www.outsystems.com/tk/redirect?g=OS-ELG-MODL-05029 - not implementing
 
+        // ----------------------------------------------- BEST PRACTICES
+
+        // --------------------- PotentialStatefulImplementationRule ---------------
+        [TestMethod]
+        public async Task PotentialStatefulImplementationRule_DetectsStaticState()
+        {
+            var test = @"
+using System.Collections.Generic;
+
+[OSInterface]
+public interface ICalculator 
+{
+    decimal Add(decimal a, decimal b);
+    int GetTotalCalculations();
+}
+
+public class Calculator : ICalculator 
+{
+    // Attempting to track state between calls
+    private static int _totalCalculations;
+    private static readonly List<decimal> _history = new();
+    public static decimal LastResult { get; private set; }
+    
+    public decimal Add(decimal a, decimal b) 
+    {
+        _totalCalculations++;
+        var result = a + b;
+        _history.Add(result);
+        LastResult = result;
+        return result;
+    }
+    
+    public int GetTotalCalculations() 
+    {
+        return _totalCalculations;
+    }
+}";
+
+            var expected = CSharpAnalyzerVerifier<Analyzer>
+                .Diagnostic(DiagnosticIds.PotentialStatefulImplementation)
+                .WithSpan(11, 14, 11, 24)
+                .WithArguments("Calculator", "_history, _totalCalculations, LastResult");
+
+            await CSharpAnalyzerVerifier<Analyzer>.VerifyAnalyzerAsync(test, TestContext, skipSDKreference: false, expected);
+        }
         // ----------------------------------------------- MIXED TESTS!
         [TestMethod]
         public async Task ComplexScenario_MultipleNamingAndStructureIssues_ReportsWarnings()