Rule S4070: Use xor operator to verify flags

sonaranalyzer-dotnet/src/SonarAnalyzer.CSharp/Rules/DoNotMarkEnumsWithFlags.cs

@@ -39,6 +39,7 @@ public sealed class DoNotMarkEnumsWithFlags : SonarDiagnosticAnalyzer
 
         private static readonly DiagnosticDescriptor rule =
             DiagnosticDescriptorBuilder.GetDescriptor(DiagnosticId, MessageFormat, RspecStrings.ResourceManager);
+
         public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(rule);
 
         protected override void Initialize(SonarAnalysisContext context)
@@ -114,20 +115,19 @@ private static bool IsCombinationOfOtherValues(ulong value, List<ulong> otherVal
             }
 
             var newValue = value;
-            foreach (var otherValue in otherValues)
+            foreach (var otherValue in otherValues.SkipWhile(v => value <= v))
             {
-                if (newValue <= 0)
-                {
-                    return true;
-                }
-
-                if (otherValue < value)
+                if (otherValue <= newValue)
                 {
-                    newValue -= otherValue;
+                    newValue ^= otherValue;
+                    if (newValue == 0)
+                    {
+                        return true;
+                    }
                 }
             }
 
             return newValue == 0;
         }
     }
-}
+}

sonaranalyzer-dotnet/src/Tests/SonarAnalyzer.UnitTest/TestCases/DoNotMarkEnumsWithFlags.cs

@@ -88,4 +88,24 @@ public enum InvalidStringEnum : string // Noncompliant
     {
         MyValue = "toto" // Secondary
     }
+
+    [Flags]
+    enum EnumFoo3
+    {
+        N1 = 1,
+        N2 = 2,
+        N3 = N1 | N2,
+        N4 = 4,
+        N5 = N4 | N1,
+        N6 = N4 | N2
+    }
+
+    [Flags]
+    enum EnumFoo4 // Noncompliant
+    {
+        N1 = 1,
+        N2 = 2,
+        N3 = N1 | N2,
+        N5 = 5 // Secondary
+    }
 }