Fix S1144 FP: Ignore unused Deconstruct methods

analyzers/src/SonarAnalyzer.CSharp/Rules/UnusedPrivateMember.cs

@@ -190,10 +190,16 @@ private static HashSet<ISymbol> GetUnusedSymbols(CSharpSymbolUsageCollector usag
             .Except(usageCollector.UsedSymbols)
             .Where(x => !IsMentionedInDebuggerDisplay(x, usageCollector)
                 && !IsAccessorUsed(x, usageCollector)
+                && !IsDeconstructMethod(x)
                 && !usageCollector.PrivateAttributes.Contains(x)
                 && !IsUsedWithReflection(x, usageCollector.TypesUsedWithReflection))
             .ToHashSet();
 
+    private static bool IsDeconstructMethod(ISymbol symbol) =>
+        symbol is IMethodSymbol { Name: "Deconstruct", Parameters.Length: > 0 } method
+        && method.ReturnType.Is(KnownType.Void)
+        && method.Parameters.All(x => x.RefKind == RefKind.Out);
+
     private static bool IsAccessorUsed(ISymbol symbol, CSharpSymbolUsageCollector usageCollector) =>
         symbol is IMethodSymbol { AssociatedSymbol: IPropertySymbol property } accessor
         && usageCollector.PropertyAccess.TryGetValue(property, out var access)

analyzers/tests/SonarAnalyzer.Test/TestCases/UnusedPrivateMember.CSharp7.cs

@@ -60,32 +60,29 @@ private sealed class PublicDeconstructWithInnerType
     {
         public void Deconstruct(out object a, out InternalDeconstruct b) { a = b = null; }
 
-        // deconstructors must be public, internal or protected internal
-        private void Deconstruct(out object a, out object b) { a = b = null; } // Noncompliant
+        private void Deconstruct(out object a, out object b) { a = b = null; } // Compliant, Deconstruct methods are ignored
     }
 
     internal sealed class InternalDeconstruct
     {
         internal void Deconstruct(out object a, out object b) { a = b = null; }
 
-        // deconstructors must be public, internal or protected internal
-        private void Deconstruct(out object a, out string b, out string c) { a = b = c = null; } // Noncompliant
+        private void Deconstruct(out object a, out string b, out string c) { a = b = c = null; } // Compliant, Deconstruct methods are ignored
     }
 
     private class PublicDeconstruct
     {
         public void Deconstruct(out object a, out object b, out object c) { a = b = c = null; }
 
-        // deconstructors must be public, internal or protected internal
-        protected void Deconstruct(out string a, out string b, out string c) { a = b = c = null; } // Noncompliant
-        private void Deconstruct(out object a, out string b, out string c) { a = b = c = null; } // Noncompliant
+        protected void Deconstruct(out string a, out string b, out string c) { a = b = c = null; } // Compliant, Deconstruct methods are ignored
+        private void Deconstruct(out object a, out string b, out string c) { a = b = c = null; } // Compliant, Deconstruct methods are ignored
     }
 
     private sealed class MultipleDeconstructors
     {
         public void Deconstruct(out object a, out object b, out object c) { a = b = c = null; }
 
-        public void Deconstruct(out object a, out object b) // Noncompliant
+        public void Deconstruct(out object a, out object b) // Compliant, Deconstruct methods are ignored
         {
             a = b = null;
         }
@@ -95,25 +92,37 @@ private class ProtectedInternalDeconstruct
     {
         protected internal void Deconstruct(out object a, out object b) { a = b = null; }
 
-        protected internal void Deconstruct(out object a, out object b, out object c) { a = b = c = null; } // Noncompliant
+        protected internal void Deconstruct(out object a, out object b, out object c) { a = b = c = null; } // Compliant, Deconstruct methods are ignored
     }
 
     private class Ambiguous
     {
         public void Deconstruct(out string a, out string b, out string c) { a = b = c = null; }
-        public void Deconstruct(out object a, out object b, out object c) { a = b = c = null; } // Noncompliant FP, actually the one above is not used
+        public void Deconstruct(out object a, out object b, out object c) { a = b = c = null; } // Compliant, Deconstruct methods are ignored
     }
 
     private class NotUsedDifferentArgumentCount
     {
-        public void Deconstruct(out string a, out string b, out string c) { a = b = c = null; } // Noncompliant
-        public void Deconstruct(out string a, out string b, out string c, out string d) { a = b = c = d = null; } // Noncompliant
+        public void Deconstruct(out string a, out string b, out string c) { a = b = c = null; } // Compliant, Deconstruct methods are ignored
+        public void Deconstruct(out string a, out string b, out string c, out string d) { a = b = c = d = null; } // Compliant, Deconstruct methods are ignored
     }
 
     private class NotUsedNotVisible
     {
-        protected void Deconstruct(out object a, out object b) { a = b = null; } // Noncompliant
-        private void Deconstruct(out string a, out string b) { a = b = null; } // Noncompliant
+        protected void Deconstruct(out object a, out object b) { a = b = null; } // Compliant, Deconstruct methods are ignored
+        private void Deconstruct(out string a, out string b) { a = b = null; } // Compliant, Deconstruct methods are ignored
+    }
+
+    public class InvalidDeconstruct
+    {
+        private void Deconstruct(object a, out object b, out object c) { b = c = a; } // Noncompliant
+        private void Deconstruct() { } // Noncompliant
+
+        private int Deconstruct(out object a, out object b, out object c) // Noncompliant
+        {
+            a = b = c = null;
+            return 42;
+        }
     }
 
     private ForMethod ReturnFromMethod() => null;

analyzers/tests/SonarAnalyzer.Test/TestCases/UnusedPrivateMember.CSharp8.cs

@@ -33,12 +33,12 @@ public void SomeMethod()
 
         private sealed class ForSwitchArm
         {
-            public void Deconstruct(out object a, out object b) { a = b = null; } // Noncompliant FP
+            public void Deconstruct(out object a, out object b) { a = b = null; } // Compliant, Deconstruct methods are ignored
         }
 
         private sealed class ForIsPattern
         {
-            public void Deconstruct(out string a, out string b) { a = b = null; } // Noncompliant FP
+            public void Deconstruct(out string a, out string b) { a = b = null; } // Compliant
         }
     }
 }