C#: Improve empty collection check to not report on collections with property writes

Author: tamasvajk

csharp/ql/src/semmle/code/csharp/commons/Collections.qll

@@ -143,8 +143,8 @@ private predicate readonlyAccess(Access a) {
   // A read-only method call
   exists(MethodCall mc | mc.getQualifier() = a | mc.getTarget().hasName(readonlyMethodName()))
   or
-  // Any property access
-  a = any(PropertyAccess pa).getQualifier()
+  // Any property read
+  a = any(PropertyRead pr).getQualifier()
   or
   // An element read
   a = any(ElementRead er).getQualifier()

csharp/ql/test/query-tests/ReadOnlyContainer/ReadOnlyContainer.cs

@@ -152,6 +152,21 @@ void f8(object arguments)
                 break;
         }
     }
+
+    void f9()
+    {
+        var l1 = new MyList(); // BAD
+        var x1 = l1[0];
+
+        var l2 = new MyList(); // GOOD
+        var x2 = l2[0];
+        l2.Prop = 42;
+    }
+
+    class MyList : List<int>
+    {
+        public int Prop { get { return 0; } set { Add(value); } }
+    }
 }
 
 // semmle-extractor-options: /r:System.Collections.dll

csharp/ql/test/query-tests/ReadOnlyContainer/ReadOnlyContainer.expected

@@ -8,4 +8,5 @@
 | ReadOnlyContainer.cs:91:13:91:14 | v8 | The contents of this container are never initialized. |
 | ReadOnlyContainer.cs:96:13:96:14 | v9 | The contents of this container are never initialized. |
 | ReadOnlyContainer.cs:99:13:99:15 | v10 | The contents of this container are never initialized. |
-| ReadOnlyContainer.cs:121:13:121:15 | v11 | The contents of this container are never initialized. |
+| ReadOnlyContainer.cs:121:13:121:15 | v11 | The contents of this container are never initialized. |
+| ReadOnlyContainer.cs:158:13:158:14 | l1 | The contents of this container are never initialized. |