C#: Use ref return getters for properties/indexers in write contexts.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/exprs/Call.qll

@@ -766,7 +766,16 @@ class PropertyCall extends AccessorCall, PropertyAccessExpr {
   }
 
   override Accessor getWriteTarget() {
-    this instanceof AssignableWrite and result = this.getProperty().getSetter()
+    this instanceof AssignableWrite and
+    exists(Property p | p = this.getProperty() |
+      result = p.getSetter()
+      or
+      result =
+        any(Getter g |
+          g = p.getGetter() and
+          g.getAnnotatedReturnType().isRef()
+        )
+    )
   }
 
   override Expr getArgument(int i) {
@@ -801,7 +810,16 @@ class IndexerCall extends AccessorCall, IndexerAccessExpr {
   }
 
   override Accessor getWriteTarget() {
-    this instanceof AssignableWrite and result = this.getIndexer().getSetter()
+    this instanceof AssignableWrite and
+    exists(Indexer i | i = this.getIndexer() |
+      result = i.getSetter()
+      or
+      result =
+        any(Getter g |
+          g = i.getGetter() and
+          g.getAnnotatedReturnType().isRef()
+        )
+    )
   }
 
   override Expr getArgument(int i) {