Merge pull request #18849 from michaelnebel/csharp/tupledefaulttostring

C#: Proper handling of value tuples in `cs/call-to-object-tostring`.

Author: michaelnebel

csharp/ql/src/Useless code/DefaultToStringQuery.qll

@@ -46,6 +46,7 @@ private predicate alwaysInvokesToString(ParameterRead pr) {
  * method from `System.Object` or `System.ValueType`.
  */
 predicate alwaysDefaultToString(ValueOrRefType t) {
+  not t instanceof TupleType and
   exists(ToStringMethod m | t.hasMethod(m) |
     m.getDeclaringType() instanceof SystemObjectClass or
     m.getDeclaringType() instanceof SystemValueTypeClass
@@ -55,6 +56,11 @@ predicate alwaysDefaultToString(ValueOrRefType t) {
     overriding.getABaseType+() = t
   ) and
   ((t.isAbstract() or t instanceof Interface) implies not t.isEffectivelyPublic())
+  or
+  exists(ValueOrRefType elem |
+    elem = t.(TupleType).getElementType(_) and
+    alwaysDefaultToString(elem)
+  )
 }
 
 class DefaultToStringType extends ValueOrRefType {

csharp/ql/src/change-notes/2025-02-24-object-tostring.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* C#: Improve precision of the query `cs/call-to-object-tostring` for value tuples.

csharp/ql/test/query-tests/Useless Code/DefaultToString/DefaultToString.cs

@@ -35,6 +35,16 @@ void M()
 
         IPublic g = null;
         Console.WriteLine(g);  // GOOD
+
+        Console.WriteLine(new ValueTuple<int, int>(1, 2)); // GOOD
+
+        Console.WriteLine((1, 2)); // GOOD
+
+        var t1 = new ValueTuple<int, DefaultToString>(1, new DefaultToString());
+        Console.WriteLine(t1); // BAD
+
+        var t2 = new ValueTuple<A, D>(new A(), new D());
+        Console.WriteLine(t2); // GOOD
     }
 
     class A

csharp/ql/test/query-tests/Useless Code/DefaultToString/DefaultToString.expected

@@ -2,7 +2,8 @@
 | DefaultToString.cs:10:28:10:28 | access to local variable d | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | DefaultToString.cs:4:14:4:28 | DefaultToString | DefaultToString |
 | DefaultToString.cs:16:27:16:30 | access to local variable ints | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | file://:0:0:0:0 | Int32[] | Int32[] |
 | DefaultToString.cs:19:24:19:27 | access to local variable ints | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | file://:0:0:0:0 | Int32[] | Int32[] |
-| DefaultToString.cs:34:27:34:27 | access to local variable f | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | DefaultToString.cs:62:23:62:30 | IPrivate | IPrivate |
+| DefaultToString.cs:34:27:34:27 | access to local variable f | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | DefaultToString.cs:72:23:72:30 | IPrivate | IPrivate |
+| DefaultToString.cs:44:27:44:28 | (...) ... | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | file://:0:0:0:0 | (Int32,DefaultToString) | (Int32,DefaultToString) |
 | DefaultToStringBad.cs:8:35:8:35 | access to local variable p | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | DefaultToStringBad.cs:14:11:14:16 | Person | Person |
 | DefaultToStringBad.cs:11:38:11:41 | access to local variable ints | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | file://:0:0:0:0 | Int32[] | Int32[] |
 | WriteLineArray.cs:7:23:7:26 | access to parameter args | Default 'ToString()': $@ inherits 'ToString()' from 'Object', and so is not suitable for printing. | file://:0:0:0:0 | String[] | String[] |