Fix failing FieldMask.Merge for well-known wrapper field types

csharp/src/Google.Protobuf.Test/FieldMaskTreeTest.cs

@@ -432,5 +432,89 @@ public void Merge(bool useDynamicMessage)
             Assert.IsNotNull(destination.Payload);
         }
 
+        [Test]
+        public void MergeWrapperFieldsWithNonNullFieldsInSource()
+        {
+            // Instantiate a destination with wrapper-based field types.
+            var destination = new TestWellKnownTypes()
+            {
+                StringField = "Hello",
+                Int32Field = 12,
+                Int64Field = 24,
+                BoolField = true,
+            };
+
+            // Set up a targeted update.
+            var source = new TestWellKnownTypes()
+            {
+                StringField = "Hi",
+                Int64Field = 240
+            };
+
+            Merge(new FieldMaskTree().AddFieldPath("string_field").AddFieldPath("int64_field"),
+                source,
+                destination,
+                new FieldMask.MergeOptions(),
+                false);
+
+            // Make sure the targeted fields changed.
+            Assert.AreEqual("Hi", destination.StringField);
+            Assert.AreEqual(240, destination.Int64Field);
+
+            // Prove that non-targeted fields stay intact...
+            Assert.AreEqual(12, destination.Int32Field);
+            Assert.IsTrue(destination.BoolField);
+
+            // ...including default values which were not explicitly set in the destination object.
+            Assert.IsNull(destination.FloatField);
+        }
+
+        [Test]
+        [TestCase(false, "Hello", 24)]
+        [TestCase(true, null, null)]
+        public void MergeWrapperFieldsWithNullFieldsInSource(
+            bool replaceMessageFields,
+            string expectedStringValue,
+            long? expectedInt64Value)
+        {
+            // Instantiate a destination with wrapper-based field types.
+            var destination = new TestWellKnownTypes()
+            {
+                StringField = "Hello",
+                Int32Field = 12,
+                Int64Field = 24,
+                BoolField = true,
+            };
+
+            // Set up a targeted update with null valued fields.
+            var source = new TestWellKnownTypes()
+            {
+                StringField = null,
+                Int64Field = null
+            };
+
+            Merge(new FieldMaskTree().AddFieldPath("string_field").AddFieldPath("int64_field"),
+                source,
+                destination,
+                new FieldMask.MergeOptions()
+                {
+                    ReplaceMessageFields = replaceMessageFields
+                },
+                false);
+
+            // Make sure the targeted fields changed according to our expectations, depending on the value of ReplaceMessageFields.
+            // When ReplaceMessageFields is false, the null values are not applied to the destination, because, although wrapped types
+            // are semantically primitives, FieldMaskTree.Merge still treats them as message types in order to maintain consistency with other Protobuf
+            // libraries such as Java and C++.
+            Assert.AreEqual(expectedStringValue, destination.StringField);
+            Assert.AreEqual(expectedInt64Value, destination.Int64Field);
+
+            // Prove that non-targeted fields stay intact...
+            Assert.AreEqual(12, destination.Int32Field);
+            Assert.IsTrue(destination.BoolField);
+
+            // ...including default values which were not explicitly set in the destination object.
+            Assert.IsNull(destination.FloatField);
+        }
     }
 }

csharp/src/Google.Protobuf/FieldMaskTree.cs

@@ -333,15 +333,24 @@ public void Merge(IMessage source, IMessage destination, FieldMask.MergeOptions
                         {
                             if (sourceField != null)
                             {
-                                var sourceByteString = ((IMessage)sourceField).ToByteString();
-                                var destinationValue = (IMessage)field.Accessor.GetValue(destination);
-                                if (destinationValue != null)
+                                // Well-known wrapper types are represented as nullable primitive types, so we do not "merge" them.
+                                // Instead, any non-null value just overwrites the previous value directly.
+                                if (field.MessageType.IsWrapperType)
                                 {
-                                    destinationValue.MergeFrom(sourceByteString);
+                                    field.Accessor.SetValue(destination, sourceField);
                                 }
                                 else
                                 {
-                                    field.Accessor.SetValue(destination, field.MessageType.Parser.ParseFrom(sourceByteString));
+                                    var sourceByteString = ((IMessage)sourceField).ToByteString();
+                                    var destinationValue = (IMessage)field.Accessor.GetValue(destination);
+                                    if (destinationValue != null)
+                                    {
+                                        destinationValue.MergeFrom(sourceByteString);
+                                    }
+                                    else
+                                    {
+                                        field.Accessor.SetValue(destination, field.MessageType.Parser.ParseFrom(sourceByteString));
+                                    }
                                 }
                             }
                         }