Ensure that FieldMask, Timestamp and Duration ToString() calls don't throw

The usage of ICustomDiagnosticMessage here is non-essential - ToDiagnosticString
doesn't actually get called by ToString() in this case, due to JsonFormatter code. It was
intended to make it clearer that it *did* have a custom format... but then arguably I should
do the same for Value, Struct, Any etc.

Moving some of the code out of JsonFormatter and into Duration/Timestamp/FieldMask likewise
feels somewhat nice, somewhat nasty... basically there are JSON-specific bits of formatting, but
also domain-specific bits of computation. <sigh>

Thoughts welcome.

1 files changed, 8 insertions, 0 deletions

diff --git a/csharp/src/Google.Protobuf.Test/WellKnownTypes/TimestampTest.cs b/csharp/src/Google.Protobuf.Test/WellKnownTypes/TimestampTest.cs
index 84717d66..9ecd24c6 100644
--- a/csharp/src/Google.Protobuf.Test/WellKnownTypes/TimestampTest.cs
+++ b/csharp/src/Google.Protobuf.Test/WellKnownTypes/TimestampTest.cs
@@ -103,5 +103,13 @@ namespace Google.Protobuf.WellKnownTypes
             Assert.AreEqual(t1, t2 + difference);
             Assert.AreEqual(t2, t1 - difference);
         }
+
+        [Test]
+        public void ToString_NonNormalized()
+        {
+            // Just a single example should be sufficient...
+            var duration = new Timestamp { Seconds = 1, Nanos = -1 };
+            Assert.AreEqual("{ \"@warning\": \"Invalid Timestamp\", \"seconds\": \"1\", \"nanos\": -1 }", duration.ToString());
+        }
     }
 }