From cea3653a2ea4fa08ba8fa59b3f06cad27b5af074 Mon Sep 17 00:00:00 2001
From: Jon Skeet <jonskeet@google.com>
Date: Wed, 20 Jan 2021 08:06:34 +0000
Subject: [PATCH] Include the size of values (and tags) for extensions, even if
 the value is the default

Fixes #8218.
---
 .../Google.Protobuf.Test/ExtensionSetTest.cs    | 17 ++++++++++++++++-
 csharp/src/Google.Protobuf/ExtensionValue.cs    |  2 +-
 csharp/src/Google.Protobuf/FieldCodec.cs        |  6 ++++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs b/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs
index aceb4a68f7..951e856a59 100644
--- a/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs
+++ b/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs
@@ -116,7 +116,22 @@ namespace Google.Protobuf
             var other = message.Clone();
 
             Assert.AreEqual(message, other);
-            Assert.AreEqual(message.CalculateSize(), message.CalculateSize());
+            Assert.AreEqual(message.CalculateSize(), other.CalculateSize());
+        }
+
+        [Test]
+        public void TestDefaultValueRoundTrip()
+        {
+            var message = new TestAllExtensions();
+            message.SetExtension(OptionalBoolExtension, false);
+            Assert.IsFalse(message.GetExtension(OptionalBoolExtension));
+            Assert.IsTrue(message.HasExtension(OptionalBoolExtension));
+
+            var bytes = message.ToByteArray();
+            var registry = new ExtensionRegistry { OptionalBoolExtension };
+            var parsed = TestAllExtensions.Parser.WithExtensionRegistry(registry).ParseFrom(bytes);
+            Assert.IsFalse(parsed.GetExtension(OptionalBoolExtension));
+            Assert.IsTrue(parsed.HasExtension(OptionalBoolExtension));
         }
     }
 }
diff --git a/csharp/src/Google.Protobuf/ExtensionValue.cs b/csharp/src/Google.Protobuf/ExtensionValue.cs
index ada5d79c80..5257c4c955 100644
--- a/csharp/src/Google.Protobuf/ExtensionValue.cs
+++ b/csharp/src/Google.Protobuf/ExtensionValue.cs
@@ -59,7 +59,7 @@ namespace Google.Protobuf
 
         public int CalculateSize()
         {
-            return codec.CalculateSizeWithTag(field);
+            return codec.CalculateUnconditionalSizeWithTag(field);
         }
 
         public IExtensionValue Clone()
diff --git a/csharp/src/Google.Protobuf/FieldCodec.cs b/csharp/src/Google.Protobuf/FieldCodec.cs
index 158739d50a..ee6bd6a0eb 100644
--- a/csharp/src/Google.Protobuf/FieldCodec.cs
+++ b/csharp/src/Google.Protobuf/FieldCodec.cs
@@ -876,6 +876,12 @@ namespace Google.Protobuf
         /// </summary>
         public int CalculateSizeWithTag(T value) => IsDefault(value) ? 0 : ValueSizeCalculator(value) + tagSize;
 
+        /// <summary>
+        /// Calculates the size required to write the given value, with a tag, even
+        /// if the value is the default.
+        /// </summary>
+        internal int CalculateUnconditionalSizeWithTag(T value) => ValueSizeCalculator(value) + tagSize;
+
         private bool IsDefault(T value) => EqualityComparer.Equals(value, DefaultValue);
     }
 }