Include the size of values (and tags) for extensions, even if the value is the default Fixes #8218.

diff --git a/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs b/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs
index aceb4a6..951e856 100644
--- a/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs
+++ b/csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs

@@ -116,7 +116,22 @@
             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 ada5d79..5257c4c 100644
--- a/csharp/src/Google.Protobuf/ExtensionValue.cs
+++ b/csharp/src/Google.Protobuf/ExtensionValue.cs

@@ -59,7 +59,7 @@
 
         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 158739d..ee6bd6a 100644
--- a/csharp/src/Google.Protobuf/FieldCodec.cs
+++ b/csharp/src/Google.Protobuf/FieldCodec.cs

@@ -876,6 +876,12 @@
         /// </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);
     }
 }