add more tests

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

@@ -132,6 +132,8 @@ namespace Google.Protobuf
             byte[] bytes = message.ToByteArray();
             Assert.AreEqual(0, bytes.Length);
 
+            MessageParsingHelpers.AssertWritingMessage(message);
+
             MessageParsingHelpers.AssertRoundtrip(TestAllTypes.Parser, message);
         }
 
@@ -164,7 +166,7 @@ namespace Google.Protobuf
                 SingleUint64 = ulong.MaxValue
             };
 
-            byte[] bytes = message.ToByteArray();
+            MessageParsingHelpers.AssertWritingMessage(message);
 
             MessageParsingHelpers.AssertRoundtrip(TestAllTypes.Parser, message);
         }
@@ -198,7 +200,7 @@ namespace Google.Protobuf
                 RepeatedUint64 = { ulong.MaxValue, uint.MinValue }
             };
 
-            byte[] bytes = message.ToByteArray();
+            MessageParsingHelpers.AssertWritingMessage(message);
 
             MessageParsingHelpers.AssertRoundtrip(TestAllTypes.Parser, message);
         }
@@ -230,7 +232,7 @@ namespace Google.Protobuf
                 }
             };
 
-            byte[] bytes = message.ToByteArray();
+            MessageParsingHelpers.AssertWritingMessage(message);
 
             MessageParsingHelpers.AssertRoundtrip(TestMap.Parser, message);
         }
@@ -246,6 +248,8 @@ namespace Google.Protobuf
             byte[] bytes = message.ToByteArray();
             Assert.AreEqual(2, bytes.Length); // Tag for field entry (1 byte), length of entry (0; 1 byte)
 
+            MessageParsingHelpers.AssertWritingMessage(message);
+
             MessageParsingHelpers.AssertReadingMessage(
                 TestMap.Parser,
                 bytes,
@@ -660,6 +664,8 @@ namespace Google.Protobuf
             var bytes = message.ToByteArray();
             Assert.AreEqual(3, bytes.Length); // 2 bytes for the tag + 1 for the value - no string!
 
+            MessageParsingHelpers.AssertWritingMessage(message);
+
             MessageParsingHelpers.AssertRoundtrip(TestAllTypes.Parser, message, parsedMessage =>
             {
                 Assert.AreEqual(TestAllTypes.OneofFieldOneofCase.OneofUint32, parsedMessage.OneofFieldCase);
@@ -675,6 +681,8 @@ namespace Google.Protobuf
             var bytes = message.ToByteArray();
             Assert.AreEqual(3, bytes.Length); // 2 bytes for the tag + 1 for the value - it's still serialized
 
+            MessageParsingHelpers.AssertWritingMessage(message);
+
             MessageParsingHelpers.AssertRoundtrip(TestAllTypes.Parser, message, parsedMessage =>
             {
                 Assert.AreEqual(TestAllTypes.OneofFieldOneofCase.OneofUint32, parsedMessage.OneofFieldCase);

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

@@ -33,6 +33,7 @@
 using NUnit.Framework;
 using System;
 using System.Buffers;
+using Google.Protobuf.Buffers;
 
 namespace Google.Protobuf
 {
@@ -81,6 +82,11 @@ namespace Google.Protobuf
         {
             var bytes = message.ToByteArray();
 
+            // also serialize using IBufferWriter and check it leads to the same data
+            var bufferWriter = new ArrayBufferWriter<byte>();
+            message.WriteTo(bufferWriter);
+            Assert.AreEqual(bytes, bufferWriter.WrittenSpan.ToArray(), "Both serialization approaches need to result in the same data.");
+
             // Load content as single segment
             var parsedBuffer = parser.ParseFrom(new ReadOnlySequence<byte>(bytes));
             Assert.AreEqual(message, parsedBuffer);
@@ -96,5 +102,18 @@ namespace Google.Protobuf
             Assert.AreEqual(message, parsedStream);
             additionalAssert?.Invoke(parsedStream);
         }
+
+        public static void AssertWritingMessage(IMessage message)
+        {
+            // serialize using CodedOutputStream
+            var bytes = message.ToByteArray();
+
+            // also serialize using IBufferWriter and check it leads to the same data
+            var bufferWriter = new ArrayBufferWriter<byte>();
+            message.WriteTo(bufferWriter);
+            Assert.AreEqual(bytes, bufferWriter.WrittenSpan.ToArray(), "Both serialization approaches need to result in the same data.");
+
+            Assert.AreEqual(message.CalculateSize(), bytes.Length);
+        }
     }
 }