Support schema evolution

Author: Havret

src/Protobuf.System.Text.Json/ProtobufConverter.cs

@@ -72,15 +72,14 @@ public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerial
             }
 
             var propertyName = reader.GetString();
-
+            reader.Read();
+            
             if (!_fieldsLookup.TryGetValue(propertyName, out var fieldInfo))
             {
                 continue;
             }
 
             fieldInfo.Converter ??= InternalConverterFactory.Create(fieldInfo);
-
-            reader.Read();
             fieldInfo.Converter.Read(ref reader, obj, fieldInfo.FieldType, options, fieldInfo.Accessor);
         }
 

test/Protobuf.System.Text.Json.Tests/ContractEvolutionTests.cs

@@ -0,0 +1,30 @@
+using System.Text.Json;
+using System.Text.Json.Protobuf.Tests;
+using Protobuf.System.Text.Json.Tests.Utils;
+using Shouldly;
+using Xunit;
+
+namespace Protobuf.System.Text.Json.Tests;
+
+public class ContractEvolutionTests
+{
+    [Fact]
+    public void Should_deserialize_a_new_version_of_a_message_using_the_old_version_of_the_contract()
+    {
+        // Arrange
+        var msg = new MessageWithVersionMismatchV2
+        {
+            DoubleProperty = 1d,
+            FloatProperty = 2f,
+        };
+        var jsonSerializerOptions = TestHelper.CreateJsonSerializerOptions();
+        
+        // Act
+        var payload = JsonSerializer.Serialize(msg, jsonSerializerOptions);
+        var deserialized = JsonSerializer.Deserialize<MessageWithVersionMismatch>(payload, jsonSerializerOptions);
+
+        // Assert
+        deserialized.ShouldNotBeNull();
+        deserialized.DoubleProperty.ShouldBe(msg.DoubleProperty);
+    }
+}

test/Protobuf.System.Text.Json.Tests/Protos/message_with_version_mismatch.proto

@@ -0,0 +1,12 @@
+syntax = "proto3";
+
+option csharp_namespace = "System.Text.Json.Protobuf.Tests";
+
+message MessageWithVersionMismatch {
+  double double_property = 1;
+}
+
+message MessageWithVersionMismatchV2 {
+  double double_property = 1;
+  float float_property = 2;
+}