Fixed #8: Forwards compatibility of enums

Tapeti/Default/FallbackStringEnumConverter.cs

@@ -0,0 +1,90 @@
+using System;
+using System.Diagnostics;
+using Newtonsoft.Json;
+
+namespace Tapeti.Default
+{
+    /// <summary>
+    /// Converts an <see cref="Enum"/> to and from its name string value. If an unknown string value is encountered
+    /// it will translate to 0xDEADBEEF (-559038737) so it can be gracefully handled.
+    /// If you copy this value as-is to another message and try to send it, this converter will throw an exception.
+    ///
+    /// This converter is far simpler than the default StringEnumConverter, it assumes both sides use the same
+    /// enum and therefore skips the naming strategy.
+    /// </summary>
+    public class FallbackStringEnumConverter : JsonConverter
+    {
+        private readonly int invalidEnumValue;
+
+
+        public FallbackStringEnumConverter()
+        {
+            unchecked { invalidEnumValue = (int)0xDEADBEEF; }
+        }
+
+
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            if (value == null)
+            {
+                writer.WriteNull();
+                return;
+            }
+
+            if ((int) value == invalidEnumValue)
+                throw new ArgumentException("Enum value was an unknown string value in an incoming message and can not be published in an outgoing message as-is");
+
+            var outputValue = Enum.GetName(value.GetType(), value);
+            writer.WriteValue(outputValue);
+        }
+
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        {
+            var isNullable = IsNullableType(objectType);
+
+            if (reader.TokenType == JsonToken.Null)
+            {
+                if (!isNullable)
+                    throw new JsonSerializationException($"Cannot convert null value to {objectType}");
+
+                return null;
+            }
+
+            var actualType = isNullable ? Nullable.GetUnderlyingType(objectType) : objectType;
+            Debug.Assert(actualType != null, nameof(actualType) + " != null");
+
+            if (reader.TokenType != JsonToken.String)
+                throw new JsonSerializationException($"Unexpected token {reader.TokenType} when parsing enum");
+
+            var enumText = reader.Value.ToString();
+            if (enumText == string.Empty && isNullable)
+                return null;
+
+            try
+            {
+                return Enum.Parse(actualType, enumText);
+            }
+            catch (ArgumentException)
+            {
+                return Enum.ToObject(actualType, invalidEnumValue);
+            }
+        }
+
+
+        public override bool CanConvert(Type objectType)
+        {
+            var actualType = IsNullableType(objectType) ? Nullable.GetUnderlyingType(objectType) : objectType;
+            return actualType?.IsEnum ?? false;
+        }
+
+
+        private static bool IsNullableType(Type t)
+        {
+            if (t == null)
+                throw new ArgumentNullException(nameof(t));
+
+            return t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>);
+        }
+    }
+}

Tapeti/Default/JsonMessageSerializer.cs

@@ -3,7 +3,6 @@ using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Text;
 using Newtonsoft.Json;
-using Newtonsoft.Json.Converters;
 using RabbitMQ.Client;
 
 namespace Tapeti.Default
@@ -25,7 +24,7 @@ namespace Tapeti.Default
                 NullValueHandling = NullValueHandling.Ignore
             };
 
-            serializerSettings.Converters.Add(new StringEnumConverter());
+            serializerSettings.Converters.Add(new FallbackStringEnumConverter());            
         }
 
 
@@ -52,7 +51,7 @@ namespace Tapeti.Default
                 throw new ArgumentException($"{ClassTypeHeader} header not present");
 
             var messageType = deserializedTypeNames.GetOrAdd(Encoding.UTF8.GetString((byte[])typeName), DeserializeTypeName);
-            return JsonConvert.DeserializeObject(Encoding.UTF8.GetString(body), messageType);
+            return JsonConvert.DeserializeObject(Encoding.UTF8.GetString(body), messageType, serializerSettings);
         }
 
 

Test/MarcoController.cs

@@ -80,12 +80,16 @@ namespace Test
             return flowProvider.YieldWithParallelRequest()
                 .AddRequestSync<PoloConfirmationRequestMessage, PoloConfirmationResponseMessage>(new PoloConfirmationRequestMessage
                 {
-                    StoredInState = StateTestGuid
+                    StoredInState = StateTestGuid,
+                    EnumValue = TestEnum.Value1,
+
                 }, HandlePoloConfirmationResponse1)
 
                 .AddRequestSync<PoloConfirmationRequestMessage, PoloConfirmationResponseMessage>(new PoloConfirmationRequestMessage
                 {
-                    StoredInState = StateTestGuid
+                    StoredInState = StateTestGuid,
+                    EnumValue = TestEnum.Value2,
+                    OptionalEnumValue = TestEnum.Value1
                 }, HandlePoloConfirmationResponse2)
 
                 .YieldSync(ContinuePoloConfirmation);
@@ -127,7 +131,9 @@ namespace Test
 
             return new PoloConfirmationResponseMessage
             {
-                ShouldMatchState = message.StoredInState
+                ShouldMatchState = message.StoredInState,
+                EnumValue = message.EnumValue,
+                OptionalEnumValue = message.OptionalEnumValue
             };
         }
 
@@ -141,6 +147,13 @@ namespace Test
     }
 
 
+    public enum TestEnum
+    {
+        Value1,
+        Value2
+    }
+
+
     [Request(Response = typeof(PoloMessage))]
     public class MarcoMessage
     {
@@ -157,6 +170,9 @@ namespace Test
     {
         [Required]
         public Guid StoredInState { get; set; }
+
+        public TestEnum EnumValue;
+        public TestEnum? OptionalEnumValue;
     }
 
 
@@ -164,5 +180,8 @@ namespace Test
     {
         [Required]
         public Guid ShouldMatchState { get; set; }
+
+        public TestEnum EnumValue;
+        public TestEnum? OptionalEnumValue;
     }
 }