Honor [JsonConverter] attributes on properties of custom params obj…

…ects

Fixes microsoft#735

src/StreamJsonRpc/JsonMessageFormatter.cs

@@ -636,35 +636,18 @@ private void TokenizeUserData(JsonRpcMessage jsonRpcMessage)
                                 throw new NotSupportedException(Resources.ParameterObjectsNotSupportedInJsonRpc10);
                             }
 
-                            JObject tokenizedArgumentsObject = new();
-                            if (request.NamedArguments is not null)
-                            {
-                                foreach (KeyValuePair<string, object?> pair in request.NamedArguments)
-                                {
-                                    Type? declaredType = null;
-                                    request.NamedArgumentDeclaredTypes?.TryGetValue(pair.Key, out declaredType);
-                                    tokenizedArgumentsObject.Add(pair.Key, this.TokenizeUserData(declaredType, pair.Value));
-                                }
-                            }
-                            else if (DefaultSerializer.ContractResolver.ResolveContract(request.Arguments.GetType()) is JsonObjectContract contract)
-                            {
-                                // The arguments should be interpreted as named arguments,
-                                // but as they were not detected as in a dictionary,
-                                // it may be an anonymous type. Allow Newtonsoft.Json to determine the property names and values.
-                                foreach (JsonProperty property in contract.Properties)
-                                {
-                                    if (property.PropertyName is not null && property.ValueProvider is not null)
-                                    {
-                                        tokenizedArgumentsObject.Add(property.PropertyName, this.TokenizeUserData(property.PropertyType, property.ValueProvider.GetValue(request.Arguments)));
-                                    }
-                                }
-                            }
-                            else
+                            // Tokenize the user data using the user-supplied serializer.
+                            JObject? paramsObject = JObject.FromObject(request.Arguments, this.JsonSerializer);
+
+                            // Json.Net TypeHandling could insert a $type child JToken to the paramsObject above.
+                            // This $type JToken should not be there to maintain Json RPC Spec compatibility. We will
+                            // strip the token out here.
+                            if (this.JsonSerializer.TypeNameHandling != TypeNameHandling.None)
                             {
-                                throw new JsonSerializationException("Unsupported arguments object type: " + request.Arguments.GetType());
+                                paramsObject.Remove("$type");
                             }
 
-                            request.Arguments = tokenizedArgumentsObject;
+                            request.Arguments = paramsObject;
                         }
 
                         break;