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POPreserving Polymorphic Types in a WCF Service using JSON
 

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    <p>I have a C# WCF service using a webHttpBinding endpoint that will receive and return data in JSON format. The data to send/receive needs to use a polymorphic type so that data of different types can be exchanged in the same "data packet". I have the following data model:</p> <pre><code>[DataContract] public class DataPacket { [DataMember] public List&lt;DataEvent&gt; DataEvents { get; set; } } [DataContract] [KnownType(typeof(IntEvent))] [KnownType(typeof(BoolEvent))] public class DataEvent { [DataMember] public ulong Id { get; set; } [DataMember] public DateTime Timestamp { get; set; } public override string ToString() { return string.Format("DataEvent: {0}, {1}", Id, Timestamp); } } [DataContract] public class IntEvent : DataEvent { [DataMember] public int Value { get; set; } public override string ToString() { return string.Format("IntEvent: {0}, {1}, {2}", Id, Timestamp, Value); } } [DataContract] public class BoolEvent : DataEvent { [DataMember] public bool Value { get; set; } public override string ToString() { return string.Format("BoolEvent: {0}, {1}, {2}", Id, Timestamp, Value); } } </code></pre> <p>My service will send/receive the sub-type events (IntEvent, BoolEvent etc.) in a single data packet, as follows:</p> <pre><code>[ServiceContract] public interface IDataService { [OperationContract] [WebGet(UriTemplate = "GetExampleDataEvents")] DataPacket GetExampleDataEvents(); [OperationContract] [WebInvoke(UriTemplate = "SubmitDataEvents", RequestFormat = WebMessageFormat.Json)] void SubmitDataEvents(DataPacket dataPacket); } public class DataService : IDataService { public DataPacket GetExampleDataEvents() { return new DataPacket { DataEvents = new List&lt;DataEvent&gt; { new IntEvent { Id = 12345, Timestamp = DateTime.Now, Value = 5 }, new BoolEvent { Id = 45678, Timestamp = DateTime.Now, Value = true } } }; } public void SubmitDataEvents(DataPacket dataPacket) { int i = dataPacket.DataEvents.Count; //dataPacket contains 2 events, but both are type DataEvent instead of IntEvent and BoolEvent IntEvent intEvent = dataPacket.DataEvents[0] as IntEvent; Console.WriteLine(intEvent.Value); //null pointer as intEvent is null since the cast failed } } </code></pre> <p>When I submit my packet to the <code>SubmitDataEvents</code> method though, I get <code>DataEvent</code> types and trying to cast them back to their base types (just for testing purposes) results in an <code>InvalidCastException</code>. My packet is:</p> <pre><code>POST http://localhost:4965/DataService.svc/SubmitDataEvents HTTP/1.1 User-Agent: Fiddler Host: localhost:4965 Content-Type: text/json Content-Length: 340 { "DataEvents": [{ "__type": "IntEvent:#WcfTest.Data", "Id": 12345, "Timestamp": "\/Date(1324905383689+0000)\/", "Value": 5 }, { "__type": "BoolEvent:#WcfTest.Data", "Id": 45678, "Timestamp": "\/Date(1324905383689+0000)\/", "Value": true }] } </code></pre> <p>Apologies for the long post, but is there anything I can do to preserve the base types of each object? I thought adding the type hint to the JSON and the KnownType attributes to <code>DataEvent</code> would allow me to preserve the types - but it doesn't seem to work.</p> <p><strong>Edit</strong>: If I send the request to <code>SubmitDataEvents</code> in XML format (with <code>Content-Type: text/xml</code> instead of <code>text/json</code>) then the <code>List&lt;DataEvent&gt; DataEvents</code> does contain the sub-types instead of the super-type. As soon as I set the request to <code>text/json</code> and send the above packet then I only get the super-type and I can't cast them to the sub-type. My XML request body is:</p> <pre><code>&lt;ArrayOfDataEvent xmlns="http://schemas.datacontract.org/2004/07/WcfTest.Data"&gt; &lt;DataEvent i:type="IntEvent" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"&gt; &lt;Id&gt;12345&lt;/Id&gt; &lt;Timestamp&gt;1999-05-31T11:20:00&lt;/Timestamp&gt; &lt;Value&gt;5&lt;/Value&gt; &lt;/DataEvent&gt; &lt;DataEvent i:type="BoolEvent" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"&gt; &lt;Id&gt;56789&lt;/Id&gt; &lt;Timestamp&gt;1999-05-31T11:20:00&lt;/Timestamp&gt; &lt;Value&gt;true&lt;/Value&gt; &lt;/DataEvent&gt; &lt;/ArrayOfDataEvent&gt; </code></pre> <p><strong>Edit 2</strong>: Updated service description after Pavel's comments below. This still doesn't work when sending the JSON packet in Fiddler2. I just get a <code>List</code> containing <code>DataEvent</code> instead of <code>IntEvent</code> and <code>BoolEvent</code>.</p> <p><strong>Edit 3</strong>: As Pavel suggested, here is the output from <code>System.ServiceModel.OperationContext.Current.RequestContext.RequestMessage.ToString()</code>. Looks OK to me.</p> <pre><code>&lt;root type="object"&gt; &lt;DataEvents type="array"&gt; &lt;item type="object"&gt; &lt;__type type="string"&gt;IntEvent:#WcfTest.Data&lt;/__type&gt; &lt;Id type="number"&gt;12345&lt;/Id&gt; &lt;Timestamp type="string"&gt;/Date(1324905383689+0000)/&lt;/Timestamp&gt; &lt;Value type="number"&gt;5&lt;/Value&gt; &lt;/item&gt; &lt;item type="object"&gt; &lt;__type type="string"&gt;BoolEvent:#WcfTest.Data&lt;/__type&gt; &lt;Id type="number"&gt;45678&lt;/Id&gt; &lt;Timestamp type="string"&gt;/Date(1324905383689+0000)/&lt;/Timestamp&gt; &lt;Value type="boolean"&gt;true&lt;/Value&gt; &lt;/item&gt; &lt;/DataEvents&gt; &lt;/root&gt; </code></pre> <p>When tracing the deserialization of the packet, I get the following messages in the trace:</p> <pre><code>&lt;TraceRecord xmlns="http://schemas.microsoft.com/2004/10/E2ETraceEvent/TraceRecord" Severity="Verbose"&gt; &lt;TraceIdentifier&gt;http://msdn.microsoft.com/en-GB/library/System.Runtime.Serialization.ElementIgnored.aspx&lt;/TraceIdentifier&gt; &lt;Description&gt;An unrecognized element was encountered in the XML during deserialization which was ignored.&lt;/Description&gt; &lt;AppDomain&gt;1c7ccc3b-4-129695001952729398&lt;/AppDomain&gt; &lt;ExtendedData xmlns="http://schemas.microsoft.com/2006/08/ServiceModel/StringTraceRecord"&gt; &lt;Element&gt;:__type&lt;/Element&gt; &lt;/ExtendedData&gt; &lt;/TraceRecord&gt; </code></pre> <p>This message is repeated 4 times (twice with <code>__type</code> as the element and twice with <code>Value</code>). Looks like the type hinting information is being ignored then the <code>Value</code> elements are ignored as the packet is deserialized to <code>DataEvent</code> instead of <code>IntEvent</code>/<code>BoolEvent</code>.</p>
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