Provide JSONP with your WCF services (using .NET 3.5)

This post contains a small correction to MS provided example for returning JSONP with over WCF. The example is available here and works in most cases correctly, except the case when you are returning a raw JSON, that is you are not returning object which is serialized in to JSON, but rather returning a Stream which represents this JSON.

Such case might occur if you are building the JSON dynamically and not just serializing objects. The exception which you might obtain will be something along these lines:

Encountered invalid root element name 'Binary'. 'root' is the only allowed root element name.

About JSONP

JSON with Padding is a transport format, which uses the ability of script tag to execute scripts from different domains to overcome the cross-domain access issue. Generally the returned JSON is wrapped by JavaScript function which can be executed cross-domain.

So before we start - JSONP support is already added to .NET 4 so the services can be configured to use JSONP only by adding the CrossDomainScriptAccessEnabled attribute.

When the problem occurs

However I am stuck with NET 3.5 - so I needed to provide JSONP manually. Actually that is not that hard because MS provides this functionality in the WCF-WF example package (Downloadable here).

As stated above, the problem is, that this example is not complete. It works only when the service returns .NET objects which are serialized to JSON by WCF. However in some cases you might be serving the JSON which is already prepared. In this case your service returns a Stream. And the provided example won’t work.

To understand the problem, we have to take a look at what exactly does the example of MS code. Well to start you can simply look at this blog.

So basically to enable JSONP you just need to add JSONPBehavior attribute to your service. In fact this behavior uses JSONPEncoderFactory class, which defines an encoder (JSONPEncoder) which converts the messages to JSONP. The encoding takes place in the override WriteMessage method. Let’s take a look at the method provided in the MS example.

public override ArraySegment&ltbyte> WriteMessage(Message message, int maxMessageSize, BufferManager bufferManager, int messageOffset)
{
    MemoryStream stream = new MemoryStream();
    StreamWriter sw = new StreamWriter(stream);

    string methodName = null;
    if (message.Properties.ContainsKey(JSONPMessageProperty.Name))
        methodName = ((JSONPMessageProperty)(message.Properties[JSONPMessageProperty.Name])).MethodName;

    if (methodName != null)
    {
        sw.Write(methodName + "( ");
        sw.Flush();
    }
    XmlWriter writer = JsonReaderWriterFactory.CreateJsonWriter(stream);
    message.WriteMessage(writer);
    writer.Flush();
    if (methodName != null)
    {
        sw.Write(" );");
        sw.Flush();
    }

    byte[] messageBytes = stream.GetBuffer();
    int messageLength = (int)stream.Position;
    int totalLength = messageLength + messageOffset;
    byte[] totalBytes = bufferManager.TakeBuffer(totalLength);
    Array.Copy(messageBytes, 0, totalBytes, messageOffset, messageLength);

    ArraySegment&ltbyte> byteArray = new ArraySegment&ltbyte>(totalBytes, messageOffset, messageLength);
    writer.Close();
    return byteArray;
}

So what is happening here: the Message object contains the object which returns your method. The WriteMessage method will take this object and write it to a Stream which is passed to it in argument. In the method the passed stream is a JsonWriter. The problem is that JsonWriter expects the structure of the message to be object represented by XML, which it will convert to JSON.

Now you can see that before we are actually writing the content of the message, we write “methodName(“ and after “);”. Generally this is the wrapping by JavaScript function. The result will be something like “methodName({JSONOBject});”.

The resulted Stream is than just converted to byte array.

This works, but the problem is that when you are returning raw JSON, in other words, that your method returns Stream, than you cannot use JsonWriter, because the Message.WriteMessage will push to the writer XML of different structure, than it expects.

To be specific the XML will have a form of <binary>asdqwetasfd</Binary> and JsonWriter will not be able to create reasonable Json object.

Solution

The solution to the problem is following:

Check the format of the message (if it Json or Raw Stream)
If it is a Raw Stream, than just convert the Stream to array of bytes
If it is Json, than use the same procedure as before

public override ArraySegment&ltbyte> WriteMessage(Message message, int maxMessageSize, BufferManager bufferManager, int messageOffset)
{
    WebContentFormat messageFormat = this.GetMessageContentFormat(message);

    MemoryStream stream = new MemoryStream();
    StreamWriter sw = new StreamWriter(stream);

    string methodName = null;
    if (message.Properties.ContainsKey(JSONPMessageProperty.Name))
        methodName = ((JSONPMessageProperty)(message.Properties[JSONPMessageProperty.Name])).MethodName;

    if (methodName != null)
    {
        sw.Write(methodName + "( ");
        sw.Flush();
    }

    XmlWriter writer = null;
    if (messageFormat == WebContentFormat.Json)
    {
        writer = JsonReaderWriterFactory.CreateJsonWriter(stream);
        message.WriteMessage(writer);
        writer.Flush();
        //writer.Close();
    }
    else if (messageFormat == WebContentFormat.Raw)
    {
        String messageBody = ReadRawBody(ref message);
        sw.Write(messageBody);
        sw.Flush();
    }

    if (methodName != null)
    {
        sw.Write(" );");
        sw.Flush();
    }

    byte[] messageBytes = stream.GetBuffer();
    int messageLength = (int)stream.Position;
    int totalLength = messageLength + messageOffset;
    byte[] totalBytes = bufferManager.TakeBuffer(totalLength);
    Array.Copy(messageBytes, 0, totalBytes, messageOffset, messageLength);

    ArraySegment&ltbyte> byteArray = new ArraySegment&ltbyte>(totalBytes, messageOffset, messageLength);
    stream.Close();

    return byteArray;
}

You can see that I am using two additional methods: GetMessageContentFormat and ReadRawBody. I did not came up with these methods, instead I have borrowed them from the blog of Carlos Figueira In his blog, he describes how to use these methods when Inspecting messages. That is not the same scenario, but actually Inspecting outgoing methods or creating own MessageEncoder are just two ways to achieve the same thing. I will add the definitions of the methods here, but the above mentioned blog post is a great source of information regarding customization of WCF Service, definitely worth checking.

private WebContentFormat GetMessageContentFormat(Message message)
{
    WebContentFormat format = WebContentFormat.Default;
    if (message.Properties.ContainsKey(WebBodyFormatMessageProperty.Name))
    {
        WebBodyFormatMessageProperty bodyFormat;
        bodyFormat = (WebBodyFormatMessageProperty)message.Properties[WebBodyFormatMessageProperty.Name];
        format = bodyFormat.Format;
    }

    return format;
}

private String ReadRawBody(ref Message message)
{
    var bodyReader = message.GetReaderAtBodyContents();

    bodyReader.ReadStartElement("Binary");
    byte[] bodyBytes = bodyReader.ReadContentAsBase64();

    string messageBody = Encoding.UTF8.GetString(bodyBytes);

    // Now to recreate the message
    MemoryStream ms = new MemoryStream();
    XmlDictionaryWriter writer = XmlDictionaryWriter.CreateBinaryWriter(ms);
    writer.WriteStartElement("Binary");
    writer.WriteBase64(bodyBytes, 0, bodyBytes.Length);
    writer.WriteEndElement();
    writer.Flush();
    ms.Position = 0;
    XmlDictionaryReader reader = XmlDictionaryReader.CreateBinaryReader(ms, XmlDictionaryReaderQuotas.Max);
    Message newMessage = Message.CreateMessage(reader, int.MaxValue, message.Version);
    newMessage.Properties.CopyProperties(message.Properties);
    message = newMessage;
    //return bodyBytes;
    return messageBody;
}

Written on July 19, 2011