Question

I am trying to find a solution to validate if XML data sent in a POST request are fulfilling a given custom XML schema.

If I use the XmlMediaTypeFormatter delivered with ASP.NET Web API I don’t have a schema validation available, as far as I can see. For example: If I have a model type…

public class Order
{
    public string Code { get; set; }
    public int Quantity { get; set; }
}

…and a POST action in an ApiController…

public HttpResponseMessage Post(Order order)
{
    if (ModelState.IsValid)
    {
        // process order...
        // send 200 OK response for example
    }
    else
        // send 400 BadRequest response with ModelState errors in response body
}

…I can post the following “wrong” XML data and will get a 200 OK response nevertheless:

User-Agent: Fiddler
Host: localhost:45678
Content-Type: application/xml; charset=utf-8

<Order> <Code>12345</Nonsense> </Order>   // malformed XML

Or:

<Order> <CustomerName>12345</CustomerName> </Order>    // invalid property

Or:

<Customer> <Code>12345</Code> </Customer>    // invalid root

Or:

"Hello World"    // no XML at all

etc., etc.

The only point where I have a validation of the request is model binding: In request example 1, 3 and 4 the order passed into the Post method is null, in example 2 the order.Code property is null which I could invalidate by testing for order == null or by marking the Code property with a [Required] attribute. I could send this validation result back in the response with a 400 “BadRequest” Http status code and validation messages in the response body. But I cannot tell exactly what was wrong and can’t distinguish between the wrong XML in example 1, 3 and 4 (no order has been posted, that’s the only thing I can see) – for instance.

Requiring that an Order has to be posted with a specific custom XML schema, for example xmlns="http://test.org/OrderSchema.xsd", I would like to validate if the posted XML is valid with respect to this schema and, if not, send schema validation errors back in the response. To achieve this I have started with a custom MediaTypeFormatter:

public class MyXmlMediaTypeFormatter : MediaTypeFormatter
{
    // constructor, CanReadType, CanWriteType, ...

    public override Task<object> ReadFromStreamAsync(Type type, Stream stream,
        HttpContentHeaders contentHeaders, IFormatterLogger formatterLogger)
    {
        var task = Task.Factory.StartNew(() =>
        {
            using (var streamReader = new StreamReader(stream))
            {
                XDocument document = XDocument.Load(streamReader);
                // TODO: exceptions must the catched here,
                // for example due to malformed XML
                XmlSchemaSet schemaSet = new XmlSchemaSet();
                schemaSet.Add(null, "OrderSchema.xsd");

                var msgs = new List<string>();
                document.Validate(schemaSet, (s, e) => msgs.Add(e.Message));
                // msgs contains now the list of XML schema validation errors
                // I want to send back in the response
                if (msgs.Count == 0)
                {
                    var order = ... // deserialize XML to order
                    return (object)order;
                }
                else
                    // WHAT NOW ?
            }
        });
        return task;
    }
}

This works so far as long as everything is correct.

But I don’t know what to do if msgs.Count > 0. How can I “transfer” this validation result list to the Post action or how can I create a Http response that contains those XML schema validation messages?

Also I am unsure if a custom MediaTypeFormatter is the best extensibility point for such a XML schema validation and if my approach isn’t the wrong way. Would possibly a custom HttpMessageHandler/DelegatingHandler be a better place for this? Or is there perhaps something much simpler out of the box?

Answer 1

By trial and error I found a solution (for the WHAT NOW ? placeholder in the question’s code):

//...
else
{
    PostOrderErrors errors = new PostOrderErrors
    {
        XmlValidationErrors = msgs
    };
    HttpResponseMessage response = new HttpResponseMessage(
        HttpStatusCode.BadRequest);
    response.Content = new ObjectContent(typeof(PostOrderErrors), errors,
        GlobalConfiguration.Configuration.Formatters.XmlFormatter);
    throw new HttpResponseException(response);
}

…with the response class like this:

public class PostOrderErrors
{
    public List<string> XmlValidationErrors { get; set; }
    //...
}

That seems to work and the response looks like this then:

HTTP/1.1 400 Bad Request
Content-Type: application/xml; charset=utf-8

<PostOrderErrors xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                 xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <XmlValidationErrors>
        <string>Some error text...</string>
        <string>Another error text...</string>
    </XmlValidationErrors>
</PostOrderErrors>