Custom WebFaultException on WCF Rest Serialization Failure?

With an object like this:

[DataContract]
public class SampleItem
{              
    private int _id;

    [DataMember(IsRequired = true)]
    public int Id
    {
        get { return _id; }
        set { _id = value; }
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    private string _stringValue;

    [DataMember()]
    public string StringValue
    {
        get { return _stringValue; }
        set { _stringValue = value; }
    }

And a REST call like this:

[WebInvoke(UriTemplate = "", Method = "POST")]        
public SampleItem Create(SampleItem instance)
{
    if (instance == null)
        throw new WebFaultException<string>("The SampleItem returned wasn't correctly formatted.",
                                            HttpStatusCode.BadRequest);               

    return instance;
}        

If I call it with an invalid SampleItem, say something without an ID like this:

<SampleItem xmlns="http://schemas.datacontract.org/2004/07/UserWebServices" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"><StringValue>SingleItem</StringValue></SampleItem>

Then the server gives me a 400 (correct) back with no useful error info (all I get is this: The server encountered an error processing the request. See server logs for more details). Ideally I want it to say something like ID is required.

How do I intercept the place where this error is being generated and throw my own WebFaultException?

Check out the WebProtocolException from the REST Start Kit. You can then send back a custom error message in your exception like this:

catch (Exception ex)
        {
            throw new WebProtocolException(HttpStatusCode.InternalServerError, "Unexpected Error.", new ServiceFault { ExceptionDetail = "Your custom error message here" }, null);
        }