Question

I’m writing an ASP.NET web application that transmits JSON between the client and the server. I have nearly everything complete, except that I cannot seem to transmit from the client the JSON to the ASMX and have it interpreted as anything but a Dictionary<string>

On the server-side, I have

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[ToolboxItem(false)]
[ScriptService]
public class WebService1: System.Web.Services.WebService
{
    [WebMethod(EnableSession = true)]
    public bool SaveExpression(ExpressionStructure Expression) {
        return true;
    }
}

On the client-side, I am sending the data using $.ajax(), where expressionObject.expression is an object of .NET class ExpressionStructure received earlier by the client:

var dataSubmit = { Expression: expressionObject.expression };
$.ajax({
    url: "WebService.asmx/SaveExpression",
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    type: 'POST',
    data: JSON.stringify(dataSubmit),
    success: function (msg) {
        alert(msg.d);
    },
    error: function (obj, status, msg) { ajaxServerError(obj, status, msg); }
});

When the SaveExpression method accepts Object, I get a dictionary of strings. When I use ExpressionStructure, I get a 500 Internal Server Error.

The expression being sent is an unmodified version of what is received from this MVC3 controller method.

public ActionResult Expression(int ExpressionID) {
    ExpressionStructure es = GetExpressionFromDatabase(ExpressionID);

    return new JsonResult {
        Data = new {
            expression = es,
            view = this.RenderPartialView("_Expression_Statement", es)
        }, 
        JsonRequestBehavior = System.Web.Mvc.JsonRequestBehavior.AllowGet
    };
}

The expression itself looks like this:

{
    "Expression": 
    {
        "Name":"Status Desc",
        "Type":0,
        "Statement": 
        {
            "FormulaItem": 
            {
                "Type":"Replace",
                "Parameters": 
                [
                    {
                        "Type":"Char",
                        "Value": 
                        {
                            "Value":"[Status]"
                        },
                        "Source":"picker"
                    },
                    {
                        "Type":"input",
                        "Value":
                        {
                            "Value":"0,1,2;Home,Driving,Away"
                        },
                        "Source":"inputBox"
                    }
                ]
            }
        }
    }
}

I’ve tried changing the ASMX method to take string, ExpressionStructure but both return a 500 Internal Server Error. I’ve also tried modifying the JSON coming from the client, doing various wrappings (including wrapping the outermost "Expression" with square braces).

What critical element am I missing that is preventing the ASMX from correctly taking the JSON and getting the ExpressionStructure? Optionally, how can I get a string either directly or from the Dictionary so I can perform the JSON convertion manually (which I’d rather not do)?

Answer 1

After building a small test case separated from the main project, I was able to get better information about why the server was returning 500 Internal Server Error. As it happens, the web app was catching a deserialization error and attempting to redirect the request to the error handler page. Since it wasn’t an actual page request, the redirect threw an error, which forced a redirect to the error handler page….until too many redirects kicked out an error.

The underlying problem was in the deserializing of the JSON to a rather complicated (collection of) classes on the .NET side. The classes, perfectly decorated with XML attribute decorators, used abstract classes when there was a choice in the schema (ie one element or another). Without similar JSON decorators, the deserialization didn’t know how to deserialize the JSON to which class.