Problem with jQuery validate plugin (remote validation)

I have a problem trying to validate a user value using the jQuery Validation plugin.

The validation seems to fire correctly and call the web service function exactly as I want but, even if the server function does work correctly and returns a true/false result the field is always invalid.

This is the validation code on the client side

$('#myForm').validate({
    errorContainer: container,
    errorLabelContainer: $("ol", container),
    wrapper: 'li',
    meta: "validate",
    rules: {
        Code: { required: true, maxlength: 15, remote: function () {
            return {
                type: "POST",
                url: GetBaseWSUrl() + 'MyService.asmx/IsValidCode',
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                data: JSON.stringify({ umltCode: $('#Code').val() })
            }
        }
        },
        Description: { required: true, maxlength: 255 },
        Notes: { maxlength: 255 }
    },
    messages: {
        // ... omitted for brevity
    },
    submitHandler: function (form) {
        saveObject(form);
    }
});

Using fiddler I am able to see that a call is made to the server and that the server is returning a json true/false value depending on the case as in the following sample:

{"d":false}

or

{"d":true}

Despite this, the plugin still mark the field as invalid. Any suggestion?

EDIT: this is my server function

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[ScriptService]
public class MyService : System.Web.Services.WebService
{
    [WebMethod]
    public object IsValidCode(string umltCode)
    {
        [...]

        if (u != null)
            return false;

        return true;
    }
}

Answers:

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Method 1

The problem is that instead of

true

your web service returns

{"d":true}

The d property should be removed.

ASMX is considered legacy so I would get rid of it at the first place. But until then you could also use the dataFilter property, like this:

remote: function() {  
    return {  
        type: "POST",
        url: GetBaseWSUrl() + 'MyService.asmx/IsValidCode',
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        data: JSON.stringify({ umltCode: $('#Code').val() }),
        dataFilter: function (data) {
            var x = (JSON.parse(data)).d;
            return JSON.stringify(x); 
        }  
    };   
}

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0