Send JSON data with jQuery

Because you haven’t specified neither request content type, nor correct JSON request. Here’s the correct way to send a JSON request:

var arr = { City: 'Moscow', Age: 25 };
$.ajax({
    url: 'Ajax.ashx',
    type: 'POST',
    data: JSON.stringify(arr),
    contentType: 'application/json; charset=utf-8',
    dataType: 'json',
    async: false,
    success: function(msg) {
        alert(msg);
    }
});

Things to notice:

• Usage of the JSON.stringify method to convert a javascript object into a JSON string which is native and built-into modern browsers. If you want to support older browsers you might need to include json2.js
• Specifying the request content type using the contentType property in order to indicate to the server the intent of sending a JSON request
• The dataType: 'json' property is used for the response type you expect from the server. jQuery is intelligent enough to guess it from the server Content-Type response header. So if you have a web server which respects more or less the HTTP protocol and responds with Content-Type: application/json to your request jQuery will automatically parse the response into a javascript object into the success callback so that you don’t need to specify the dataType property.

Things to be careful about:

• What you call arr is not an array. It is a javascript object with properties (City and Age). Arrays are denoted with [] in javascript. For example [{ City: 'Moscow', Age: 25 }, { City: 'Paris', Age: 30 }] is an array of 2 objects.