public void ProcessRequest(HttpContext context)
    {
      context.Response.ContentType = "application/json";
      context.Response.ContentEncoding = Encoding.UTF8;

      Stream inputStream = context.Request.InputStream;
      Encoding encoding = context.Request.ContentEncoding;
      StreamReader streamReader = new StreamReader(inputStream, encoding);

      string strJson = streamReader.ReadToEnd();

      Param p = JsonConvert.DeserializeObject<Param>(strJson);

      int top = Convert.ToInt32(p.Top);
      string term = p.Term;

      
      //var result = ...
      //context.Response.Write(result.ToJson());
    }

Source Code

$.ajax({
           url: '<%= ResolveUrl("~/Handlers/xxx.ashx") %>',
              dataType: "json",
              type: "POST",
              contentType: "application/json; charset=utf-8",
              data: JSON.stringify({
                top: 10,
                term: request.term
              }),
              success: function (data) {
                //...
              }
            });

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

/// <summary>
/// Summary description for Param
/// </summary>
namespace Insus.NET
{
  public class Param
  {
    private int _top;
    public int Top
    {
      get { return _top; }
      set { _top = value; }
    }


    private string _term;

    public string Term
    {
      get { return _term; }
      set { _term = value; }
    }
  }
}