public class Handler : IHttpHandler, IRequiresSessionState

string method = Request["METHAD"];//namespace.class.method//包名.类名.方法名
string ClassNamespace = method.Substring(0, method.LastIndexOf("."));// "</SPAN>< style="FONT-FAMILY: 宋体; COLOR: green; FONT-SIZE: 9pt" SPAN >?;< SPAN>
object objType = CreateObjectNoCache(Request["DLL"], ClassNamespace);//反射
Type ht = objType.GetType();
List<string> likey = new List<string>();
foreach (string item in Request.Params.AllKeys)
{
    if (item.StartsWith("[__DOTNET__]"))
        likey.Add(item);
}
int length = likey.Count;//Request.Form.AllKeys.Length;
//int length=Request
//反射方法参数,和类型
object[] obj = new object[length];// 数据
Type[] objTypes = new Type[length];//类型

for (int i = 0; i < length; i++)
{
    string ReqKey = likey[i].Substring(12);//Request.Form.AllKeys[i];
    Type tType = null;
    if (ReqKey.IndexOf("[") != -1)
    {
        tType = Type.GetType(ReqKey.Remove(ReqKey.LastIndexOf("[")), true, true);
    }
    else
    {
        tType = Type.GetType(ReqKey, true, true);
    }
    objTypes[i] = tType;//类型赋值
    //如果它是值类型,或者String
    if (tType.Equals(typeof(string)) || tType.Equals(typeof(string[])) || (!tType.IsInterface && !tType.IsClass))
    {
        obj[i] = Convert.ChangeType(Request.Params[likey[i]], tType);
    }
    else if (tType.IsClass)//如果是类,将它的json字符串转换成对象
    {
        obj[i] = JavaScriptConvert.DeserializeObject(Request.Params[likey[i]], tType); //Serialize.Deserialize(Request.Form[i], tType);
    }
}
//TODO
MethodInfo methodInfo = ht.GetMethod(method.Substring(method.LastIndexOf(".") + 1), objTypes);
object returnValue = null;
foreach (object attribute in methodInfo.GetCustomAttributes(true))
{
    if (attribute is AjaxMethod)
    {
        returnValue = methodInfo.Invoke(objType, obj);
        break;
    }
}
//执行方法
//object returnValue = methodInfo.Invoke(objType, obj);
if (returnValue == null)
    throw new Exception("你没用权限执行该方法，是否加了[XAjaxMethod]自定义属性");

#region 处理返回值
if (returnValue.GetType().IsClass && !returnValue.GetType().Equals(typeof(string)))
{
    if (returnValue.GetType().Equals(typeof(XmlDocument)))
    {
        //如果是XML文档,则写入xml文档
        Response.ContentType = "text/xml";
        Response.Write(((XmlDocument)returnValue).OuterXml);
    }
    else if (returnValue.GetType().IsClass && returnValue.GetType().Equals(typeof(DataTable)))
    {
        if (!string.IsNullOrEmpty(Request["callback"]) && Request["callback"].StartsWith("jsonp"))
        {
            Response.ContentType = "application/json";
            Response.ContentEncoding = Encoding.Default;
            Response.Write(Request["callback"] + "(" + Json.DataTableToJson("Rows", (DataTable)returnValue) + ")");
        }
        else
            Response.Write(Json.DataTableToJson("Rows", (DataTable)returnValue));
    }
    else
    {
        if (!string.IsNullOrEmpty(Request["callback"]) && Request["callback"].StartsWith("jsonp"))
        {
            Response.ContentType = "application/json";
            Response.ContentEncoding = Encoding.Default;
            Response.Write(Request["callback"] + "(" + JavaScriptConvert.SerializeObject(returnValue) + ")");
        }
        else
            Response.Write(JavaScriptConvert.SerializeObject(returnValue));
    }
}
else
{
    if (!string.IsNullOrEmpty(Request["callback"]) && Request["callback"].StartsWith("jsonp"))
    {
        Response.ContentType = "application/json";
        Response.ContentEncoding = Encoding.Default;
        Response.Write(Request["callback"] + "(\"" + returnValue.ToString() + "\")");
    }
    else
        Response.Write(returnValue.ToString());
}
#endregion

public class AjaxMethod : Attribute
{
    public override string ToString()
    {
        return "XAjaxMethod";
    }
}

<httpHandlers>
    <add path="DotNet2.ashx" verb="*" type="AjaxHandler.Handler,AjaxHandler"/>
</httpHandlers>

$.ajax({
    url: "/DotNet2.ashx?METHAD=AjaxTest._Default.Test&DLL=AjaxTest",
    data: {
        "[__DOTNET__]System.String": "测试测试一"
    },
    success: function(json) {
        alert(json);
    }
});

METHAD:命名空间.类名.方法名
DLL:DLL名称

data: {
    "[__DOTNET__]System.String": "测试测试二",
    "[__DOTNET__]System.Int32": "12345678"
}
需要加上[__DOTNET__]前缀，后面是参数类型，前缀是为了处理get方式

AjaxHendler.rar