asp.net - Building AJAX Responses -

i building unhandled exception handler ajax callbacks ran across parameter can not understand is. through fiddler noticed responses sent asp.net built so:

100|[__eventvalidation][__callbackparam] 

i know how read __eventvalidation , __callbackparam, 100| , being read from? if don't include in responses don't work. here code:

//check if current request ajax 1 if (context.request["__callbackid"] != null) {     server.clearerror();      response.statuscode = 200;     response.contenttype = "application/html";      string response = "100|" + context.request["__eventvalidation"] + "there unhandled error.";      response.write(response); } 

it works this, want read 100| server if possible , not hardcode it.

thanks.

edit: here typical response actual data:

100|/wedaamnrqqzxzxxcef8iaqo1ebpves4afnrewzq25wfmnghn3sz7wk+fyprn1wwdwtmoysytayzdybmykophfidquojfbjrrq==success 

it seems me length of response body / first parameter.