Json - Get The JsonResult In Function?

How can I get the result of a method that return a JsonResult and cast into string.

[HttpPost]
public JsonResult AnalyseNbPayment(DateTime dt, DateTime dt2,int FrequencyID) {
if (FrequencyID == ApplConfig.Frequency.WEEKLY) {
return Json(new { val = GetNbWeek(dt, dt2) });
}
else if (FrequencyID == ApplConfig.Frequency.MONTHLY) {
return Json(new { val =GetDateDiffInMonth(dt, dt2) });
}
else if (FrequencyID == ApplConfig.Frequency.QUARTELY) {
return Json(new { val = GetQuarterLy(dt, dt2) });
}
else if (FrequencyID == ApplConfig.Frequency.BI_MONTLY) {
return Json(new { val = GetBiMontlhy(dt, dt2) });
}
else if(FrequencyID == ApplConfig.Frequency.YEARLY)
{
return Json(new { val = GetNbYear(dt, dt2) });
}
return Json(new { val =0 });
}

I want to call my method like this

string MyValue = AnalyseNbPayment(Convert.ToDateTime(ViewModel.oRent.DateFrom), Convert.ToDateTime(ViewModel.oRent.DateTo), Convert.ToInt32(oLease.FrequencyID)).val.ToString(); <br />