Question

I need to send an error message from controller, in case that,an error happens in ajax function.
Here is my Controller code:

//check if the day is already approved
string check1 = "SELECT approve_status_id FROM table_approvals WHERE approve_date=" + date + " AND user_id=" + Session["userID"];
MySqlCommand status_cmd = new MySqlCommand(check1, con);
MySqlDataReader rdrStatus = status_cmd.ExecuteReader();
while (rdrStatus.Read())
{
    if (rdrStatus.GetString(0) == "5")
    {
        valid = false;
        error = "You cannot edit already approved dates!";
        response.Message = error;
     return View(Json(new { success = false,error},JsonRequestBehavior.AllowGet));
    }
}

my ajax function:

$.ajax({
    url: '@Url.Action("SaveLine", "AddLine")',
    type: 'post',
    data: { ids: IDs },
    dataType: 'json',
    traditional: true,
    success: function () {
        window.location.href = '@Url.Action("Index", "AddLine")';
    },
    error: function () {
        $.getJSON('/AddLine/SaveLine', function (json) {
            alert('error');
            $("#ajaxPostMessage").html(json.Message);
        });

Answer 1

Wrap everything in try catch. You can simple call the JSON method which will create a JSON response and return that to the client code. Also , you do not need JsonRequestBehavior.AllowGet to return JSON when it is a HttpPost Action method. You need that for GET only.($.getJSON, $.get ..)

try
{
   //code for getting data to reader    
     while (rdrStatus.Read())
     {
        if (rdrStatus.GetString(0) == "5")
        {
          valid = false;
          var errorMsg = "You cannot edit already approved dates!";             
          return Json( new { success = false,Error=errorMsg });           
        }
     }
 }
 catch(Exception ex)
 {
   //log Error
     return Json(new { success = false,Error="Some Error!"});
 }

This will return a JSON like this to the client

{  "success ": "True",    "Error": "SomeError"  }

Now in your client code, you can check the JSON And do whatever you want.

success: function (data) {
    if(data.success=="True")
    {
       window.location.href = '@Url.Action("Index", "AddLine")';
    }
    else
    {
      alert(data.Error);
    }
},