c# - How to get alert message with pop-up? -

here when select same date , same batch should give me alert message pop-up. tried try catch method it's not working.tell me went wrong.

my code far

using (sqlconnection con2 = new sqlconnection(configurationmanager.connectionstrings["connectionstring"].connectionstring))         {             var fromdate = datetime.parse(txtfromdate.text);             var todate = datetime.parse(txttodate.text);             var datedif1 = (todate - fromdate).days;              var sqlinsert = new sqlcommand("insert datelist ([datedif],[batch],[daywk],[semester],[weekbatch],[subject],[facultyname],[facultyid],[weekmonth])  select * (select @datedif datedif, @batch batch,@daywk daywk,@semester semster,@weekbatch weekbatch,@subject subject,@facultyname facultyname,@facultyid facultyid,@weekmonth weekmonth) temp not exists (select datedif,batch datelist batch=@batch , datedif=@datedif)", con2);              var sqlparamater = sqlinsert.parameters.add("@datedif", sqldbtype.date);             var sqlparameter1 = sqlinsert.parameters.add("@batch", sqldbtype.nvarchar);             var sqlparameter2 = sqlinsert.parameters.add("@daywk", sqldbtype.nvarchar);             var sqlparameter3 = sqlinsert.parameters.add("@semester", sqldbtype.nvarchar);             var sqlparameter4 = sqlinsert.parameters.add("@weekbatch", sqldbtype.nvarchar);             var sqlparameter5 = sqlinsert.parameters.add("@subject", sqldbtype.nvarchar);             var sqlparameter6 = sqlinsert.parameters.add("@facultyname", sqldbtype.nvarchar);             var sqlparameter7 = sqlinsert.parameters.add("@facultyid", sqldbtype.nvarchar);             var sqlparameter8 = sqlinsert.parameters.add("@weekmonth", sqldbtype.nvarchar);              con2.open();            try             {                 (var = 0; <= datedif1; i++)                 {                     var consecutivedate = fromdate.adddays(i);                      sqlparamater.value  = consecutivedate;                     sqlparameter1.value = batch1;                     sqlparameter2.value = dayweek;                     sqlparameter3.value = semester;                     sqlparameter4.value = weekbatch;                     sqlparameter5.value = subject;                     sqlparameter6.value = faculty;                     sqlparameter7.value = facultyid;                     sqlparameter8.value = weekmonth;                      int s = sqlinsert.executenonquery();                 }              }              catch             {                 response.write("<script>alert('already date selected.select date')</script>");                 con2.close();            }         }     } 

1 - please first check going inside catch block put debugger there.
2 - put debugger on page load check when happening.

you can use registerstartupscript() method
at server side

 catch         { string msg="already date selected.select date"; scriptmanager.registerstartupscript(this, gettype(), "myfunction",      "myfunction('"+msg+"');", true);   } 

at client side

<script>         function myfunction(a) {              alert(a);                 }     </script> 

also better make store procedure.