Error message for data duplication in asp.net

I have tried this below code but it show unique key constraint error but i want it as a pop-up message see my below code.please help me out from these problem

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 INTO datelist ([datedif],[batch],[daywk],[semester],[weekbatch],[subject],[facultyname],[facultyid],[WeekMonth])  VALUES  (@datedif,@batch,@daywk,@semester,@weekbatch,@subject,@facultyname,@facultyid,@weekMonth)", 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 { 
            for (var i = 0; i <= 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();
               }
            con2.Close();
                }
            catch(ConstraintException ex)
            {
                throw new ApplicationException("data are duplicated");
            }
        }
    }

And these is my screen shot of error which am getting.look at these