How to Add, Delete and Update the SQL Database through Gantt?

[ASPX] 
<form id="form1" runat="server" onsubmit="return false">
//…
<asp:SqlDataSource runat="server" ID="SqlDataSource1" ConnectionString='<%$ ConnectionStrings:ConnectionString %>' SelectCommand="SELECT * FROM [GanttData]"></asp:SqlDataSource>
<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="True" />
</form>
<script type="text/javascript">
        function ActionComplete(args) {
            var ganttRecord = args.data;
            if (args.requestType === 'save' && args._cAddedRecord) {
                PageMethods.AddIt(ganttRecord);
            }            
           else if (args.requestType === 'save') {
                ganttRecord.ParentId = args._cModifiedData.item.ParentId;
                PageMethods.UpdateIt(ganttRecord);
            }            
           else if (args.requestType === 'delete') {
                var children = ganttRecord.childRecords;
                PageMethods.DeleteIt(ganttRecord.taskId);
                if (ganttRecord.hasChildRecords) {
                    for (var i = 0; i < children.length; i++)
                        PageMethods.DeleteIt(children[i].taskId);
                }
            }
        }
    </script>

[ASPX.CS]
   public partial class GanttSamples : System.Web.UI.Page
    {
        string connectionString;
        protected void Page_Load(object sender, EventArgs e)
        {
        }
        public void Update(TaskData Task)
        {
            //Update Edited TaskDetails
            //Using UPDATE SQL Query to Save the Modified taskData into the Table
            int IDNumber = Task.TaskId;
            string cmdString = "UPDATE GanttData SET TaskId=@TaskId, TaskName=@TaskName,StartDate=@StartDate,EndDate=@EndDate,Duration=@Duration,Progress=@Progress,ParentId=@ParentId WHERE TaskId = '" + IDNumber + "'";
            SqlConnection con = new SqlConnection(WebConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);//connectionString
            con.Open();
            using (SqlCommand sqlCommand = new SqlCommand(cmdString, con))
            {
                sqlCommand.Parameters.AddWithValue("@TaskName", Task.TaskName);
                sqlCommand.Parameters.AddWithValue("@Duration", Task.Duration);
                sqlCommand.Parameters.AddWithValue("@StartDate", Task.StartDate);
                sqlCommand.Parameters.AddWithValue("@TaskId", Task.TaskId);
                sqlCommand.Parameters.AddWithValue("@EndDate", Task.EndDate);
                sqlCommand.Parameters.AddWithValue("@Progress", Task.Progress);
                sqlCommand.Parameters.AddWithValue("@ParentId", Task.ParentId);
                sqlCommand.ExecuteNonQuery();
            }
            con.Close();
        }
        public void Add(TaskData Task)
        {
            //Here Task is the added New Task
            //Add Task 
            //Using INSERT Query to add the New Record to SQL Table
            string cmdString = "INSERT INTO GanttData ([TaskName],[TaskId],[StartDate],[EndDate],[Duration],[Progress],[ParentId])" + "VALUES(@TaskName,@TaskId,@StartDate,@EndDate,@Duration,@Progress,@ParentId)";
            SqlConnection con = new SqlConnection(WebConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);//connectionString
            con.Open();
            using (SqlCommand sqlCommand = new SqlCommand(cmdString, con))
            {
                sqlCommand.Parameters.AddWithValue("@TaskName", Task.TaskName);
                sqlCommand.Parameters.AddWithValue("@Duration", Task.Duration);
                sqlCommand.Parameters.AddWithValue("@StartDate", Task.StartDate);
                sqlCommand.Parameters.AddWithValue("@TaskId", Task.TaskId);
                sqlCommand.Parameters.AddWithValue("@EndDate", Task.EndDate);
                sqlCommand.Parameters.AddWithValue("@Progress", Task.Progress);
                if (Task.ParentId == 0)
                {
                    sqlCommand.Parameters.AddWithValue("@ParentId", Task.ParentId).Value = "1";
                }
                else
                {
                    sqlCommand.Parameters.AddWithValue("@ParentId", Task.ParentId);
                }
                int test = sqlCommand.ExecuteNonQuery();
            }
            con.Close();
        }
        [WebMethod]
         public void Delete(int taskId)
         {
             //Delete Task 
             //Using Delete Query to delete Record from SQL Table
             //TaskData._TaskId = taskId;
             int IDNumber = taskId;
             SqlConnection con = new SqlConnection(WebConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);//connectionString
             con.Open();
             SqlCommand cmd = new SqlCommand("delete from GanttData where TaskId = '" + IDNumber + "'", con);
             //int result = cmd.ExecuteNonQuery();
             SqlDataAdapter adapter = new SqlDataAdapter();     
             adapter.DeleteCommand = con.CreateCommand();
             adapter.DeleteCommand = cmd;
             adapter.DeleteCommand.ExecuteNonQuery();        
             con.Close();
            // HttpContext.Current.Response.Redirect("~/Default.aspx");
         }
        [WebMethod]
        public static void AddIt(TaskData record)
        {
            GanttSamples sample = new GanttSamples();
            sample.Add(record);
        }
        [WebMethod]
        public static void UpdateIt(TaskData record)
        {
            GanttSamples sample1 = new GanttSamples();
            sample1.Update(record);
        }
        [WebMethod]
        public static int DeleteIt(int Id)
        {
            GanttSamples sample2 = new GanttSamples();
            sample2.Delete(Id);
            return 0;
        }
    }