Regarding strongly typed dataset in windows form application

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using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace GST.Masters
{
    public partial class Customers : Heptanesia.Winforms.Ui.Forms.MainUC
    {

        public static string recCode
        { get; set; }
        public string RC;
        public bool insertFlag;
        public Data.GSTDb.CustomersRow r;
        public Data.GSTDb.CustomersGSTNosRow gr;
        public Data.GSTDb.CustomersDataTable dt = new Data.GSTDb.CustomersDataTable();

        public Customers()
        {
            InitializeComponent();
            recCode = Guid.NewGuid().ToString();
            this.Load += new EventHandler(this.Customers_Load);
            this.Ts.SaveClicked += new Heptanesia.Winforms.Ui.Menu.ToolStrip.SaveClickedEventHandler(this.Ts_SaveClicked);
            this.Ts.AddNewClicked += new Heptanesia.Winforms.Ui.Menu.ToolStrip.AddNewClickedEventHandler(this.Ts_AddNewClicked);
            this.Ts.SearchClicked += new Heptanesia.Winforms.Ui.Menu.ToolStrip.SearchClickedEventHandler(this.Ts_SearchClicked);
            this.DgGST.CellValidated += (sender, e) =>
            {
                string name = this.DgGST.Columns[e.ColumnIndex].Name;
                if (name == "DGCCountryCode")
                {
                    name = this.DgGST.Rows[e.RowIndex].Cells[name].Value.ToString();
                    this.statesBindingSource.Filter = "ParentCode = '" + name + "'";
                    if (!this.FormIsLoading)
                    {
                        this.DgGST.Rows[e.RowIndex].Cells["DGCStateCode"].Value = null;
                        this.DgGST.Rows[e.RowIndex].Cells["DGCCityCode"].Value = null;
                    }
                }
                else if (name == "DGCStateCode")
                {
                    name = this.DgGST.Rows[e.RowIndex].Cells[name].Value.ToString();
                    this.citiesBindingSource.Filter = "ParentCode = '" + name + "'";
                    if (!this.FormIsLoading)
                    {
                        this.DgGST.Rows[e.RowIndex].Cells["DGCCityCode"].Value = null;
                    }
                }
            };
        }

        private void Ts_AddNewClicked(object sender, EventArgs e)
        {
            this.gSTDb.Customers.Rows.Clear();
            r = this.gSTDb.Customers.NewCustomersRow();
            this.gSTDb.Customers.AddCustomersRow(r);
            Data.GSTDbTableAdapters.CustomersGSTNosTableAdapter CGT = new Data.GSTDbTableAdapters.CustomersGSTNosTableAdapter();
            CGT.FillByParentCode(gSTDb.CustomersGSTNos, r.RecCode);
        }

        private void Ts_SearchClicked(object sender, EventArgs e)
        {
            Heptanesia.Winforms.Ui.Forms.SearchForm sf = new Heptanesia.Winforms.Ui.Forms.SearchForm();
            sf.DataSource = new Data.GSTDbTableAdapters.CustomersTableAdapter().GetData();
            sf.DisplayColumns = new string[] { "Name" };
            sf.DisplayHeaders = new string[] { "Customers Name" };
            sf.DisplayWidths = new int[] { 200 };
            if (sf.ShowDialog(this) == DialogResult.OK)
            {
                this.RC = sf.ReturnRow["RecCode"].ToString();
                this.customersTableAdapter.FillByRecCode(this.gSTDb.Customers, RC);
            }
        }

        private void Ts_SaveClicked(object sender, EventArgs e)
        {
            string message = "Error: ";
            try
            {
                this.customersBindingSource.EndEdit();
                //if (this.DgGST.CurrentRow.DataBoundItem != null)
                this.fkCustomerGSTNosCustomersBindingSource.EndEdit();
                if (this.gSTDb.Customers.HasErrors)
                    message += Classes.Global.SetBindingSourcePositionAndGetError(this.customersBindingSource, this.gSTDb.Customers.GetErrors()[0]);
                else if (this.gSTDb.CustomersGSTNos.HasErrors)
                    message += Classes.Global.SetBindingSourcePositionAndGetError(this.fkCustomerGSTNosCustomersBindingSource, this.gSTDb.CustomersGSTNos.GetErrors()[0]);
                else
                {
                    this.gSTDb.Customers.BeforeSave();
                    this.gSTDb.CustomersGSTNos.BeforeSave();
                    Data.GSTDbTableAdapters.TableAdapterManager tm = new Data.GSTDbTableAdapters.TableAdapterManager();
                    tm.CustomersTableAdapter = this.customersTableAdapter;
                    tm.CustomersGSTNosTableAdapter = this.customersGSTNosTableAdapter;
                    tm.UpdateAll(this.gSTDb);
                    message = "Record(s) Saved Successfully";
                }
            }
            catch (Exception ex)
            {
                message += ex.Message;
            }
            MessageBox.Show(message, "Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
        }


        private void Customers_Load(object sender, EventArgs e)
        {
            this.FormIsLoading = true;
            this.customersTableAdapter.Fill(this.gSTDb.Customers);
            this.customersGSTNosTableAdapter.Fill(this.gSTDb.CustomersGSTNos);
            this.countriesTableAdapter.Fill(this.gSTDb.Countries);
            this.statesTableAdapter.Fill(this.gSTDb.States);
            this.citiesTableAdapter.Fill(this.gSTDb.Cities);
            this.gSTDb.Customers.Rows.Clear();
            if (this.gSTDb.Customers.Rows.Count == 0)
            {
                Data.GSTDb.CustomersRow r = this.gSTDb.Customers.NewCustomersRow();
                this.gSTDb.Customers.AddCustomersRow(r);
            }
            this.FormIsLoading = false;
        }


        private void DgGST_DataError(object sender, DataGridViewDataErrorEventArgs e)
        {
            if (e.Exception is ArgumentException)
            {
                object value = DgGST.Rows[e.RowIndex].Cells[e.ColumnIndex].Value;
                if (!((DataGridViewComboBoxColumn)DgGST.Columns[e.ColumnIndex]).Items.Contains(value))
                {
                    ((DataGridViewComboBoxCell)DgGST[e.ColumnIndex, e.RowIndex]).Value = DBNull.Value;
                    //((DataGridViewComboBoxColumn)dgv.Columns[e.ColumnIndex]).Items.Add(value);
                    e.ThrowException = false;
                }
            }
            else
            {
                MessageBox.Show(e.Exception.Message);
                e.Cancel = true;
            }
        }
    }
}

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Quote:

The main issue here is that it is not clearing the rows??

Use the debugger to see what is done, then you will know what is missing or go wrong, it is the first step to correction.

There is a tool that allow you to see what your code is doing, its name is debugger. It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]
Debugging C# Code in Visual Studio - YouTube[^]
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.