I solved like this

  if(myGridView.Rows.Count > 0)
            {
                var dt= new DataTable();
               
dt.Columns.Add("Column1", typeof(string));
               
dt.Columns.Add("Column2", typeof(Int64));
               
dt.Columns.Add("Column3", typeof(string));
               
foreach (GridViewRow row in gd_endYearSchool.Rows)
                {
                   
var id = row.Cells[1].Text;
         //for find textbox
          var tb1 =row.Cells[2].FindControl("tbNr") as TextBox;
                   
              int nrord = 0;
                    if(tb1 != null)
                    {
                       
             var ord = tb1.Text;
                       
if (!Int64.TryParse(ord, out nrord))                       
{                           
       nrord = 0;
                       
}
                    }
                   
var text=row.Cell[3].text;
                   
dt.Rows.Add(id,nrord,text);
                }
            }