Please try with the below code snippet.
private void DefineGridStructure()
{
DataTable current = (DataTable)Session["GridData"];
RadGrid grid = new RadGrid();
grid.ID = "RadGrid123";
grid.DataSource = current;
if (!IsPostBack)
{
grid.AutoGenerateEditColumn = true;
grid.AutoGenerateDeleteColumn = true;
}
grid.Width = Unit.Percentage(100);
grid.PageSize = 15;
grid.AllowPaging = true;
grid.PagerStyle.Mode = GridPagerMode.NextPrevAndNumeric;
grid.AutoGenerateColumns = false;
grid.MasterTableView.Width = Unit.Percentage(100);
grid.MasterTableView.CommandItemDisplay = GridCommandItemDisplay.TopAndBottom;
grid.MasterTableView.DataKeyNames = new string[] { "RowNumber" };
GridBoundColumn boundColumn = new GridBoundColumn();
boundColumn.DataField = "RowNumber";
boundColumn.HeaderText = "RowNumber";
grid.MasterTableView.Columns.Add(boundColumn);
boundColumn = new GridBoundColumn();
boundColumn.DataField = "Size";
boundColumn.HeaderText = "Size";
grid.MasterTableView.Columns.Add(boundColumn);
boundColumn = new GridBoundColumn();
boundColumn.DataField = "Description";
boundColumn.HeaderText = "Description";
grid.MasterTableView.Columns.Add(boundColumn);
boundColumn = new GridBoundColumn();
boundColumn.DataField = "Quantity";
boundColumn.HeaderText = "Quantity";
grid.MasterTableView.Columns.Add(boundColumn);
boundColumn = new GridBoundColumn();
boundColumn.DataField = "Duration";
boundColumn.HeaderText = "Duration";
grid.MasterTableView.Columns.Add(boundColumn);
boundColumn = new GridBoundColumn();
boundColumn.DataField = "DurationType";
boundColumn.HeaderText = "DurationType";
grid.MasterTableView.Columns.Add(boundColumn);
boundColumn = new GridBoundColumn();
boundColumn.DataField = "Amount";
boundColumn.HeaderText = "Amount";
grid.MasterTableView.Columns.Add(boundColumn);
grid.DataBind();
PlaceHolder1.Controls.Add(grid);
}