DataTable dt = new DataTable
dt.Columns.Add(new System.Data.DataColumn("EMPNAME", typeof(String)));
dt.Columns.Add(new System.Data.DataColumn("EMPID", typeof(String)));
dt.Columns.Add(new System.Data.DataColumn("DEPARTMENT", typeof(String)));
Session["Data"] = dt;
int C = 0;
DataTable tmp = CType(Session["Data"], DataTable);
DataRow dr = tmp.NewRow;
dr["EMPNAME"] = Txtaddname.Text;
dr["EMPID"] = Txtaddempid.Text;
dr["DEPARTMENT"] = Drpadddept.SelectedItem.Text;
gvEmployees.DataSource = tmp;
gvEmployees.DataBind();
Session["Data"] = tmp;
Alternative to this, you could use DataTable.Merge method to add your single row table (the way you're doing it) to gridview datasource.