DataTable dt = (dataGridView1.DataSource as DataTable);

如未绑定过数据源：

 public DataTable GetDgvToTable(DataGridView dgv)
        {
            DataTable dt = new DataTable();

            // 列强制转换
            for (int count = 0; count < dgv.Columns.Count; count++)
            {
                DataColumn dc = new DataColumn(dgv.Columns[count].Name.ToString());
                dt.Columns.Add(dc);
            }

            // 循环行
            for (int count = 0; count < dgv.Rows.Count; count++)
            {
                DataRow dr = dt.NewRow();
                for (int countsub = 0; countsub < dgv.Columns.Count; countsub++)
                {
                    dr[countsub] = Convert.ToString(dgv.Rows[count].Cells[countsub].Value);
                }
                dt.Rows.Add(dr);
            }
            return dt;
        }