#region Join
    /// <summary>
    /// 连接表
    /// </summary>
    /// <param name="dt">将要被连接的表</param>
    /// <param name="col">主键字段</param>
    /// <param name="dt2">带有主键的表</param>
    /// <param name="insertAt">插入到的位置，-1不需要插入</param>
    /// <param name="cols">连接的字段</param>
    public static DataTable Join(this DataTable dt, string col, DataTable dt2, int insertAt, params string[] cols) {
        foreach (var it in cols) {
            var c = dt.Columns.Add(it, dt2.Columns[it].DataType);
            if (insertAt >= 0) c.SetOrdinal(insertAt++);
        }
        foreach (DataRow row in dt.Rows) {
            var row2 = dt2.Find(row[col]);
            if (row2 == null) continue;
            cols.For(it => row[it] = row2[it]);
        }
        return dt;
    }
    #endregion