程序中实现两个DataTable的Left Join效果(修改了，网上第二个DataTable为空，所处的异常)

      public static DataTable Join(DataTable First, DataTable Second, DataColumn[] FJC, DataColumn[] SJC)
      {
          DataTable table = new DataTable("Join");
          using (DataSet ds = new DataSet())
          {
              ds.Tables.AddRange(new DataTable[] { First.Copy(), Second.Copy() });
              DataColumn[] First_columns = new DataColumn[FJC.Length];
              for (int i = 0; i < First_columns.Length; i++)
              {
                  First_columns[i] = ds.Tables[0].Columns[FJC[i].ColumnName];
              }
              DataColumn[] Second_columns = new DataColumn[SJC.Length];
              for (int i = 0; i < Second_columns.Length; i++)
              {
                  Second_columns[i] = ds.Tables[1].Columns[SJC[i].ColumnName];
              }
              DataRelation r = new DataRelation(string.Empty, First_columns, Second_columns, false);
              ds.Relations.Add(r);

              for (int i = 0; i < First.Columns.Count; i++)
              {
                  table.Columns.Add(First.Columns[i].ColumnName, First.Columns[i].DataType);
              }

              for (int i = 0; i < Second.Columns.Count; i++)
              {

                  //看看有没有重复的列，如果有在第二个DataTable的Column的列明后加_Second
                  if (!table.Columns.Contains(Second.Columns[i].ColumnName))
                      table.Columns.Add(Second.Columns[i].ColumnName, Second.Columns[i].DataType);
                  else
                      table.Columns.Add(Second.Columns[i].ColumnName + "_1", Second.Columns[i].DataType);
              }
              table.BeginLoadData();

              int itable2Colomns = 0;
              if (ds.Tables[1].Rows.Count > 0) {
                  itable2Colomns = ds.Tables[1].Rows[0].ItemArray.Length;
              }
                  
              foreach (DataRow firstrow in ds.Tables[0].Rows)
              {
                  //得到行的数据
                  DataRow[] childrows = firstrow.GetChildRows(r);//第二个表关联的行
                  if (childrows != null && childrows.Length > 0)
                  {
                      object[] parentarray = firstrow.ItemArray;
                      foreach (DataRow secondrow in childrows)
                      {
                          object[] secondarray = secondrow.ItemArray;
                          object[] joinarray = new object[parentarray.Length + secondarray.Length];
                          Array.Copy(parentarray, 0, joinarray, 0, parentarray.Length);
                          Array.Copy(secondarray, 0, joinarray, parentarray.Length, secondarray.Length);
                          table.LoadDataRow(joinarray, true);
                      }
                  }
                  else//如果有外连接(Left Join)添加这部分代码
                  {
                      object[] table1array = firstrow.ItemArray;//Table1
                      object[] table2array = new object[itable2Colomns];
                      object[] joinarray = new object[table1array.Length + itable2Colomns];
                      Array.Copy(table1array, 0, joinarray, 0, table1array.Length);
                      Array.Copy(table2array, 0, joinarray, table1array.Length, itable2Colomns);
                      table.LoadDataRow(joinarray, true);
                      DataColumn[] dc = new DataColumn[2];
                      dc[0] = new DataColumn("");
                  }
              }
              table.EndLoadData();
          }
          return table;
      }