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;

      }