上一篇中我们可以看出Fill方法最后都调用FillFromCommand和FillFromReader方法。
那我们接着探讨FillFromCommand方法
private int FillFromCommand(object data, int startRecord, int maxRecords, string srcTable, IDbCommand command, CommandBehavior behavior)
{
      IDbConnection connection1 = DbDataAdapter.GetConnection(command, "Fill");//返回connection
      ConnectionState state1 = ConnectionState.Open;
      if (MissingSchemaAction.AddWithKey == base.MissingSchemaAction)

    //添加必需的列和主键信息以完成架构。

      {
            behavior |= CommandBehavior.KeyInfo;
      }
      int num1 = 0;
      try
      {
            try
            {
                  DbDataAdapter.QuietOpen(connection1, out state1);//打开连接
                  using (IDataReader reader1 = command.ExecuteReader(behavior | CommandBehavior.SequentialAccess))//读取数据
                  {
                        if (data is DataTable)
                        {
                              num1 = this.Fill((DataTable) data, reader1);//这个方法我们已经

    //说过它最后调用FillFromReader(dataTable, null, dataReader, 0, 0, null, null);

                        }
                        else
                        {
                              num1 = this.Fill((DataSet) data, srcTable, reader1, startRecord, maxRecords);

//这个方法最后调用  return this.FillFromReader(dataSet, srcTable, dataReader,

//startRecord, maxRecords, null, null);

                        }
                  }
            }
            finally
            {
                  DbDataAdapter.QuietClose(connection1, state1);//关闭连接
            }
      }
      catch
      {
            throw;
      }
      return num1;
}
这是就不难看出FillFromCommand其实也是在调用了FillFromReader，也就是说Fill方法的所有操作将由FillFromReader方法来完成。

看看FillFromReader方法：
internal int FillFromReader(object data, string srcTable, IDataReader dataReader, int startRecord, int maxRecords, DataColumn parentChapterColumn, object parentChapterValue)
{
      int num1 = 0;
      int num2 = 0;//schemaCount
Label_0004:
      if (0 < dataReader.FieldCount)
      {
            SchemaMapping mapping1 = this.FillSchemaMappingTry(data, srcTable, dataReader, num2, parentChapterColumn, parentChapterValue);//创建结构映射
            num2++;
            if (((mapping1 != null) && (mapping1.DataValues != null)) && (mapping1.DataTable != null))
            {
                  try
                  {
                        mapping1.DataTable.BeginLoadData();//加载数据时关闭通知、索引维护和约束
                        try
                        {
                              if ((1 == num2) && ((0 < startRecord) || (0 < maxRecords)))
                              {
                                    num1 = this.FillLoadDataRowChunk(mapping1, startRecord, maxRecords);//填充从

//startRecord开始的maxRecords条记录，并返回记录个数
                              }
                              else
                              {
                                    int num3 = this.FillLoadDataRow(mapping1);//填充数据
                                    if (1 == num2)
                                    {

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