月份：2004年7月

在CSDN论坛上看到下列问题，如果把DataReader当作DataGrid的DataSource，在DataGrid.DataBind()之后，在下列两种情形下，DataReader的IsClosed的值是不同的

1.
IDataReader reader = SomeDBCommand.ExecuteReader();
DataGrid1.DataSource = reader;
DataGrid1.DataBind();

在这里，reader的IsClosed的值是false

2.
IDataReader reader = SomeDBCommand.ExecuteReader(CommandBehavior.CloseConnection);
DataGrid1.DataSource = reader;
DataGrid1.DataBind();

在这里，reader的IsClosed的值是true

结论是，如果DataReader的CommandBehavior是CloseConnection的话，DataGrid1.DataBind()在某种(也许是非直接的)方式下，调用了DataReader的Close方法。在网上找到了一个类似的讨论，但他们没有给出原因。

把DataReader作为DataGrid的DataSource时，DataReader是当作IEnumerable来看待的，从IEnumerable可以获取IEnumerator，然后可以用它的MoveNext/Current来依次获取每个记录，难道MoveNext调用了DataReader的Close方法？做了个试验

using System;
using System.Data;
using System.Data.OleDb;
using System.Data.SqlClient;
using System.Collections;

class TestClose
{
  static void Main()
  {
 SqlConnection conn = new SqlConnection(“server=localhost;database=pubs;uid=sa;pwd=;”);
 SqlCommand cmd = new SqlCommand(“select * from authors”, conn);

 conn.Open();
 SqlDataReader reader = cmd.ExecuteReader(CommandBehavior.CloseConnection);
 TestRead(reader);
 reader.Close();

 conn.Open();
 reader = cmd.ExecuteReader(CommandBehavior.CloseConnection);
 TestMoveNext(reader);
 reader.Close();

   }

  static void TestRead(IDataReader reader)
  {
 while (reader.Read());
 Console.WriteLine(“after Read(), reader is closed? {0}”, reader.IsClosed);
  }

  static void TestMoveNext(IDataReader reader)
  {
 IEnumerator e = ((IEnumerable)reader).GetEnumerator();
 while (e.MoveNext());
 Console.WriteLine(“after MoveNext(), reader is closed?{0}”, reader.IsClosed);
  }

}

输出是
after Read(), reader is closed? False
after MoveNext(), reader is closed? True

果不其然啊，用Lutz Roeder的Reflector(大力推荐！)工具看了一下.NET里SqlDataReader的实现

IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
      return new DbEnumerator(this, (CommandBehavior.Default != (CommandBehavior.CloseConnection & this._behavior)));
}
 

可见SqlDataReader的CommandBehavior确实是传进去的，再看DbEnumerator的实现

public DbEnumerator(IDataReader reader, bool closeReader)
{
      if (reader == null)
      {
            throw ADP.ArgumentNull(“reader”);
      }
      this._reader = reader;
      this.closeReader = closeReader;
}

 

public bool MoveNext()
{
      object[] objArray1;
      if (this._schemaInfo == null)
      {
            this.BuildSchemaInfo();
      }
      this._current = null;
      if (this._reader.Read())
      {
            objArray1 = new object[this._schemaInfo.Length];
            this._reader.GetValues(objArray1);
            this._current = new DbDataRecord(this._schemaInfo, objArray1, this._descriptors, this._fieldNameLookup);
            return true;
      }
      if (this.closeReader)
      {
            this._reader.Close();
      }
      return false;
}

果然， 如果DataReader的CommandBehavior是CommandBehavior.CloseConnection的话，MoveNext调用了Close方法!