SqlDataReader，SqlDataAdapter与SqlCommand的一点总结.

【http://www.cnblogs.com/liuzhendong/archive/2012/01/28/2330689.html】

1.SqlDataReader，在线应用，需要conn.open()，使用完之后要关闭.

SqlConnection conn = new SqlConnection(connStr);
 //conn.Open();
SqlCommand cmd = new SqlCommand("select top 10 * from tuser", conn);
SqlDataReader reader = cmd.ExecuteReader(CommandBehavior.CloseConnection);
while (reader.Read())
{
    Console.WriteLine(reader.GetValue(2));
}
这段代码报错:ExecuteReader requires an open and available Connection. The connection's current state is closed.
应该将conn.Open()打开.

2.SqlDataAdapter，离线应用，不需要用conn.open(), 它把这部分功能给封装到自己内部了，不需要你来显式的去调用, 它直接将数据fill到dataset中.

SqlCommand与ADO时代的Command一样，SqlDataAdapter则是ADO.NET中的新事物，它配合DataSet来使用。其实，DataSet就像是驻留在内存中的小数据库，在DataSet中可以有多张DataTable，这些DataTable之间可以相互关联，就像在数据库中表关联一样！SqlDataAdapter的作用就是将数据从数据库中提取出来，放在DataSet中，当DataSet中的数据发生变化时，SqlDataAdapter再将数据库中的数据更新，以保证数据库中的数据和DataSet中的数据是一致的！ 
用微软顾问的话讲：DataAdapter就像是一把铁锹，它负责把数据从数据库“铲”到DataSet中，或者将数据从DataSet“铲”到数据库中！

调用DataAdapter的Fill方法时, 它会打开到数据库的SqlConnection, 再通过创建一个SqlCommand和调用ExecuteReader的方式来执行命令, 然后, 通过一个隐式士创建的SqlDataReader, 从数据库读取数据, 结束行的读取之后, SqlDataReader和SqlConnection会被关闭.

DefaultView是DataTable类的一个属性。可用DataView类为每个DataTable定义多个视图。

用Reflacter察看一下SqlDataAdapter及其父类的原代码，其方法调用脉络如下：

Fill(DataSet dataSet)->

Fill(dataSet, 0, 0, "Table", selectCommand, fillCommandBehavior)->

FillInternal(dataSet, null, startRecord, maxRecords, srcTable, command, behavior)->

Fill(dataset, srcTable, reader, startRecord, maxRecords)->

Fill(dataset, srcTable, reader, startRecord, maxRecords)->

FillFromReader(dataSet, null, srcTable, container, startRecord, maxRecords, null, null)->

FillLoadDataRow(mapping)->

while (dataReader.Read())

因此个人认为最精髓的一句总结就是：SqlDataAdapter内部获取数据是通过调用SqlDataReader来实现的，而两者都需要使用SqlConnection和SqlCommand。

 

具体Reflacter察看一下SqlDataAdapter及其父类的原代码如下:

2.1 SqlDataAdapter是DbDataAdapter的子类.

public sealed class SqlDataAdapter : DbDataAdapter, IDbDataAdapter, IDataAdapter, ICloneable

2.2 DbDataAdapter是一个抽象类，里面包含了Fill的具体实现.

public abstract class DbDataAdapter : DataAdapter, IDbDataAdapter, IDataAdapter, ICloneable
{
  public override int Fill(DataSet dataSet)
  {
    int num;
    IntPtr ptr;
    Bid.ScopeEnter(out ptr, "<comm.DbDataAdapter.Fill|API> %d#, dataSet\n", base.ObjectID);
    try
    {
        IDbCommand selectCommand = this._IDbDataAdapter.SelectCommand;
        CommandBehavior fillCommandBehavior = this.FillCommandBehavior;
        num = this.Fill(dataSet, 0, 0, "Table", selectCommand, fillCommandBehavior);
    }
    finally
    {
        Bid.ScopeLeave(ref ptr);
    }
    return num;
  }

  protected virtual int Fill(DataSet dataSet, int startRecord, int maxRecords, string srcTable, IDbCommand command, CommandBehavior behavior)
  {
    int num;
    IntPtr ptr;
    Bid.ScopeEnter(out ptr, "<comm.DbDataAdapter.Fill|API> %d#, dataSet, startRecord, maxRecords, srcTable, command, behavior=%d{ds.CommandBehavior}\n", base.ObjectID, (int) behavior);
    try
    {
        if (dataSet == null)
        {
            throw ADP.FillRequires("dataSet");
        }
        if (startRecord < 0)
        {
            throw ADP.InvalidStartRecord("startRecord", startRecord);
        }
        if (maxRecords < 0)
        {
            throw ADP.InvalidMaxRecords("maxRecords", maxRecords);
        }
        if (ADP.IsEmpty(srcTable))
        {
            throw ADP.FillRequiresSourceTableName("srcTable");
        }
        if (command == null)
        {
            throw ADP.MissingSelectCommand("Fill");
        }
        num = this.FillInternal(dataSet, null, startRecord, maxRecords, srcTable, command, behavior);
    }
    finally
    {
        Bid.ScopeLeave(ref ptr);
    }
    return num;
  }

  private int FillInternal(DataSet dataset, DataTable[] datatables, int startRecord, int maxRecords, string srcTable, IDbCommand command, CommandBehavior behavior)
  {
    bool flag = null == command.Connection;
    try
    {
        IDbConnection connection = GetConnection3(this, command, "Fill");
        ConnectionState open = ConnectionState.Open;
        if (MissingSchemaAction.AddWithKey == base.MissingSchemaAction)
        {
            behavior |= CommandBehavior.KeyInfo;
        }
        try
        {
            QuietOpen(connection, out open);
            behavior |= CommandBehavior.SequentialAccess;
            using (IDataReader reader = null)
            {
                reader = command.ExecuteReader(behavior);
                if (datatables != null)
                {
                    return this.Fill(datatables, reader, startRecord, maxRecords);
                }
                return this.Fill(dataset, srcTable, reader, startRecord, maxRecords);
            }
        }
        finally
        {
            QuietClose(connection, open);
        }
    }
    finally
    {
        if (flag)
        {
            command.Transaction = null;
            command.Connection = null;
        }
    }

  protected virtual int Fill(DataSet dataSet, string srcTable, IDataReader dataReader, int startRecord, int maxRecords)
  {
    int num;
    IntPtr ptr;
    Bid.ScopeEnter(out ptr, "<comm.DataAdapter.Fill|API> %d#, dataSet, srcTable, dataReader, startRecord, maxRecords\n", this.ObjectID);
    try
    {
        if (dataSet == null)
        {
            throw ADP.FillRequires("dataSet");
        }
        if (ADP.IsEmpty(srcTable))
        {
            throw ADP.FillRequiresSourceTableName("srcTable");
        }
        if (dataReader == null)
        {
            throw ADP.FillRequires("dataReader");
        }
        if (startRecord < 0)
        {
            throw ADP.InvalidStartRecord("startRecord", startRecord);
        }
        if (maxRecords < 0)
        {
            throw ADP.InvalidMaxRecords("maxRecords", maxRecords);
        }
        if (dataReader.IsClosed)
        {
            return 0;
        }
        DataReaderContainer container = DataReaderContainer.Create(dataReader, this.ReturnProviderSpecificTypes);
        num = this.FillFromReader(dataSet, null, srcTable, container, startRecord, maxRecords, null, null);
    }
    finally
    {
        Bid.ScopeLeave(ref ptr);
    }
    return num;
  }

  internal int FillFromReader(DataSet dataset, DataTable datatable, string srcTable, DataReaderContainer dataReader, int startRecord, int maxRecords, DataColumn parentChapterColumn, object   parentChapterValue)
  {
    int num2 = 0;
    int schemaCount = 0;
    do
    {
        if (0 < dataReader.FieldCount)
        {
            SchemaMapping mapping = this.FillMapping(dataset, datatable, srcTable, dataReader, schemaCount, parentChapterColumn, parentChapterValue);
            schemaCount++;
            if (((mapping != null) && (mapping.DataValues != null)) && (mapping.DataTable != null))
            {
                mapping.DataTable.BeginLoadData();
                try
                {
                    if ((1 == schemaCount) && ((0 < startRecord) || (0 < maxRecords)))
                    {
                        num2 = this.FillLoadDataRowChunk(mapping, startRecord, maxRecords);
                    }
                    else
                    {
                        int num3 = this.FillLoadDataRow(mapping);
                        if (1 == schemaCount)
                        {
                            num2 = num3;
                        }
                    }
                }
                finally
                {
                    mapping.DataTable.EndLoadData();
                }
                if (datatable != null)
                {
                    return num2;
                }
            }
        }
    }
    while (this.FillNextResult(dataReader));
    return num2;
  }

  private int FillLoadDataRow(SchemaMapping mapping)
  {
    int num = 0;
    DataReaderContainer dataReader = mapping.DataReader;
    if (!this._hasFillErrorHandler)
    {
        while (dataReader.Read())
        {
            mapping.LoadDataRow();
            num++;
        }
        return num;
    }
    while (dataReader.Read())
    {
        try
        {
            mapping.LoadDataRowWithClear();
            num++;
            continue;
        }
        catch (Exception exception)
        {
            if (!ADP.IsCatchableExceptionType(exception))
            {
                throw;
            }
            ADP.TraceExceptionForCapture(exception);
            this.OnFillErrorHandler(exception, mapping.DataTable, mapping.DataValues);
            continue;
        }
    }
    return num;
  }

}