区别：

DataReader：和数据库处于一直连接状态。只读只能向前读取，一次只能读取一行信息。内存占用少，高效率！ 

Dateset：和数据库是断开的离线状态。返回都是数据集，可以对其中的数据做任意操作，内存占用多，效率稍低。

选择使用DataReader的情形： 

1)不需要缓存数据。 

2)要处理的结果集太大，内存中放不下。 

3)一旦需要以仅向前、只读方式快速访问数据。

DataReader转换到DataSet

public static DataSet ConvertDataReaderToDataSet(SqlDataReader reader) 
{ 
DataSet dataSet = new DataSet(); 
do 
{ 
// Create new data table 

DataTable schemaTable = reader.GetSchemaTable(); 
DataTable dataTable = new DataTable(); 

if ( schemaTable != null ) 
{ 
// A query returning records was executed 

for ( int i = 0; i < schemaTable.Rows.Count; i++ ) 
{ 
DataRow dataRow = schemaTable.Rows[ i ]; 
// Create a column name that is unique in the data table 
string columnName = ( string )dataRow[ "ColumnName" ]; //+ " // Add the column definition to the data table 
DataColumn column = new DataColumn( columnName, ( Type )dataRow[ "DataType" ] ); 
dataTable.Columns.Add( column ); 
} 

dataSet.Tables.Add( dataTable ); 

// Fill the data table we just created 

while ( reader.Read() ) 
{ 
DataRow dataRow = dataTable.NewRow(); 

for ( int i = 0; i < reader.FieldCount; i++ ) 
dataRow[ i ] = reader.GetValue( i ); 

dataTable.Rows.Add( dataRow ); 
} 
} 
else 
{ 
// No records were returned 

DataColumn column = new DataColumn("RowsAffected"); 
dataTable.Columns.Add(column); 
dataSet.Tables.Add( dataTable ); 
DataRow dataRow = dataTable.NewRow(); 
dataRow[0] = reader.RecordsAffected; 
dataTable.Rows.Add( dataRow ); 
} 
} 
while ( reader.NextResult() ); 
return dataSet; 
}