【转】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;
}
}
