擷取大型資料 (ADO.NET)
更新： November 2007
DataReader 的預設行為是一旦有整列資料可用時，便將內送資料載入為資料列。但是二進位大型物件 (BLOB) 需要不同的處理方式，因為它們所包含的資料可能高達數 GB，無法包含在單一資料列內。Command.ExecuteReader 方法具有多載，會採用 CommandBehavior 引數以修改 DataReader 的預設行為。您可將 SequentialAccess 傳遞給 ExecuteReader 方法以修改 DataReader 的預設行為，這樣可在接收到資料時依序載入資料，而不必載入資料列。這種方式相當適於載入 BLOB 或其他大型資料結構。請注意，這個行為取決於您的資料來源。例如，從 Microsoft Access 傳回 BLOB 會將整個正在載入的 BLOB 載入記憶體，而不是在接收時循序載入。
設定 DataReader 來使用 SequentialAccess 時，您必須要注意存取傳回欄位的順序。DataReader 的預設行為 (一旦有整列資料可用時立即將其載入) 能讓您在讀取下個資料列前用任何順序存取傳回欄位。但是使用 SequentialAccess 時，您就必須依序存取 DataReader 傳回的欄位。例如，如果您的查詢傳回三個資料行，而第三個是 BLOB，則您必須在存取第三個欄位內的 BLOB 資料前，先把第一和第二個欄位的值傳回。如果您在尚未存取第一和第二個欄位前先存取第三個欄位，便無法再使用第一和第二個欄位的值。這是因為 SequentialAccess 已修改了 DataReader 來循序傳回資料，而 DataReader 讀過資料後，就無法取得資料。
存取 BLOB 欄位的資料時，請使用 DataReader 的 GetBytes 或 GetChars 具型別存取子，以資料填入陣列。您也可以對字元資料使用 GetString，然而為了節省系統資統，您可能不想要將整個 BLOB 值載入單一字串變數。您可以指定傳回資料的特定緩衝區大小，以及從傳回資料讀取的第一個位元組或字元的開始位置。GetBytes 和 GetChars 會傳回 long 值，表示傳回的位元組數或字元數。如果您將 Null 陣列傳遞給 GetBytes 或 GetChars，則傳回的長整數 (Long) 值將會是 BLOB 內的總位元組數或總字元數。您可以選擇將陣列內的索引指定為要讀取資料的開始位置。
範例
下列範例從 Microsoft SQL Server 2000 的 pubs 範例資料庫傳回發行者 ID 和標誌。發行者 ID (pub_id) 是字元欄位，而標誌是影像 (為 BLOB)。由於 logo 欄位是點陣圖，因此範例會使用 GetBytes 傳回二進位資料。請注意，由於欄位必須被循序存取，所以您要先存取目前資料列的發行者 ID，再存取標誌。
' Assumes that connection is a valid SqlConnection object.
Dim command As SqlCommand = New SqlCommand( _
"SELECT pub_id, logo FROM pub_info", connection)
' Writes the BLOB to a file (*.bmp).
Dim stream As FileStream
' Streams the binary data to the FileStream object.
Dim writer As BinaryWriter
' The size of the BLOB buffer.
Dim bufferSize As Integer = 100
' The BLOB byte() buffer to be filled by GetBytes.
Dim outByte(bufferSize - 1) As Byte
' The bytes returned from GetBytes.
Dim retval As Long
' The starting position in the BLOB output.
Dim startIndex As Long = 0
' The publisher id to use in the file name.
Dim pubID As String = ""
' Open the connection and read data into the DataReader.
connection.Open()
Dim reader As SqlDataReader = command.ExecuteReader(CommandBehavior.SequentialAccess)
Do While reader.Read()
' Get the publisher id, which must occur before getting the logo.
pubID = reader.GetString(0)
' Create a file to hold the output.
stream = New FileStream( _
"logo" & pubID & ".bmp", FileMode.OpenOrCreate, FileAccess.Write)
writer = New BinaryWriter(stream)
' Reset the starting byte for a new BLOB.
startIndex = 0
' Read bytes into outByte() and retain the number of bytes returned.
retval = reader.GetBytes(1, startIndex, outByte, 0, bufferSize)
' Continue while there are bytes beyond the size of the buffer.
Do While retval = bufferSize
writer.Write(outByte)
writer.Flush()
' Reposition start index to end of the last buffer and fill buffer.
startIndex += bufferSize
retval = reader.GetBytes(1, startIndex, outByte, 0, bufferSize)
Loop
' Write the remaining buffer.
writer.Write(outByte, 0 , retval - 1)
writer.Flush()
' Close the output file.
writer.Close()
stream.Close()
Loop
' Close the reader and the connection.
reader.Close()
connection.Close()
// Assumes that connection is a valid SqlConnection object.
SqlCommand command = new SqlCommand(
"SELECT pub_id, logo FROM pub_info", connection);
// Writes the BLOB to a file (*.bmp).
FileStream stream;
// Streams the BLOB to the FileStream object.
BinaryWriter writer;
// Size of the BLOB buffer.
int bufferSize = 100;
// The BLOB byte[] buffer to be filled by GetBytes.
byte[] outByte = new byte[bufferSize];
// The bytes returned from GetBytes.
long retval;
// The starting position in the BLOB output.
long startIndex = 0;
// The publisher id to use in the file name.
string pubID = "";
// Open the connection and read data into the DataReader.
connection.Open();
SqlDataReader reader = command.ExecuteReader(CommandBehavior.SequentialAccess);
while (reader.Read())
{
// Get the publisher id, which must occur before getting the logo.
pubID = reader.GetString(0);
// Create a file to hold the output.
stream = new FileStream(
"logo" + pubID + ".bmp", FileMode.OpenOrCreate, FileAccess.Write);
writer = new BinaryWriter(stream);
// Reset the starting byte for the new BLOB.
startIndex = 0;
// Read bytes into outByte[] and retain the number of bytes returned.
retval = reader.GetBytes(1, startIndex, outByte, 0, bufferSize);
// Continue while there are bytes beyond the size of the buffer.
while (retval == bufferSize)
{
writer.Write(outByte);
writer.Flush();
// Reposition start index to end of last buffer and fill buffer.
startIndex += bufferSize;
retval = reader.GetBytes(1, startIndex, outByte, 0, bufferSize);
}
// Write the remaining buffer.
writer.Write(outByte, 0, (int)retval - 1);
writer.Flush();
// Close the output file.
writer.Close();
stream.Close();
}
// Close the reader and the connection.
reader.Close();
connection.Close();