读clob数据时出现乱码
请问,我在读取Clob数据时 出现乱码,望高手解决。
DataReader 的默认行为是在整个数据行可用时立即以行的形式加载传入数据。但是,对于二进制大对象 (BLOB) 则需要进行不同的处理,因为它们可能包含数十亿字节的数据,而单个行中无法包含如此多的数据。Command.ExecuteReader 方法具有一个重载,它将采用 CommandBehavior 参数来修改 DataReader 的默认行为。您可以将 CommandBehavior.SequentialAccess 传递到 ExecuteReader 方法来修改 DataReader 的默认行为,以便让 DataReader 按照顺序在接收到数据时立即将其加载,而不是加载数据行。这是加载 BLOB 或其他大数据结构的理想方案。请注意,该行为可能会因数据源的不同而不同。例如,从 Microsoft Access 中返回 BLOB 将导致整个 BLOB 加载到内存中,而不是按接收数据的顺序加载数据。 在将 DataReader 设置为使用 SequentialAccess 时,务必要注意访问所返回字段的顺序。DataReader 的默认行为是在整个行可用时立即加载该行,这使您能够在读取下一行之前按任何顺序访问所返回的字段。但是,当使用 SequentialAccess 时,必须按顺序访问由 DataReader 返回的不同字段。例如,如果查询返回三个列,其中第三列是 BLOB,则必须在访问第三个字段中的 BLOB 数据之前返回第一个和第二个字段的值。如果在访问第一个或第二个字段之前访问第三个字段,则第一个和第二个字段值将不再可用。这是因为 SequentialAccess 已修改 DataReader,使其按顺序返回数据,当 DataReader 已经读取超过特定数据时,该数据将不可用。 在访问 BLOB 字段中的数据时,请使用 DataReader 的 GetBytes 或 GetChars 类型化访问器,它们将用数据来填充数组。还可以对字符数据使用 GetString,但是为了节省系统资源,您可能不希望将整个 BLOB 值加载到单个字符串变量中。您可以指定要返回的特定数据缓冲区大小,以及从返回的数据中读取的第一个字节或字符的起始位置。GetBytes 和 GetChars 将返回一个 long 值,它表示返回的字节或字符数。如果将一个空数组传递给 GetBytes 或 GetChars,则返回的长值将是 BLOB 中字符或字符的总数。您可以选择将数组中的某个索引指定为所读取数据的起始位置。 以下示例从 Microsoft SQL Server 中的 pubs 示例数据库中返回发行者 ID 和徽标。发行者 ID (pub_id) 是字符字段,而徽标则是图形,即 BLOB。由于 logo 字段是位图,因此该示例使用 GetBytes 返回二进制数据。请注意,由于必须按顺序访问字段,所以将在访问徽标之前访问当前数据行的发行者 ID。 [Visual Basic] Dim pubsConn As SqlConnection = New SqlConnection("Data Source=localhost;Integrated Security=SSPI;Initial Catalog=pubs;") Dim logoCMD As SqlCommand = New SqlCommand("SELECT pub_id, logo FROM pub_info", pubsConn) Dim fs As FileStream ' Writes the BLOB to a file (*.bmp). Dim bw As BinaryWriter ' Streams the binary data to the FileStream object. Dim bufferSize As Integer = 100 ' The size of the BLOB buffer. Dim outbyte(bufferSize - 1) As Byte ' The BLOB byte() buffer to be filled by GetBytes. Dim retval As Long ' The bytes returned from GetBytes. Dim startIndex As Long = 0 ' The starting position in the BLOB output. Dim pub_id As String = "" ' The publisher id to use in the file name. ' Open the connection and read data into the DataReader. pubsConn.Open() Dim myReader As SqlDataReader = logoCMD.ExecuteReader(CommandBehavior.SequentialAccess) Do While myReader.Read() ' Get the publisher id, which must occur before getting the logo. pub_id = myReader.GetString(0) ' Create a file to hold the output. fs = New FileStream("logo" & pub_id & ".bmp", FileMode.OpenOrCreate, FileAccess.Write) bw = New BinaryWriter(fs) ' Reset the starting byte for a new BLOB. startIndex = 0 ' Read bytes into outbyte() and retain the number of bytes returned. retval = myReader.GetBytes(1, startIndex, outbyte, 0, bufferSize) ' Continue reading and writing while there are bytes beyond the size of the buffer. Do While retval = bufferSize bw.Write(outbyte) bw.Flush() ' Reposition the start index to the end of the last buffer and fill the buffer. startIndex += bufferSize retval = myReader.GetBytes(1, startIndex, outbyte, 0, bufferSize) Loop ' Write the remaining buffer. bw.Write(outbyte, 0 , retval - 1) bw.Flush() ' Close the output file. bw.Close() fs.Close() Loop ' Close the reader and the connection. myReader.Close() pubsConn.Close() [C#] SqlConnection pubsConn = new SqlConnection("Data Source=localhost;Integrated Security=SSPI;Initial Catalog=pubs;"); SqlCommand logoCMD = new SqlCommand("SELECT pub_id, logo FROM pub_info", pubsConn); FileStream fs; // Writes the BLOB to a file (*.bmp). BinaryWriter bw; // Streams the BLOB to the FileStream object. int bufferSize = 100; // Size of the BLOB buffer. byte[] outbyte = new byte[bufferSize]; // The BLOB byte[] buffer to be filled by GetBytes. long retval; // The bytes returned from GetBytes. long startIndex = 0; // The starting position in the BLOB output. string pub_id = ""; // The publisher id to use in the file name. // Open the connection and read data into the DataReader. pubsConn.Open(); SqlDataReader myReader = logoCMD.ExecuteReader(CommandBehavior.SequentialAccess); while (myReader.Read()) { // Get the publisher id, which must occur before getting the logo. pub_id = myReader.GetString(0); // Create a file to hold the output. fs = new FileStream("logo" + pub_id + ".bmp", FileMode.OpenOrCreate, FileAccess.Write); bw = new BinaryWriter(fs); // Reset the starting byte for the new BLOB. startIndex = 0; // Read the bytes into outbyte[] and retain the number of bytes returned. retval = myReader.GetBytes(1, startIndex, outbyte, 0, bufferSize); // Continue reading and writing while there are bytes beyond the size of the buffer. while (retval == bufferSize) { bw.Write(outbyte); bw.Flush(); // Reposition the start index to the end of the last buffer and fill the buffer. startIndex += bufferSize; retval = myReader.GetBytes(1, startIndex, outbyte, 0, bufferSize); } // Write the remaining buffer. bw.Write(outbyte, 0, (int)retval - 1); bw.Flush(); // Close the output file. bw.Close(); fs.Close(); } // Close the reader and the connection. myReader.Close(); pubsConn.Close();