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Obtaining BLOB Values from a Database

The default behavior of the DataReader is to load incoming data as a row as soon as an entire row of data is available. Binary large objects (BLOBs) need to be treated differently, however, because they can contain gigabytes of data that cannot be contained in a single row. The Command.ExecuteReader method has an overload which will take a CommandBehavior argument to modify the default behavior of the DataReader. You can pass CommandBehavior.SequentialAccess to the ExecuteReader method to modify the default behavior of the DataReader so that instead of loading rows of data, it will load data sequentially as it is received. This is ideal for loading BLOBs or other large data structures. Note that this behavior may differ depending on your data source. For example, returning a BLOB from Microsoft Access will result in the entire BLOB being loaded into memory rather than loading data sequentially as it is received.

When setting the DataReader to use SequentialAccess, it is important to note the sequence in which you access the fields returned. The default behavior of the DataReader, which loads an entire row as soon as it is available, allows you to access the fields returned in any order until the next row is read. When using SequentialAccess however, you must access the different fields returned by the DataReader in order. For example, if your query returns three columns, the third of which is a BLOB, you must return the values of the first and second fields before accessing the BLOB data in the third field. If you access the third field before the first or second fields, the first and second field values will no longer be available. This is because SequentialAccess has modified the DataReader to return data in sequence and the data will not be available after the DataReader has read past it.

When accessing the data in the BLOB field, use the GetBytes or GetChars typed accessors of the DataReader, which fill an array with data. You can also use GetString for character data, however to conserve system resources you may not want to load an entire BLOB value into a single string variable. You can specify a specific buffer size of data to be returned, and a starting location for the first byte or character to be read from the returned data. GetBytes and GetChars will return a long value, which represents the number of bytes or characters returned. If you pass a null array to GetBytes or GetChars, the long value returned will be the total number of bytes or characters in the BLOB. You can optionally specify an index in the array as a starting position for the data being read.

The following example returns the publisher id and logo from the pubs sample database in Microsoft SQL Server. The publisher id (pub_id) is a character field, and the logo is an image, which is a BLOB. Because the logo field is a bitmap, the example returns binary data using GetBytes. Notice that the publisher id is accessed for the current row of data before the logo, because the fields must be accessed sequentially.

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();

See Also

Using .NET Framework Data Providers to Access Data | Writing BLOB Values to a Database | OleDbDataReader Class | OleDbCommand Class | OdbcDataReader Class | OdbcCommand Class | SqlDataReader Class | SqlCommand Class | CommandBehavior Enumeration

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