What's the most memory effective way to read an SQL 2005 image field using C# 3.5?
Right now I have a (byte[])cm.ExecuteScalar("...")
.
If I could not read all field content into memory, that would be nice.
What's the most memory effective way to read an SQL 2005 image field using C# 3.5?
Right now I have a (byte[])cm.ExecuteScalar("...")
.
If I could not read all field content into memory, that would be nice.
See this excellent article here or this blog post for a long explanation how to do it.
Basically, you need to use a SqlDataReader and specify SequentialAccess
to it when you create it - then you can read (or write) the BLOB from the database in chunks of whatever size is best for you.
Basically something like:
SqlDataReader myReader = getEmp.ExecuteReader(CommandBehavior.SequentialAccess);
while (myReader.Read())
{
int 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)
{
// write the buffer to the output, e.g. a file
....
// 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 last buffer to the output, e.g. a file
....
}
// Close the reader and the connection.
myReader.Close();
Marc
The trick here is to use ExecuteReader in sequential mode, and read the data from the IDataReader
. Here's a version for CLOBs - BLOBs are virtually identical, but with a byte[]
and GetBytes(...)
.
Something like:
using (var reader = cmd.ExecuteReader(CommandBehavior.SequentialAccess))
{
byte[] buffer = new byte[8040]; // or some multiple (sql server page size)
while (reader.Read()) // each row
{
long dataOffset = 0, read;
while ((read = reader.GetBytes(
colIndex, dataOffset, buffer, 0, buffer.Length)) > 0)
{
// TODO: process "read"-many bytes from "buffer"
dataOffset += read;
}
}
}