Trying to read a blob

Question

I am trying to read a BLOB from an Oracle database. The function GetFileContent take p_file_id as a paramater and return a BLOB. The BLOB is a DOCX-file that needs to be written in a folder somewhere. But I can't quite figure out how to read the BLOB. There is definitely something stored in the return_value-paramater after

OracleDataReader reader = cmd.ExecuteReader(CommandBehavior.SequentialAccess);

The value is {byte[9946]}. But I get an error when executing

long retrievedBytes = reader.GetBytes(1, startIndex, buffer, 0, ChunkSize);

It says InvalidOperationException was caught: "No data exists for the row or column."

Here is the code:

cmd = new OracleCommand("GetFileContent", oraCon);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add("p_file_id", OracleType.Number).Direction = ParameterDirection.Input;
cmd.Parameters[0].Value = fileID;
cmd.Parameters.Add("return_value", OracleType.Blob).Direction = ParameterDirection.ReturnValue;
cmd.Connection.Open();

OracleDataReader reader = cmd.ExecuteReader(CommandBehavior.SequentialAccess);
reader.Read();

MemoryStream memory = new MemoryStream();
long startIndex = 0;
const int ChunkSize = 256;
while (true)
{
   byte[] buffer = new byte[ChunkSize];
   long retrievedBytes = reader.GetBytes(1, startIndex, buffer, 0, ChunkSize); //FAILS
   memory.Write(buffer, 0, (int)retrievedBytes);
   startIndex += retrievedBytes;
   if (retrievedBytes != ChunkSize)
      break;
}
cmd.Connection.Close();
byte[] data = memory.ToArray();
memory.Dispose();

How can I read the BLOB from the function?

Answer 1

Looks like you are using the Microsoft Oracle Client. You probably want to use the LOB objects rather than using GetBytes(...).

I think the first link below would be the easiest for you. Here is an excerpt:

using(reader)
{
      //Obtain the first row of data.
      reader.Read();
      //Obtain the LOBs (all 3 varieties).
      OracleLob BLOB = reader.GetOracleLob(1);
      ...

      //Example - Reading binary data (in chunks).
      byte[] buffer = new byte[100];
      while((actual = BLOB.Read(buffer, 0, buffer.Length)) >0)
         Console.WriteLine(BLOB.LobType + ".Read(" + buffer + ", " + buffer.Length + ") => " + actual);

      ...
}

OracleLob::Read Method

OracleLob Class

OracleDataReader::GetOracleLob Method

On a side note, the Microsoft Oracle client is being depreciated. You may want look into switching to Oracle's ODP.net, as that will be the only "Officially Supported" client moving forward.