GetOracleBlobForUpdate(int)

This method returns an updatable OracleBlob object of the specified BLOB column.

Declaration

// C#
public OracleBlob GetOracleBlobForUpdate(int index);

Parameters

index

The zero-based column index.

Return Value

An updatable OracleBlob object.

Exceptions

InvalidOperationException - The connection is closed, the reader is closed, Read() has not been called, or all rows have been read.

IndexOutOfRangeException - The column index is invalid.

InvalidCastException - The accessor method is invalid for this column type or the column value is NULL.

Remarks

When the OracleCommand's ExecuteReader() method is invoked, all the data fetched by the OracleDataReader is from a particular snapshot. Therefore, calling an accessor method on the same column always returns the same value. However, the GetOracleBlobForUpdate() method incurs a database round-trip to obtain a reference to the current BLOB data while also locking the row using the FOR UPDATE clause. This means that the OracleBlob obtained from GetOracleBlob() can have a different value than the OracleBlob obtained from GetOracleBlobForUpdate() since it is not obtained from the original snapshot.

The returned OracleBlob object can be used to safely update the BLOB because the BLOB column has been locked after a call to this method.

Invoking this method internally executes a SELECT..FOR UPDATE statement without a WAIT clause. Therefore, the statement can wait indefinitely until a lock is acquired for that row.

IsDBNull should be called to check for NULL values before calling this method.

Example

The following example gets the OracleBlob object for update from the reader, updates the OracleBlob object, and then commits the transaction.

/* Database Setup, if you have not done so yet.
connect scott/tiger@oracle 
CREATE TABLE empInfo (
empno NUMBER(4) PRIMARY KEY,
empName VARCHAR2(20) NOT NULL,
hiredate DATE,
salary NUMBER(7,2),
jobDescription Clob,
byteCodes BLOB
);
 
Insert into empInfo(EMPNO,EMPNAME,JOBDESCRIPTION,byteCodes) values 
(1,'KING','SOFTWARE ENGR', '5657');
Insert into empInfo(EMPNO,EMPNAME,JOBDESCRIPTION,byteCodes) values 
(2,'SCOTT','MANAGER', '5960');
commit;
 
*/
// C#
 
using System;
using System.Data;
using Oracle.DataAccess.Client; 
using Oracle.DataAccess.Types;
 
class GetOracleBlobForUpdateSample
{
  static void Main()
  {
    string constr = "User Id=scott;Password=tiger;Data Source=oracle";
    OracleConnection con = new OracleConnection(constr);
    con.Open();
 
    // Get the ByteCodes for empno = 1
    string cmdstr = "SELECT BYTECODES, EMPNO FROM EMPINFO where EMPNO = 1";
    OracleCommand cmd = new OracleCommand(cmdstr, con);
 
    // Since we are going to update the OracleBlob object, we will
    //have to create a transaction
    OracleTransaction txn = con.BeginTransaction();
 
    // Get the reader
    OracleDataReader reader = cmd.ExecuteReader();
 
    // Declare the variables to retrieve the data in EmpInfo
    OracleBlob byteCodesBlob;
 
    // Read the first row
    reader.Read();
    if (!reader.IsDBNull(0))
    {
      byteCodesBlob = reader.GetOracleBlobForUpdate(0);
 
      // Close the reader
      reader.Close();
 
      // Update the ByteCodes object
      byte[] addedBytes = new byte[2] {0, 0};
      byteCodesBlob.Append(addedBytes, 0, addedBytes.Length);
 
      // Now commit the transaction
      txn.Commit();
      Console.WriteLine("Blob Column successfully updated");
    }
    else
      reader.Dispose();
 
    // Close the connection
    con.Dispose();
  }
}