This method returns an updatable OracleBlob object of the specified BLOB column using a WAIT clause.
Declaration
// C# public OracleBlob GetOracleBlobForUpdate(int index, int wait);
Parameters
index
The zero-based column index.
wait
The number of seconds the method waits to acquire a lock.
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.
IsDBNull should be called to check for NULL values before calling this method.
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 which locks the row.
Different WAIT clauses are appended to the statement, depending on the wait value. If the wait value is:
0
"NOWAIT" is appended at the end of a SELECT..FOR UPDATE statement. The statement executes immediately whether the lock is acquired or not. If the lock is not acquired, an exception is thrown.
n
"WAIT n" is appended at the end of a SELECT..FOR UPDATE statement. The statement executes as soon as the lock is acquired. However, if the lock cannot be acquired by n seconds, this method call throws an exception.
The WAIT n" feature is only available for Oracle9i or later. For any version lower than Oracle9i, n is implicitly treated as -1 and nothing is appended at the end of a SELECT..FOR UPDATE statement.
-1
Nothing is appended at the end of the SELECT..FOR UPDATE. The statement execution waits indefinitely until a lock can be acquired.
Example
The GetOracleBlobForUpdate methods are comparable. See "Example" for a code example demonstrating usage.