The Art of SQL Server FILESTREAM - Red Gate Software

Chapter 3: Accessing FILESTREAM Data from Client ApplicationsTo avoid additional round trips to the database, the stored procedure or T-SQL batchthat is used to perform the INSERT operation can also return the FILESTREAM transactioncontext of the current transaction and the logical path to the FILESTREAM datafile associated with the newly inserted record. The client application can then use thesevalues to access the FILESTREAM data.Since it is not possible to use the streaming API to create a new FILESTREAM data fileand associate it with a FILESTREAM column, to create a new record and save the BLOBdata into the FILESTREAM column the client application must use two steps.1. Execute a T-SQL statement to create a new record along with a dummy FILESTREAMvalue. Inserting a fake value such as 0x into the FILESTREAM column will force SQLServer to create a FILESTREAM data file and associate it with the column.2. Once the FILESTREAM data file has been created, the client application can use thePathName() value of the FILESTREAM column in the newly inserted record to accessthe FILESTREAM data file and write the BLOB data into it.An alternative solution involves not allowing NULL values in the FILESTREAM columnand providing a default value of 0x. This ensures that the FILESTREAM cell will alwaysbe associated with a valid file, and can remove some repetitive code from the applicationthat is writing FILESTREAM data.Replacing the FILESTREAM data completelyWhen the FILESTREAM data is to be replaced completely, this is usually done usingthe streaming APIs. A client application can perform a write operation using either theSqlFileStream class (.NET) or the OpenSqlFilestream() Win32 API function. Whenthe FILESTREAM data file is opened for write access, the file pointer is at the beginningof the file. The write operation will write into a new disk file, and the reference on theFILESTREAM column will be updated to point to the new file when the file handle is103