Question

I have a .csv file that is frequently updated (about 20 to 30 times per minute). I want to insert the newly added lines to a database as soon as they are written to the file.

The FileSystemWatcher class listens to the file system change notifications and can raise an event whenever there is a change in a specified file. The problem is that the FileSystemWatcher cannot determine exactly which lines were added or removed (as far as I know).

One way to read those lines is to save and compare the line count between changes and read the difference between the last and second last change. However, I am looking for a cleaner (perhaps more elegant) solution.

Answer 1

I’ve written something very similar. I used the FileSystemWatcher to get notifications about changes. I then used a FileStream to read the data (keeping track of my last position within the file and seeking to that before reading the new data). Then I add the read data to a buffer which automatically extracts complete lines and then outputs then to the UI.

Note: ‘this.MoreData(..) is an event, the listener of which adds to the aforementioned buffer, and handles the complete line extraction.

Note: As has already been mentioned, this will only work if the modifications are always additions to the file. Any deletions will cause problems.

Hope this helps.

   public void File_Changed( object source, FileSystemEventArgs e )     {         lock ( this )         {             if ( !this.bPaused )             {                 bool bMoreData = false;                  // Read from current seek position to end of file                 byte[] bytesRead = new byte[this.iMaxBytes];                 FileStream fs = new FileStream( this.strFilename, FileMode.Open, FileAccess.Read, FileShare.ReadWrite );                  if ( 0 == this.iPreviousSeekPos )                 {                     if ( this.bReadFromStart )                     {                         if ( null != this.BeginReadStart )                         {                             this.BeginReadStart( null, null );                         }                         this.bReadingFromStart = true;                     }                     else                     {                         if ( fs.Length > this.iMaxBytes )                         {                             this.iPreviousSeekPos = fs.Length - this.iMaxBytes;                         }                     }                 }                  this.iPreviousSeekPos = (int)fs.Seek( this.iPreviousSeekPos, SeekOrigin.Begin );                 int iNumBytes = fs.Read( bytesRead, 0, this.iMaxBytes );                 this.iPreviousSeekPos += iNumBytes;                  // If we haven't read all the data, then raise another event                 if ( this.iPreviousSeekPos < fs.Length )                 {                     bMoreData = true;                 }                  fs.Close();                  string strData = this.encoding.GetString( bytesRead );                 this.MoreData( this, strData );                  if ( bMoreData )                 {                     File_Changed( null, null );                 }                 else                 {                     if ( this.bReadingFromStart )                     {                         this.bReadingFromStart = false;                         if ( null != this.EndReadStart )                         {                             this.EndReadStart( null, null );                         }                     }                 }             }         }