.net – How to avoid thousands of needless ListView.SelectedIndexChanged events

listviewnetselectedindexchangedwinforms

If a user select all items in a .NET 2.0 ListView, the ListView will fire a SelectedIndexChanged event for every item, rather than firing an event to indicate that the selection has changed.

If the user then clicks to select just one item in the list, the ListView will fire a SelectedIndexChanged event for every item that is getting unselected, and then an SelectedIndexChanged event for the single newly selected item, rather than firing an event to indicate that the selection has changed.

If you have code in the SelectedIndexChanged event handler, the program will become pretty unresponsive when you begin to have a few hundred/thousand items in the list.

I've thought about dwell timers, etc.

But does anyone have a good solution to avoid thousands of needless ListView.SelectedIndexChange events, when really one event will do?

Best Answer

Good solution from Ian. I took that and made it into a reusable class, making sure to dispose of the timer properly. I also reduced the interval to get a more responsive app. This control also doublebuffers to reduce flicker.

  public class DoublebufferedListView : System.Windows.Forms.ListView
  {
     private Timer m_changeDelayTimer = null;
     public DoublebufferedListView()
        : base()
     {
        // Set common properties for our listviews
        if (!SystemInformation.TerminalServerSession)
        {
           DoubleBuffered = true;
           SetStyle(ControlStyles.ResizeRedraw, true);
        }
     }

     /// <summary>
     /// Make sure to properly dispose of the timer
     /// </summary>
     /// <param name="disposing"></param>
     protected override void Dispose(bool disposing)
     {
        if (disposing && m_changeDelayTimer != null)
        {
           m_changeDelayTimer.Tick -= ChangeDelayTimerTick;
           m_changeDelayTimer.Dispose();
        }
        base.Dispose(disposing);
     }

     /// <summary>
     /// Hack to avoid lots of unnecessary change events by marshaling with a timer:
     /// http://stackoverflow.com/questions/86793/how-to-avoid-thousands-of-needless-listview-selectedindexchanged-events
     /// </summary>
     /// <param name="e"></param>
     protected override void OnSelectedIndexChanged(EventArgs e)
     {
        if (m_changeDelayTimer == null)
        {
           m_changeDelayTimer = new Timer();
           m_changeDelayTimer.Tick += ChangeDelayTimerTick;
           m_changeDelayTimer.Interval = 40;
        }
        // When a new SelectedIndexChanged event arrives, disable, then enable the
        // timer, effectively resetting it, so that after the last one in a batch
        // arrives, there is at least 40 ms before we react, plenty of time 
        // to wait any other selection events in the same batch.
        m_changeDelayTimer.Enabled = false;
        m_changeDelayTimer.Enabled = true;
     }

     private void ChangeDelayTimerTick(object sender, EventArgs e)
     {
        m_changeDelayTimer.Enabled = false;
        base.OnSelectedIndexChanged(new EventArgs());
     }
  }

Do let me know if this can be improved.

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