Shell Notifications in C#

Introduction

When you build an application that shows files or directories of the local computer, you must know if the files already exist. You can add refresh functions but there is another way to know in real time when something has changed by using the SHChangeNotifyRegister API.

For a complete overview of shell change notifications, James Holderness made a very useful "Shell Notifications" page.

The structures and enumerations are not listed here but are accessible in the sources.

SHChangeNotifyRegister and SHChangeNotifyUnregister declarations

The API calls are hidden: SHChangeNotifyRegister uses an entry point value of 2 and SHChangeNotifyUnregister uses 4.

using System.Runtime.InteropServices;

[DllImport("shell32.dll", EntryPoint="#2", CharSet=CharSet.Auto)]
private static extern uint SHChangeNotifyRegister(
    IntPtr hWnd,
    SHCNF fSources,
    SHCNE fEvents,
    uint wMsg,
    int cEntries,
    ref SHChangeNotifyEntry pFsne);

[DllImport("shell32.dll", EntryPoint="#4", CharSet=CharSet.Auto)]
[return:MarshalAs(UnmanagedType.Bool)]
private static extern Boolean SHChangeNotifyUnregister(
    ulong hNotify);

Register a handle

When a modification occurs, the shell sends a notification to each registered handle. You have to specify the folder root by the pidl retrieved by the SHGetSpecialFolderLocation API.

[DllImport("shell32.dll", CharSet=CharSet.Auto)]
private static extern uint SHGetSpecialFolderLocation(
    IntPtr hWnd,
    CSIDL nFolder,
    out IntPtr Pidl);

public ulong RegisterChangeNotify(IntPtr hWnd, CSIDL FolderID,
    bool Recursively)
{
    if(notifyid != 0) return(0);
    SHChangeNotifyEntry changeentry = new SHChangeNotifyEntry();

    changeentry.pIdl = GetPidlFromFolderID(hWnd, FolderID);
    changeentry.Recursively = Recursively;

    notifyid = SHChangeNotifyRegister(
        hWnd,
        SHCNF.SHCNF_TYPE | SHCNF.SHCNF_IDLIST,
        SHCNE.SHCNE_ALLEVENTS | SHCNE.SHCNE_INTERRUPT,
        WM_SHNOTIFY,
        1,
        ref changeentry);

    return(notifyid);
}

public Boolean UnregisterChangeNotify()
{
    if(notifyid == 0) return(false);

    if(SHChangeNotifyUnregister(notifyid))
    {
        notifyid = 0;
        return(true);
    }

    return(false);
}

public static IntPtr GetPidlFromFolderID(IntPtr hWnd, CSIDL Id)
{
    IntPtr pIdl = IntPtr.Zero;

    SHGetFolderLocationReturnValues res = 
	    (SHGetFolderLocationReturnValues)
		SHGetSpecialFolderLocation( hWnd,
            Id,
            out pIdl);

    return(pIdl);
}

Notification reception

When you receive a notification, you want to know the type of the operation, the concerned item, and the new value of the item. This information is stored in the SHNOTIFYSTRUCT which the WParam points to. You can then add the new notification to an ArrayList that contains each notification.

public System.Collections.ArrayList NotificationsReceived = 
    new System.Collections.ArrayList();

public bool NotificationReceipt(IntPtr wParam, IntPtr lParam)
{
    SHNOTIFYSTRUCT shNotify = (SHNOTIFYSTRUCT) Marshal.PtrToStructure(
        wParam,
        typeof(SHNOTIFYSTRUCT));

    NotifyInfos info = new NotifyInfos((SHCNE)(int) lParam);
    
    // These notifications are Not supported
    if(info.Notification == SHCNE.SHCNE_FREESPACE ||
        info.Notification == SHCNE.SHCNE_UPDATEIMAGE)
        return(false);
    
    info.Item1 = GetPathFromPidl(shNotify.dwItem1);
    info.Item2 = GetPathFromPidl(shNotify.dwItem2);
    
    // Was this notification in the received notifications ?
    if(NotificationsReceived.Contains(info))
	    return(false);

    NotificationsReceived.Add(info);

    return(true);
}

If the notification was not in the NotificationsReceived list, the NotificationReceipt method adds it to the list and returns true.

Now, a new case must be added in the WndProc of the Form, so that the notifications can be handled:

The code below handles the notification by checking if it has been added to the list. if it isn't, NotificationReceipt returns true and the code calls NewOperation passing in the NotifyInfos struct corresponding to the latest event.

private ShellNotifications Notifications = new ShellNotifications();

protected override void WndProc(ref Message m)
{
    switch(m.Msg)
    {
        case (int) ShellNotifications.WM_SHNOTIFY:
            bool bAdded = Notifications.NotificationReceipt(
                m.WParam, m.LParam);

            if( bAdded )
            {
                NotifyInfos nis = (NotifyInfos) 
                    Notifications.NotificationsReceived[
                        Notifications.NotificationsReceived.Count - 1];

                NewOperation(nis);
            }

            break;
    }
    base.WndProc(ref m);
}

private void NewOperation(NotifyInfos infos)
{
    //
    // You can add your code here.
    // The line below is an example : 
    // it adds the operation informations in a listview.
    //
    listView1.Items.Add(
        new ListViewItem(
            new string[] {
                infos.Notification.ToString(),
                infos.Item1,
                infos.Item2}));
}