How do I detect changes to Known Folder Redirection

Question

When OneDrive is installed with Known Folder Move enabled, the first logon of a user starts with no redirection of known folders then midway through the session, they are redirected to the OneDrive folders.

How can I detect that?

I've already tried WM_SETTINGCHANGE and SHChangeNotifyRegister on CSIDL_Desktop to no avail.

Answer 1

It is a dirty undocumented hack, but if you monitor HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders for changes, it will trigger on redirection.

A C# example of how to do so is:

public sealed class KnownFolderWatcher : IDisposable
{
    private readonly SynchronizationContext SyncCtx;
    private readonly RegistryKey Key;
    private readonly Thread thRead;
    private readonly AutoResetEvent mreTriggered;

    public event EventHandler KeyChanged;

    private Exception Exception;
    private bool isDisposed;


    public KnownFolderWatcher(SynchronizationContext syncCtx)
    {
        this.SyncCtx = syncCtx ?? throw new ArgumentNullException(nameof(syncCtx));

        this.Key = Registry.CurrentUser.OpenSubKey(@"Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders", false)
            ?? throw new InvalidOperationException("Could not open User Shell Folders key");

        this.mreTriggered = new AutoResetEvent(false);

        this.thRead = new Thread(thRead_Main);
        this.thRead.Name = "Registry Change Reader";
        this.thRead.IsBackground = true;
        this.thRead.Start();
    }

    private void thRead_Main()
    {
        try
        {
            while (true)
            {
                NativeMethods.RegNotifyChangeKeyValue(Key.Handle, false, 4 /* REG_NOTIFY_CHANGE_LAST_SET */, mreTriggered.SafeWaitHandle, true);
                mreTriggered.WaitOne();
                if (isDisposed)
                {
                    break;
                }

                SyncCtx.Post(_1 =>
                {
                    KeyChanged?.Invoke(this, EventArgs.Empty);
                }, null);
            }
        }
        catch (Exception ex)
        {
            this.Exception = ex;
        }
    }

    public void Dispose()
    {
        if (isDisposed)
        {
            throw new ObjectDisposedException(nameof(KnownFolderWatcher));
        }
        isDisposed = true;

        mreTriggered.Set();
        thRead.Join();

        if (this.Exception != null)
        {
            throw new InvalidOperationException("Exception from read thread", Exception);
        }
    }
}

internal static class NativeMethods
{
    [DllImport("advapi32.dll", ExactSpelling = true, SetLastError = true)]
    internal static extern uint RegNotifyChangeKeyValue(SafeRegistryHandle key, bool watchSubTree, uint notifyFilter, SafeWaitHandle regEvent, bool async);
}