How to tell if user has modified data using bindingsource?

If your object within the List support the INotifyPropertyChanged event and you replace the List<T> by a BindingList<T> you can subscribe to the ListChanged event of the BindingList to get informed about any changes made by the user.

If you're bound to a DataSet then you're in luck: it has a HasChanges Property. You can get the actual changes by calling GetChanges on the dataset. This returns a new dataset containing a copy of all changed rows

After trying different thing I ended up with this piece of code:

private MyClass currentItem = null;
private bool itemDirty = false; // can be used for "do you want to save?"

private void bindingSource_CurrentChanged(object sender, EventArgs e)
{
    var handler = new PropertyChangedEventHandler((s, e2) => itemDirty = true);

    var crnt = currentItem as INotifyPropertyChanged;
    if(crnt != null) crnt.PropertyChanged -= handler;

    currentItem = (MyClass)bindingSource.Current;

    crnt = currentItem as INotifyPropertyChanged;
    if(crnt != null) crnt.PropertyChanged += handler;

    itemDirty = false;
}

It works fine for me, although I save lots of state information in the Windows Form's instance fields. However, twiddling with CurrentChanged and CurrentItemChanged did not help me.