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copied!<p>I got this to work by implementing INotifyCollectionChanged explicitly (instead of using ObservableCollection). Furthermore, I found that using the Update action resulted in the same "not supported" error, but that I could use the Add and Remove actions. So my swap function ends up looking like this:</p> <pre><code>class MyCollection<T> : List<T>, INotifyCollectionChanged { public event NotifyCollectionChangedEventHandler CollectionChanged; private void swap( int i, int j ) { T a = this[i]; T b = this[j]; // swap my own internal data storage this[i] = b; this[j] = a; // and also let my CollectionChanged listener know that I have done so. if( CollectionChanged != null ) { NotifyCollectionChangedEventArgs arg; arg = new NotifyCollectionChangedEventArgs( NotifyCollectionChangedAction.Remove, a, i ); CollectionChanged( this, arg ); arg = new NotifyCollectionChangedEventArgs( NotifyCollectionChangedAction.Add, b, i ); CollectionChanged( this, arg ); arg = new NotifyCollectionChangedEventArgs( NotifyCollectionChangedAction.Remove, b, j ); CollectionChanged( this, arg ); arg = new NotifyCollectionChangedEventArgs( NotifyCollectionChangedAction.Add, a, j ); CollectionChanged( this, arg ); } } } </code></pre> <p>The dynamic changes are fairly local, so fortunately using a slower handwritten sort in response to changes is working OK for me. In other words, when updates arrive, I invoke another member function (in the same collection) that looks something like this:</p> <pre><code>public void ProcessUpdates( List<T> updateList ) { // use the contents of updateList to modify my internal store // ... // and now resort myself sort(); } private void sort() { // implement your favorite stable sorting algorithm here, calling // swap() whenever you swap two elements. // (this is an intentionally facetious sorting algorithm, because I // don't want to get into the long and irrelevant details of my own // data storage.) while( i_am_not_sorted() ) { int i = random_index(); int j = random_index(); if( out_of_order(i,j) ) { // modify my internal data structure and // also let my CollectionChanged listener know that I have done so swap( i, j ); } } } </code></pre> <p>Don't forget that it's also necessary to fire an "Add" notification when adding elements to the collection! I sort the initial list and then add in sorted order, which lets me use a more efficient library sort when I first populate the data. </p>

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