Virtualizing WPF Wrap Panel Issue

Question

There are not very many options for a virtualizing wrap panel for use in WPF. For one reason or another MS decided to not ship one in the standard library.

If anyon

Answer 1

First, beware that in general, if you are removing an object from a collection and you don't have it's reference, that object is dead at the point of removal. So at the very least RemoveInternalChildRange call is illegal after removal but that's not the core issue.

Second, you might be having a little race condition, even if it's not strictly multi-threaded. Have to check (with breakpoint) if that event handler is reacting too eagerly - you don't want event handler running while you are still in the middle of a removal even if it's a single item.

Third, check for null after:

UIElement child = _generator.GenerateNext(out newlyRealized) as UIElement;

and for the first trial change the code to have a graceful exit, which in this case means gracefull continue - have to use for loop and increments in the loop to be able to do continue at all.

Also check InternalChildren whne you see that null to see if that access path gives the same result as your _children (as in size, internal data, null in the same place).

If just skipping a null survives (renders without exceptions) stop it in debugger right after that and check if these arrays/collections got settled (no nulls inside).

Also, post the fully compilable sample project that gives the repro (as a zip file) somewhere - reduces random assumprions and allows ppl to just build/run and see.

Speaking of assumptions - check what's your "observable collection" doing. If you are removing an item from a collection, any and every iterator/enumerator from a prior state of that collection has the right to throw or give nulls and in a UI that tries to be too smart, having a stale iterator can happen easily.