Silverlight Error "Layout Cycle Detected Layout could not complete" when using custom control

Question

I'm building a custom control in Silverlight by deriving from ContentControl and doing some special formatting to put a dropshadow behind the contents.

I've nearly got it working but have recently ran into a bizarre error. It works fine if it contains anything besides a Border, or a Grid/Stackpanel/etc that does not have an explicitly defined height and width.

I get a JavaScript error in IE, and the text says:

Runtime Error 4008... Layout Cycle Detected... Layout Could Not Complete.

If I specify a height and width on the contained grid/stackpanel/etc it works fine.

There is a ton on the web about this error when too many textboxes are used (over 250), but I'm able to reproduce my error with a single button in a grid.

I have no textboxes at all on the page. The error has to do with a detected infinite loop. I set a few breakpoints in the code and it seems that the "SizeChanged" event is getting called a lot during rendering, and each time the height/width increments by 10.

I'm assuming that setting a default height/width causes it to skip this incrementing of the number, but I have no idea why this error is happening.

Has anyone ran into this or have any ideas?

Answer 1

There is a good blog post on this error here.

Bascially what can happen is you're changing some size in a MeasureOverride somewhere which causes another measure, which changes the size, which causes a measure and so on. I ran into this once before and fixed it by removing any code that caused a layout update or triggered a layout update during the layout cycle.

Update: Since the blog post is gone, quoting it here in full:

Continuing my series of gotchas for Silverlight 2, I wanted to talk about a common error that people are seeing. This error is something new that you might see when moving code from Beta 2 to the Release Candidate or later. In Beta 2, if the layout engine detected a cycle, it didn't throw any errors; as I understand it, the layout was just aborted. But with post Beta2 bits, an error is thrown.

The error you'll get will specify "Layout Cycle Detected" as the message. This error message is very accurate--the layout engine detected a cycle within your layout; or another way to say it, you have an infinite loop in your layout.

The biggest culprit that leads to this error is code within the LayoutUpdated event handler. If your LayoutUpdated event handler does anything to alter the layout of your control, then that will cause the LayoutUpdated event to fire again, and again, and again... :-)

Sometimes you need to have layout altering code within this event handler though, so what is one to do?

First, you should consider whether you really need the layout changes to occur on every call to LayoutUpdated. Would it suffice to handle the Loaded event as well as the Application.Current.Host.Content.Resized event. Between these two events, you'll get notified when the control is loaded into the visual tree, and you'll get notified any time the host is resized, which could cause you to need to change your layout again. Scenarios like modal dialogs should fall into this category.

Second, if you really do need to use LayoutUpdated, you might just need to put some conditions around your layout changes. For instance, if you are calculating a new width and height for your control, before you actually set the width and height, check to make sure the current values differ from what you calculated. This will allow the first LayoutUpdated event to resize your control, which triggers another LayoutUpdated event, but that event will recognize that there's no work to do, and the cycle will end.

These same rules will apply when you're handling the SizeChanged event, or if you're doing any other overrides on the layout of your control.

Answer 2

A common cause is handling SizeChanged and then in the handler doing something that affects the size of the element. Sometimes this is not obvious - it could be modifying child elements which affect the size of their container for instance.