<p>For example: </p>

<p>First, say I have a Silverlight app with Windowless=true so that I can place ASP.NET controls on top of it.  Then I place an ASP.NET button on the page.  How can I have say the text of a control in the Silverlight app change when the user presses the ASP.NET button?  How would I send the Silverlight app an update message from the C# code that catches the Click of the ASP.NET button?</p>

<p>Thanks,
Jeff</p>

Splitting the content of your file into multiple files - one file per class may help.
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=283434
Up-to-date link:
https://developercommunity.visualstudio.com/content/problem/389773/vs-2017-keeps-removing-and-re-adding-aspxcodebehin.html