Within the WPF Designer, what does Enabling/Disabling Project Code do?

Question

I have no clue how long this feature/option has been around but my mouse happened to pause over this button within the WPF designer early and now I'm curious what it's purpose is.

Hovering over the button, Visual Studio pops up a tooltip that says Enable project code. When I click on it, the designer seems to reload the xaml contents but I can't tell what's changed.

What does this feature do?

2020 EDIT

It's been years since I've had to work in XAML, but I've received a new project where I need to work in XAML. This feature/button seems to have disappeared. Where has it migrated?

Answer 1

What does this feature do?

It enables or prevents the designer from running your code in the background.

Disabling project code can lead to a loss of design time data and enabling it can lead to unhandled exceptions in the XAML designer. These exceptions are usually caused by code that attempts to access properties or methods which work differently when your application is running in the designer compared to when it runs as a built executable.

Debugging or Disabling Project Code in XAML Designer: https://msdn.microsoft.com/en-us/library/mt622752.aspx?f=255&MSPPError=-2147217396