How to debug Visual Studio 2012 instance design-time

In Visual Studio 2012, the WPF designer runs in a separate process. Try fiddling with that process. It should be easy to find out the process name.

The process is called XDesProc, and it is created with each new Preview Window. Therefor: to debug a preview window, open a second instance of Visual Studio 2012 with the same solution, set your breakpoints and attach to XDesProc. It's recommended to have only one preview window open to attach to the right process. Otherwise use Process Explorer to find the correct process.

Make sure you have the "Enable/Disable Project Code" toggle set to "enabled"...in VS/Blend 2015 it's a small icon below the XAML editoras shown here.

If it's disabled, this could be the reason your design time instance doesn't seem to be working.

Further, if it's disabled and you attempt to debug using a second VS instance, then when you attach to the XDesProc any breakpoints you set in the code-behind will report that they "will not currently be hit. [because] No symbols have been loaded for this document".You might think that you need to somehow load the symbols, but if you open up the Modules window attempting to do so, you won't even see your module in the listing.

I lost a couple hours on this issue because of this setting being disabled. Hope this helps others not do the same. Because of that, I'll post this on a few of the related questions.

BTW, there's an excellent description of this debugging procedure by James McNellis on one of those related questions.