Will add the error to the debug output.
Any chance you could provide a repro application for this? It's quite strange that snoop tries to attach to an application that does not seem to be a WPF application.

This is a WPF element hosted in ElementHost inside System.Windows.Forms.UserControl hosted in an ActiveX container, i.e. VSTO add-in.

Snoop 2 showed a message box instead (maybe that's an alternative for when STA is not available), saying Can't find a current application or a PresentationSource root visual!.

Probably a separate issue, but if that is still the error it is trying to show, can we get at least the tree and properties up without a PresentationSource? The Visual Studio visualizer can do that even in this case (not the Live Tree though).

Could you try the latest build from appveyor and see if your issue is really fixed?
There should be no error dialog if only one app domain fails.

You can grab the latest source build from https://ci.appveyor.com/project/batzen/snoopwpf/branch/develop/artifacts

Okay, so I confirm that if I choose to not snoop multiple domains, I get an error dialog now (with the error as expected above).

If I choose to debug multiple domains, I get the new output and a new exception I believe:

Starting snoop UI...
Could not snoop a specific app domain with friendly name of "DefaultDomain" in multiple app domain mode.
System.InvalidOperationException: The calling thread must be STA, because many UI components require this.
   at System.Windows.Input.InputManager..ctor()
   at System.Windows.Input.InputManager.GetCurrentInputManagerImpl()
   at System.Windows.Input.KeyboardNavigation..ctor()
   at System.Windows.FrameworkElement.FrameworkServices..ctor()
   at System.Windows.FrameworkElement.EnsureFrameworkServices()
   at System.Windows.FrameworkElement..ctor()
   at System.Windows.Controls.Control..ctor()
   at System.Windows.Window..ctor()
   at Snoop.Windows.SnoopBaseWindow..ctor() in C:\projects\snoopwpf\Snoop.Core\Windows\SnoopBaseWindow.cs:line 13
   at Snoop.SnoopUI..ctor() in C:\projects\snoopwpf\Snoop.Core\SnoopUI.xaml.cs:line 52
   at Snoop.Infrastructure.SnoopManager.<>c.<GetInstanceCreator>b__3_0() in C:\projects\snoopwpf\Snoop.Core\Infrastructure\SnoopManager.cs:line 155
   at Snoop.Infrastructure.SnoopManager.SnoopApplication(TransientSettingsData settingsData, Func`1 instanceCreator) in C:\projects\snoopwpf\Snoop.Core\Infrastructure\SnoopManager.cs:line 177
   at Snoop.Infrastructure.SnoopManager.GoBabyGoForCurrentAppDomain(TransientSettingsData settingsData) in C:\projects\snoopwpf\Snoop.Core\Infrastructure\SnoopManager.cs:line 130
'WINWORD.EXE' (CLR v4.0.30319: MyAddin.vsto|vstolocal): Loaded 'C:\Users\Myself\AppData\Local\assembly\dl3\POJM97RW.QQ0\B0ROJTNR.DQQ\d21304c4\005419d9_5ed3d501\Snoop.Core.dll'. Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
System.InvalidCastException: Unable to cast transparent proxy to type 'Snoop.Infrastructure.SnoopManager'.
   at Snoop.Infrastructure.SnoopManager.StartSnoopInstance(String settingsFile) in C:\projects\snoopwpf\Snoop.Core\Infrastructure\SnoopManager.cs:line 97
   at Snoop.Infrastructure.SnoopManager.StartSnoop(String settingsFile) in C:\projects\snoopwpf\Snoop.Core\Infrastructure\SnoopManager.cs:line 22

The element I am trying to snoop is in MyAddin.vsto|vstolocal AppDomain by the way, i.e. not DefaultDomain. I wonder if rather than default or all domains, I could pick which AppDomain I want to snoop.

I added more error handling and more logging. Could you give it another try?
If you could provide a repro it would be easier to fix this ;-)

Indeed, got the message box in both cases now!

But were you able to see your elements?
Snoop should be able to work in the case you described.

No. I have a repro but it's basically same as in #116. I am running x64 Office though, could that make a difference?

You are right.
I broke the feature completely.
Will investigate and fix it.

After some investigation it seems as if Microsoft changed something on their side which currently blocks Snoop from working correctly.
The code i wrote for cross app domain to work made some assumptions which are not always true.
So it's not really Microsofts fault but mine because i didn't think about every possible way someone could create an app domain.
It will take me a few iterations to get it working correctly.
Will mention you here if i think that it's fixed.

@miloush This should work now. At least it works on my machine. ;-)
Could you give it another try?

@batzen it does indeed, thank you very much!