Debug.WriteLine() in Android .NET 9 no longer creates any output (regression, previously working in .NET 8)

[jonmdev]

Description

Since updating to .NET 9 I have seen I am not getting anything showing from Debug.WriteLine() when I run Android on a local device by USB in Debug mode.

To verify this, I created a simple project which is the base .NET 9 project with just the following added to App.xaml.cs:

IDispatcherTimer timer = Application.Current.Dispatcher.CreateTimer();
timer.Interval = TimeSpan.FromSeconds(1);
timer.Tick += delegate {
    Debug.WriteLine("SECOND PASSED: " + DateTime.UtcNow.Second);
};
Debug.WriteLine("START TIMER");
timer.Start();

In Windows I see the START TIMER and SECOND PASSED: output easily in my Output section of Visual Studio. But with an attached Samsung Android device and debugging to that, I do not see any such output anymore.

The same Android device allows Debug.WriteLine() to work with similar code in .NET 8, so this is not a device issue. Rather, I believe this is a .NET 9 issue.

Obviously we need Debug.WriteLine() to figure out issues as we are coding in Android so this would be very helpful to have fixed.

Steps to Reproduce

Open demo project, run in Debug mode in Windows and Android to device, see output in Windows only.

Link to public reproduction project repository

https://github.com/jonmdev/AndroidDotNet9DebugBug

Version with bug

9.0.21 SR2.1

Is this a regression from previous behavior?

Yes, this used to work in .NET MAUI

Last version that worked well

8.0.0-preview.1.7762

Affected platforms

Android

[Zhanglirong-Winnie]

This issue has been verified using Visual Studio 17.13 preview 2 (9.0.12 & 9.0.0 & 8.0.100 & 8.0.83 & 8.0.3). Can repro this issue on Samsung Android device and emulator platforms. The problem reappears in both .NET9 and .NET8.

[jpobst]

From my testing, this seems to work correctly using net9.0-android in Visual Studio 17.12.3 but does not work in Visual Studio 17.13 Preview 2. If this is the same thing others are seeing, then it feels like an issue with VS 17.13 P2.

Thus this issue likely occurs in the Visual Studio integration, which is owned by a different team who does not use GitHub for issue tracking.

Please use the tools available inside of VS to report this issue. This allows the issue to be routed to the correct team, and provides them the correct logs they need to diagnose it.

Definitely let them know that is it a VS 17.13 Preview regression and that it works in VS 17.12.3.