Thread.CurrentPrincipal not flowing via Task.Run but does flow with Task.Factory.StartNew long running

[dazinator]

Steps to Reproduce

1. Within an Xamarin.Android application, on the Main Thread, Set Thread.CurrentPrincipal to a custom Principle which is serializable.
2. await a Task.Run()
3. Inside the Task, if it executes on the same thread then Thread.CurrentPrincipal is still set correctly.
4. Inside the Task, if it is executing on a different thread from the main thread, it's Thread.CurrentPrincipal value is lost (it's a GenericPrincipal again).

If you do the same, but use Task.Factory.StartNew with TaskCreationOptions.LongRunning, instead of Task.Run, then the Custom Principal does flow to the new thread.

Is this a bug?

Logged stack overflow here: https://stackoverflow.com/questions/47926132/xamarin-android-task-run-vs-task-factory-startnew-and-thread-currentprincipal

               // on main thread.
                var threadId = Thread.CurrentThread.ManagedThreadId; // == 1              
                var principle = Thread.CurrentPrincipal; // == Custom Principle set earlier via login screen.

                await Task.Run(() =>
                {

                    var afterThreadId = Thread.CurrentThread.ManagedThreadId; // != 1 (i.e different thread.)
                    var afterPriniple = Thread.CurrentPrincipal; // == != Custom Principle has not flowed.
                  
                });

This is different from the behaviour exhibited by .NET 4.7 (haven't tried other .NET framework versions).

Here is a unit test that passes under .NET 4.7 and fails under Xamarin.Android

        [Fact]
        public async Task LoginService_WhenUserLogsIn_PrincipalFlowsToAsyncTask()
        {    

            var mockIdentity = new MockIdentity(true);
            var mockPrincipal = new MockPrincipal(mockIdentity);          
            Thread.CurrentPrincipal = mockPrincipal ;          

            await Task.Factory.StartNew(async () =>
            {
                var newThreadId = Thread.CurrentThread.ManagedThreadId; // on different thread.
                Assert.True(Thread.CurrentPrincipal.Identity.IsAuthenticated);
                Assert.Equal(mockPrincipal, Thread.CurrentPrincipal);
               
                await Task.Factory.StartNew(() =>
                {
                     // still works even when nesting..
                    newThreadId = Thread.CurrentThread.ManagedThreadId;
                    Assert.True(Thread.CurrentPrincipal.Identity.IsAuthenticated);
                    Assert.Equal(mockPrincipal, Thread.CurrentPrincipal);

                }, TaskCreationOptions.LongRunning);

            }, TaskCreationOptions.LongRunning);

            await Task.Run(() =>
            {
               // Following works on NET4.7 and fails under Xamarin.Android.
                var newThreadId = Thread.CurrentThread.ManagedThreadId;
                Assert.True(Thread.CurrentPrincipal.Identity.IsAuthenticated);
                Assert.Equal(mockPrincipal, Thread.CurrentPrincipal);

            });

        }

Expected Behavior

Custom CurrentPrincipal should flow via Execution Context

Actual Behavior

Custom CurrentPrincipal isn't flowing..

Version Information

Microsoft Visual Studio Professional 2017
Version 15.5.2
VisualStudio.15.Release/15.5.2+27130.2010
Microsoft .NET Framework
Version 4.7.02556

Installed Version: Professional

Visual Basic 2017 00369-60000-00001-AA768
Microsoft Visual Basic 2017

Visual C# 2017 00369-60000-00001-AA768
Microsoft Visual C# 2017

Visual C++ 2017 00369-60000-00001-AA768
Microsoft Visual C++ 2017

Visual F# 4.1 00369-60000-00001-AA768
Microsoft Visual F# 4.1

.NET Portability Analyzer 1.1.10808.0
Evaluates portability of assemblies across .NET platforms.

Application Insights Tools for Visual Studio Package 8.10.01106.1
Application Insights Tools for Visual Studio

ASP.NET and Web Tools 2017 15.0.31125.0
ASP.NET and Web Tools 2017

ASP.NET Core Razor Language Services 1.0
Provides languages services for ASP.NET Core Razor.

ASP.NET Web Frameworks and Tools 2017 5.2.51007.0
For additional information, visit https://www.asp.net/

Azure App Service Tools v3.0.0 15.0.31106.0
Azure App Service Tools v3.0.0

Common Azure Tools 1.10
Provides common services for use by Azure Mobile Services and Microsoft Azure Tools.

JavaScript Language Service 2.0
JavaScript Language Service

JavaScript Project System 2.0
JavaScript Project System

JavaScript UWP Project System 2.0
JavaScript UWP Project System

Merq 1.1.17-rc (cba4571)
Command Bus, Event Stream and Async Manager for Visual Studio extensions.

Microsoft Azure Tools 2.9
Microsoft Azure Tools for Microsoft Visual Studio 2017 - v2.9.51120.3

Microsoft Continuous Delivery Tools for Visual Studio 0.3
Simplifying the configuration of continuous build integration and continuous build delivery from within the Visual Studio IDE.

Microsoft JVM Debugger 1.0
Provides support for connecting the Visual Studio debugger to JDWP compatible Java Virtual Machines

Microsoft MI-Based Debugger 1.0
Provides support for connecting Visual Studio to MI compatible debuggers

Microsoft Visual C++ Wizards 1.0
Microsoft Visual C++ Wizards

Microsoft Visual Studio Tools for Containers 1.1
Develop, run, validate your ASP.NET Core applications in the target environment. F5 your application directly into a container with debugging, or CTRL + F5 to edit & refresh your app without having to rebuild the container.

Microsoft Visual Studio VC Package 1.0
Microsoft Visual Studio VC Package

Mono Debugging for Visual Studio 4.8.4-pre (3fe64e3)
Support for debugging Mono processes with Visual Studio.

NuGet Package Manager 4.5.0
NuGet Package Manager in Visual Studio. For more information about NuGet, visit http://docs.nuget.org/.

SettingsWindow Extension 1.0
SettingsWindow Visual Studio Extension Detailed Info

SQL Server Data Tools 15.1.61710.120
Microsoft SQL Server Data Tools

TypeScript Tools 15.5.11025.1
TypeScript Tools for Microsoft Visual Studio

Visual Studio Code Debug Adapter Host Package 1.0
Interop layer for hosting Visual Studio Code debug adapters in Visual Studio

Visual Studio Tools for CMake 1.0
Visual Studio Tools for CMake

Visual Studio Tools for Universal Windows Apps 15.0.27128.01
The Visual Studio Tools for Universal Windows apps allow you to build a single universal app experience that can reach every device running Windows 10: phone, tablet, PC, and more. It includes the Microsoft Windows 10 Software Development Kit.

VisualStudio.Mac 1.0
Mac Extension for Visual Studio

Xamarin 4.8.0.753 (6575bd113)
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.

Xamarin Designer 4.8.188 (c5813fa34)
Visual Studio extension to enable Xamarin Designer tools in Visual Studio.

Xamarin.Android SDK 8.1.0.25 (HEAD/d8c6e504f)
Xamarin.Android Reference Assemblies and MSBuild support.

Xamarin.iOS and Xamarin.Mac SDK 11.6.1.2 (6857dfc)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

Log File

The above command doesn't work on Windows using VS.

[dazinator]

I have updated this with more information, as I can now replicate this.

[dazinator]

Added unit test code.

[dazinator]

I have added more detail to the stack overflow including screenshots of what things look like whilst debugging. (Thread Id's, Default and Current Schedulers, SynchronizationContext etc).

[dazinator]

Closing as this has now been addressed via PR: mono/mono#6326