Upgrading from Xamarin.Android to net7.0-android

[vividos]

I'm currently evaluating how to upgrade existing apps built with Xamarin.Android (and Xamarin.Forms) to .NET 7. I found an article from @jamesmontemagno in the DevBlogs that was very interesting:
https://devblogs.microsoft.com/xamarin/upgrade-xamarin-ios-android-to-dotnet6-dotnet7/

The article also points to an interesting section in the Docs:
https://learn.microsoft.com/dotnet/maui/migration/
and especially:
https://learn.microsoft.com/dotnet/maui/migration/native-projects

I always thought that the build stuff used in Xamarin.Android was also used as base for the .NET 5/6/7 implementation for Android, and I think my experiments confirm this.

I have a few questions, though, and I think it's a good place here in the Discussions to ask and discuss and also ask other developers what your experiences are.

Note that this is not about converting a Forms app to .NET MAUI.

1. Compiling Xamarin.Forms apps on Android using net7.0-android?
   I just tried out if that path even works, so I created an empty "Android Application" project (not an "Android App (Xamarin)"), and it generates me a nice net7.0-android targeted app project. My next step was to create a "Mobile App (Xamarin.Forms)", which creates me the platform specific project(s) and a netstandard2.0 project with Forms code in it. I immediately throw away the platform specific projects, as I want to use the net7.0-android project for building. I have to copy over some code in MainActivity.cs from the non-SDK-style Android project (e.g. deriving from FormsAppCompatActivity, Forms.Init and loading the Forms app). To my surprise, everything compiles, links and deploys and I get a basic Forms app using .NET 7.
   Some questions already arise here:

• How does that even work? Am I just lucky?
• The Forms project still targets netstandard2.0 - is this a good thing? Or would converting it to net7.0 make sense? Or does it have to be net7.0-android?

2. What to tell maintainers of NuGet packages?
   Now I wanted to try upgrading actual apps of me (both personal and work projects) in order to get some experience. For that, I have to actually upgrade some projects with code and NuGet packages in it.
   James writes in his blog article that some NuGet packages must be updated to work with the migrated net7.0-android targeted projects. Sometimes, when the original maintainer of a NuGet package is not available, I even might need to fork the project and do the changes (if any?) myself.
   Also, some time ago I saw that NuGet.org's packages page (e.g. here: https://www.nuget.org/packages/Xamarin.CommunityToolkit/) lists all supported TargetFrameworks (in the "Frameworks" tab). The blue target frameworks are directly supported ("Compatible target framework(s)"), and the light blue target frameworks are derived from them somehow ("Additional computed target framework(s)").

• So what does that mean when a NuGet package only has net7.0-android listed as Additional computed target framework? Does the package restore correctly and can be used in a net7.0-android project?
• If the answer is yes, I guess there is a mapping somewhere, where the build maps "MonoAndroid" to net7.0-android. Does it work that way? If yes, can we see that mapping (logic) somewhere?
• What do I tell NuGet package maintainers so that they can upgrade their packages to work with the shiny new .NET 7 Android apps?
• Or what changes do I have to do if I'm the maintainer of such a project myself?

3. What to do with MSBuild.Sdk.Extras based projects?
   I have some projects where I wanted to put all code, platform specific and platform agnostic, in one project. I copied Xamarin.Essentials' approach and use the MSBuild.Sdk.Extras SDK to do "poor-devs multi-targeting". I guess some NuGet package maintainers used the same approach to structure their packages.

• What do I do with those projects? As the MSBuild.Sdk.Extras isn't updated anymore and most probably not supported by Microsoft officially, I guess they should be converted to real SDK-style projects? Can I still target MonoAndroid there? My guess is no...

4. NETSDK1136: The target platform must be set to Windows
   I know this paragraph might better be an Issue, but maybe it plays a role in this discussion as well. My story continues with adding some NuGet packages, which most of them not aware of the new .NET 5/6/7 world, since they were created in the Xamarin.Forms era. After some of the NuGet packages are added, the NETSDK1136 error appears, saying I must target net7.0-windows as well. Is that an effect of the NuGet packages not supporting some target frameworks? The whole error line in Diagnostic verbosity is, btw:

Target _CheckForTransitiveWindowsDesktopDependencies:
  Using "NetSdkError" task from assembly "C:\Program Files\dotnet\sdk\7.0.203\Sdks\Microsoft.NET.Sdk\targets\..\tools\net472\Microsoft.NET.Build.Tasks.dll".
  Task "NetSdkError"
    C:\Program Files\dotnet\sdk\7.0.203\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.DefaultItems.Shared.targets(250,5): error NETSDK1136: The target platform must be set to Windows (usually by including '-windows' in the TargetFramework property) when using Windows Forms or WPF, or referencing projects or packages that do so.
  Done executing task "NetSdkError" -- FAILED.
Done building target "_CheckForTransitiveWindowsDesktopDependencies" in project "Net7AndroidAppTest.csproj" -- FAILED.

Thanks for reading through all of this! I'm sorry it's that long, but I can imagine that some developers have the same questions during their evaluation of how to migrate their apps!

[davidortinau]

@jonathanpeppers can probably explain the tech best. Short version is that Android libraries are largely compatible with .NET 6+ and don't require a recompilation like Xamarin.iOS libraries do.

what does that mean when a NuGet package only has net7.0-android listed as Additional computed target framework?

NuGet.org - the light blues are not guaranteed. NuGet.org is guessing. So you have to try them to see if the package is indeed compatible. Our guidance for library developers is to explicitly add support for .NET 6+ TFMs. This will make them dark blue and provide that confidence to consumers of the packages.

Yes there is an inheritance relationship.

what changes do I have to do if I'm the maintainer of such a project myself?

Add the .NET 7+ TFM, recompile and repack. See @Sweekriti91 's comment here dotnet/docs-maui#289 (comment)

I'll defer the rest of your questions to others.

fyi @Redth @mattleibow

[jonathanpeppers]

On porting a Xamarin.Forms/Xamarin.Android app to .NET 7:

How does that even work? Am I just lucky?

We've kept Mono.Android.dll binary compatible from Xamarin.Android -> .NET 6/7/8.

However, it doesn't mean that everything will still work. Note that Xamarin.Forms projects are not officially supported in .NET 6+. When you have a Xamarin.Android class library, it was compiled against Xamarin/Mono's BCL. This is similar to migrating from .NET framework to .NET Core -- the BCL is different, so some random System.* classes may cause builds to fail or not function the same at runtime. netstandard2.0 libraries are a bit safer, in that they at least are compiled against a subset of the BCL and the .NET team has better compatibility there, in general.

For the best results, I would recommend moving to at least net6.0-* projects if you are a NuGet library author and you have .NET 6 customers. If you are just building apps, I would move to .NET 7 (or latest when ready).

[jamesmontemagno]

Yes, you don't want to port a Xamarin.Forms app to .NET 6/7, you want to port it to .NET MAUI. Many of those things apply...

• Xamarin.Essentials -> Now built in with .NET MAUI Essentials
• Xamarin Community Toolkit -> .NET MAUI Community Toolkit

My article was more about non-Xamarin.Forms apps and migrating them up.

For Libraries you can still multi-target both older Xamarin targets and new .NET targets. See what I do here for Xamarin.Essentials and .NET MAUI Essentials for the Android bits -> https://github.com/jamesmontemagno/InAppBillingPlugin/blob/master/src/Plugin.InAppBilling/Plugin.InAppBilling.csproj

[Regis-Caelum]

Hi @jamesmontemagno,

I've been trying to create an app using .NET MAUI where I'm implementing Firebase authentication for mobile phone number verification. I'm using .NET 7 SDK, and the package I'm using for Firebase authentication is Xamarin.Firebase.Auth version 121.0.8. This is because Firebase Admin SDK doesn't support mobile phone verification.

However, I'm encountering issues when trying to upgrade the package to the latest version. Whenever I attempt to upgrade the package, it results in the following conflict:

Error NU1605 : Warning As Error: Detected package downgrade: Xamarin.AndroidX.Browser from 1.5.0.2 to 1.4.0.3. Reference the package directly from the project to select a different version. 
 YabieFrontend -> Xamarin.Firebase.Auth 122.0.0.2 -> Xamarin.AndroidX.Browser (>= 1.5.0.2) 
 YabieFrontend -> Xamarin.AndroidX.Browser (>= 1.4.0.3)

I also attempted to manually upgrade Xamarin.AndroidX.Browser, but this led to the following runtime exception:

Error JAVA0000 java: Error in /Users/inshalkhan/.nuget/packages/xamarin.androidx.lifecycle.common/2.6.2.1/buildTransitive/net6.0-android31.0/../../jar/androidx.lifecycle.lifecycle-common.jar:androidx/lifecycle/DispatchQueue.class:
Type androidx.lifecycle.DispatchQueue is defined multiple times: /Users/inshalkhan/.nuget/packages/xamarin.androidx.lifecycle.common/2.6.2.1/buildTransitive/net6.0-android31.0/../../jar/androidx.lifecycle.lifecycle-common.jar:androidx/lifecycle/DispatchQueue.class, obj/Debug/net7.0-android/lp/152/jl/classes.jar:androidx/lifecycle/DispatchQueue.class
Compilation failed
java.lang.RuntimeException: com.android.tools.r8.CompilationFailedException: Compilation failed to complete, origin: /Users/inshalkhan/.nuget/packages/xamarin.androidx.lifecycle.common/2.6.2.1/buildTransitive/net6.0-android31.0/../../jar/androidx.lifecycle.lifecycle-common.jar
androidx/lifecycle/DispatchQueue.class
	at com.android.tools.r8.internal.Fj.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:98)
	at com.android.tools.r8.D8.main(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:4)
Caused by: com.android.tools.r8.CompilationFailedException: Compilation failed to complete, origin: /Users/inshalkhan/.nuget/packages/xamarin.androidx.lifecycle.common/2.6.2.1/buildTransitive/net6.0-android31.0/../../jar/androidx.lifecycle.lifecycle-common.jar:androidx/lifecycle/DispatchQueue.class
	at Version.fakeStackEntry(Version_3.3.75.java:0)
	at com.android.tools.r8.internal.Fj.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:75)
	at com.android.tools.r8.internal.Fj.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:28)
	at com.android.tools.r8.internal.Fj.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:27)
	at com.android.tools.r8.internal.Fj.b(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:2)
	at com.android.tools.r8.D8.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:22)
	at com.android.tools.r8.D8.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:17)
	at com.android.tools.r8.internal.Fj.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:85)
	... 1 more
Caused by: com.android.tools.r8.internal.f: Type androidx.lifecycle.DispatchQueue is defined multiple times: /Users/inshalkhan/.nuget/packages/xamarin.androidx.lifecycle.common/2.6.2.1/buildTransitive/net6.0-android31.0/../../jar/androidx.lifecycle.lifecycle-common.jar:androidx/lifecycle/DispatchQueue.class, obj/Debug/net7.0-android/lp/152/jl/classes.jar:androidx/lifecycle/DispatchQueue.class
	at com.android.tools.r8.internal.JT.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:14)
	at com.android.tools.r8.internal.JT.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:22)
	at com.android.tools.r8.internal.HN.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:33)
	at com.android.tools.r8.internal.HN.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:10)
	at java.base/java.util.concurrent.ConcurrentHashMap.merge(ConcurrentHashMap.java:2048)
	at com.android.tools.r8.internal.HN.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:6)
	at com.android.tools.r8.graph.B2$a.e(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:4)
	at com.android.tools.r8.dex.b.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:110)
	at com.android.tools.r8.dex.b.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:28)
	at com.android.tools.r8.D8.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:25)
	at com.android.tools.r8.D8.d(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:606)
	at com.android.tools.r8.D8.c(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:1)
	at com.android.tools.r8.internal.Fj.a(R8_3.3.75_b7a6ff6b13548611571508fe72282c9167faa649161ca0013edfc92e19bd7e58:24)
	... 5 more
Directory 'obj/Debug/net7.0-android/lp/152' is from 'androidx.lifecycle.lifecycle-runtime-ktx.aar'.

I've tried various solutions found on Stack Overflow, such as cleaning and rebuilding the project, but nothing seems to be resolving the issue. When I revert to the older version I'm currently using, I encounter a runtime exception related to the setPhoneNumber method.

System.InvalidCastException: Specified cast is not valid.
   at YabieFrontend.Services.AuthenticationService.AuthenticatePhoneNumber(String phoneNumber) in /Users/inshalkhan/RiderProjects/YabieFrontend/Platforms/Android/Services/AuthenticationService.cs:line 19
   at YabieFrontend.ViewModel.LoginViewModel.Login() in /Users/inshalkhan/RiderProjects/YabieFrontend/ViewModel/LoginViewModel.cs:line 26
   at CommunityToolkit.Mvvm.Input.AsyncRelayCommand.AwaitAndThrowIfFailed(Task executionTask) in /_/src/CommunityToolkit.Mvvm/Input/AsyncRelayCommand.cs:line 351
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_0(Object state)
   at Android.App.SyncContext.<>c__DisplayClass2_0.<Post>b__0() in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.App/SyncContext.cs:line 36
   at Java.Lang.Thread.RunnableImplementor.Run() in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Java.Lang/Thread.cs:line 36
   at Java.Lang.IRunnableInvoker.n_Run(IntPtr jnienv, IntPtr native__this) in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/obj/Release/net7.0/android-33/mcw/Java.Lang.IRunnable.cs:line 84
   at Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PP_V(_JniMarshal_PP_V callback, IntPtr jnienv, IntPtr klazz) in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:line 22

Here's my code for reference:

public Task<bool> AuthenticatePhoneNumber(string phoneNumber)
    {
        _verificationCompletionSource = new TaskCompletionSource<bool>();

        
            var authOption = PhoneAuthOptions
                .NewBuilder()
                .SetPhoneNumber(phoneNumber)
                .SetTimeout((Long)60, TimeUnit.Seconds)
                .SetActivity(Platform.CurrentActivity)
                .SetCallbacks(this)
                .Build();

            PhoneAuthProvider.VerifyPhoneNumber(authOption);
        

        return _verificationCompletionSource.Task;
    }

Any assistance with resolving these issues would be greatly appreciated.

[vividos]

Maybe it's best when you open a new discussion topic, I think nobody looks at old and closed discussions...

[Regis-Caelum]

Thank you for pointing that out!.I appreciate your guidance, and I'll create a new discussion right away.

[vividos]

Thank you for all the answers - this makes it clearer!
Btw, I openend a ticket for the NETSDK1136 error: dotnet/android#7987