[templates] Default $(SupportedOSPlatformVersion) to 24.

[jpobst]

Fixes: #9517
Context: dotnet/android-libraries#767
Context: dotnet/android-libraries#767 (comment)

For .NET 10, update the dotnet new android template so that $(SupportedOSPlatformVersion) is 24, bumping from the current default of 21.

The reason for this is that "desugaring" "moves" Java methods to locations that we don't expect, which can result in AbstractMethodErrors at runtime. Setting the minimum SDK version to >= 24 avoids this desugaring step, preventing the Java methods from being moved in a manner we don't expect, and thus avoiding the AbstractMethodError.

Note that 21 will still be the supported minimum for those that need it, however this will keep most users who do not need to support devices that old from having desugaring issues.

[jpobst]

This "fixes" the issue going forward. Should we also add a warning when $(SupportedOSPlatformVersion) < 24 to encourage users to update their existing projects?

Edit: I guess this is part of the #9527 proposal.

[jonathanpeppers]

If $(SupportedOSPlatformVersion) is blank, should we also hardcode 24 here?

• android/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.DefaultProperties.targets

  Lines 35 to 36 in ae63447

  	<!-- Use $(AndroidMinimumSupportedApiLevel) for $(SupportedOSPlatformVersion) if unset -->
  	<SupportedOSPlatformVersion Condition=" '$(SupportedOSPlatformVersion)' == '' ">$(AndroidMinimumSupportedApiLevel)</SupportedOSPlatformVersion>

[jonpryor]

@jonathanpeppers asked:

If $(SupportedOSPlatformVersion) is blank, should we also hardcode 24 here?

My gut feeling is "no", because it makes sense to me that if it isn't set, it should default to the minimum supported, vs. the "minimum recommended".

My sense of what makes sense may be completely wrong.

Any idea in what scenarios $(SupportedOSPlatformVersion) wouldn't be set? All I can think of is someone manually migrating a legacy Xamarin.Android project to a .NET project.

[jpobst]

My gut feeling is "no", because it makes sense to me that if it isn't set, it should default to the minimum supported, vs. the "minimum recommended".

This may depend on if we intend to add a warning when targeting 21. If we do, we probably shouldn't default to a version that generates a warning.

[jonathanpeppers]

I think we can take this as-is for now, and follow up with more with:

• [.NET 10] invent a "recommended min OS version" for 24 #9527