[One .NET] support latest C# 10 language features

[jonathanpeppers]

Context: dotnet/sdk#19521
Fixes: #6075
Fixes: #6076

We need to make two sets of changes for C# 10:

1. Support "global usings". Our .NET 6 templates should have no
   using statements at the top of .cs files.
2. Use $(Nullable)enable by default in project templates.

To test this, our .NET 6 MSBuild tests use Nullable=enable and
ImplicitUsings=enable by default and do not include using
statements in .cs files.

I've made a new MainActivity.cs for our .NET 6 MSBuild tests. The
"legacy" Xamarin.Android tests will use the original file.

Our default global using are:

globalusingglobal::Android.App;globalusingglobal::Android.Widget;globalusingBundle=global::Android.OS.Bundle;

The last one is intentionally not bringing in Android.OS, because
Android.OS.Environment would conflict with System.Environment.

So AutoImport.props should become:

<ItemGroupCondition=" '$(TargetPlatformIdentifier)' == 'android' and ('$(ImplicitUsings)' == 'true' or '$(ImplicitUsings)' == 'enable') ">
<UsingInclude="Android.App" />
<UsingInclude="Android.Widget" />
<UsingInclude="Android.OS.Bundle"Alias="Bundle" />
</ItemGroup>

So these items are present at the time .csproj files are evaluated.

Any templates will add:

<Nullable>enable</Nullable>
<ImplicitUsings>enable</ImplicitUsings>

If users want to configure these settings, they can remove
$(ImplicitUsings) from the .csproj completely or remove specific
@(Using) items:

<ItemGroup>
<UsingRemove="Android.App" />
</ItemGroup>

[jonathanpeppers]

This is a draft, because I suspect we might get test failures on this one.

[jonathanpeppers]

One complication that came up in a test:

error CS0104: 'Application' is an ambiguous reference between 'Android.App.Application' and 'Xamarin.Forms.Application'

So I suspect that dotnet/maui is going to set $(DisableImplicitNamespaceImports_Android). Because it has to? There are so many types that will conflict.

[jonpryor]

@jonathanpepperswrote:

One complication that came up in a test:

error CS0104: 'Application' is an ambiguous reference between 'Android.App.Application' and 'Xamarin.Forms.Application'

This suggests that we might not want Android.App in the default @(Import)either, since lots of our customers will be using MAUI.

Will Android.Widget conflict with anything?

The "better answer" here may be to just not add anything to @(Import), as anything we add will likely conflict with MAUI…

[jonathanpeppers]

Adding Nullable=enable, increased this assembly's size enough to fail the test...

Error: apkdiff: File 'assemblies/UnnamedProject.dll' has changed by 358 bytes (10.14 %). This exceeds the threshold of 5.00 %.

[jonathanpeppers]

We need to wait on this, because there are changes to the .NET SDK coming:

1. @(Import) will be renamed to @(Using)
2. $(DisableImplicitNamespaceImports)=false will be $(ImplicitUsings)=true. Projects will have to opt into it, so we will put $(ImplicitUsings)=true in our project templates.
3. They might support our global using Bundle = global::Android.OS.Bundle; use case.

See: dotnet/sdk#19535 (comment)

I'd rather wait for the final implementation to land before we opt into the changes here.

[mhutch]

The "better answer" here may be to just not add anything to @(Import), as anything we add will likely conflict with MAUI…

The MAUI targets can remove the Android global usings, or make aliasing choices.

[jonathanpeppers]

I found this test was not using a Forms-based MainActivity.cs before, but it was a Xamarin.Forms app! So we need to use ${AFTER_FORMS_INIT} now and the line number changed.

[jonpryor]

Should we update AndroidBinding1.csproj to also set $(AndroidGenerateResourceDesigner)=False, as per #6224?

[jonathanpeppers]

You can have an .aar with resources, then you might want to use Resource.designer.cs values from C#.

So I don't think we need this by default, but it's there if you need to turn it off.