[One .NET] support latest C# 10 language features (#6118)

Fixes: #6075
Fixes: #6076

Context: dotnet/sdk#19521

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

 1. Support ["global usings"][0]. Our .NET 6 templates should have no
    `using` statements at the top of `.cs` files.

 2. Set `$(Nullable)`=enable by default in project templates, i.e.
    enable [C#8 nullable reference types][1] by default.

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:

	global using global::Android.App;
	global using global::Android.Widget;
	global using Bundle = global::Android.OS.Bundle;

The last one is intentionally not bringing in `Android.OS`, because
`Android.OS.Environment` would conflict with `System.Environment`,
and the `System` namespace will be in `@(Using)` by default.

`AutoImport.props` should become:

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

so that these `using`s are present at the time `.csproj` files are
compiled.

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>
	  <Using Remove="Android.App" />
	</ItemGroup>

[0]: https://github.com/dotnet/csharplang/blob/b89d4c934041db923f7238b1427cd5f3ae71ed4b/proposals/csharp-10.0/GlobalUsingDirective.md#global-using-alias-directives
[1]: https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references

Author: jonathanpeppers

src/Microsoft.Android.Templates/android-activity/Activity1.cs

@@ -1,14 +1,9 @@
-using Android.App;
-using Android.OS;
-using Android.Runtime;
-using Android.Widget;
-
 namespace AndroidApp1
 {
     [Activity(Label = "@string/app_name", MainLauncher = true)]
     public class Activity1 : Activity
     {
-        protected override void OnCreate(Bundle savedInstanceState)
+        protected override void OnCreate(Bundle? savedInstanceState)
         {
             base.OnCreate(savedInstanceState);
 

src/Microsoft.Android.Templates/android-bindinglib/AndroidBinding1.csproj

@@ -3,5 +3,7 @@
     <TargetFramework>net6.0-android</TargetFramework>
     <SupportedOSPlatformVersion>SUPPORTED_OS_PLATFORM_VERSION</SupportedOSPlatformVersion>
     <RootNamespace Condition="'$(name)' != '$(name{-VALUE-FORMS-}safe_namespace)'">AndroidBinding1</RootNamespace>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
   </PropertyGroup>
 </Project>

src/Microsoft.Android.Templates/android/AndroidApp1.csproj

@@ -4,5 +4,7 @@
     <SupportedOSPlatformVersion>SUPPORTED_OS_PLATFORM_VERSION</SupportedOSPlatformVersion>
     <RootNamespace Condition="'$(name)' != '$(name{-VALUE-FORMS-}safe_namespace)'">AndroidApp1</RootNamespace>
     <OutputType>Exe</OutputType>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
   </PropertyGroup>
 </Project>

src/Microsoft.Android.Templates/android/MainActivity.cs

@@ -1,14 +1,9 @@
-using Android.App;
-using Android.OS;
-using Android.Runtime;
-using Android.Widget;
-
 namespace AndroidApp1
 {
     [Activity(Label = "@string/app_name", MainLauncher = true)]
     public class MainActivity : Activity
     {
-        protected override void OnCreate(Bundle savedInstanceState)
+        protected override void OnCreate(Bundle? savedInstanceState)
         {
             base.OnCreate(savedInstanceState);
 

src/Microsoft.Android.Templates/androidlib/AndroidLib1.csproj

@@ -3,5 +3,7 @@
     <TargetFramework>net6.0-android</TargetFramework>
     <SupportedOSPlatformVersion>SUPPORTED_OS_PLATFORM_VERSION</SupportedOSPlatformVersion>
     <RootNamespace Condition="'$(name)' != '$(name{-VALUE-FORMS-}safe_namespace)'">AndroidLib1</RootNamespace>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
   </PropertyGroup>
 </Project>

src/Microsoft.Android.Templates/androidlib/Class1.cs

@@ -1,5 +1,3 @@
-using System;
-
 namespace AndroidLib1
 {
     public class Class1

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/Sdk/AutoImport.props

@@ -20,6 +20,12 @@ https://github.com/dotnet/designs/blob/4703666296f5e59964961464c25807c727282cae/
     <_DefaultJavaSourceJarPattern>**\*-source.jar;**\*-sources.jar;**\*-src.jar</_DefaultJavaSourceJarPattern>
   </PropertyGroup>
 
+  <ItemGroup Condition=" '$(TargetPlatformIdentifier)' == 'android' and ('$(ImplicitUsings)' == 'true' or '$(ImplicitUsings)' == 'enable') ">
+    <Using Include="Android.App" />
+    <Using Include="Android.Widget" />
+    <Using Include="Android.OS.Bundle" Alias="Bundle" />
+  </ItemGroup>
+
   <ItemGroup Condition=" '$(EnableDefaultAndroidItems)' == 'true' ">
     <!-- Default Resource file inclusion -->
     <!-- https://developer.android.com/guide/topics/resources/providing-resources -->

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/Utilities/DeviceTest.cs

@@ -67,6 +67,7 @@ protected override void CleanupTest ()
 		{
 			if (HasDevices && TestContext.CurrentContext.Result.Outcome.Status == TestStatus.Failed &&
 					TestOutputDirectories.TryGetValue (TestContext.CurrentContext.Test.ID, out string outputDir)) {
+				Directory.CreateDirectory (outputDir);
 				string local = Path.Combine (outputDir, "screenshot.png");
 				string deviceLog = Path.Combine (outputDir, "logcat-failed.log");
 				string remote = "/data/local/tmp/screenshot.png";

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/XASdkTests.cs

@@ -205,6 +205,7 @@ public void DotNetNew ([Values ("android", "androidlib", "android-bindinglib")]
 			Assert.IsTrue (dotnet.New ("android-activity"), "`dotnet new android-activity` should succeed");
 			Assert.IsTrue (dotnet.New ("android-layout", Path.Combine (dotnet.ProjectDirectory, "Resources", "layout")), "`dotnet new android-layout` should succeed");
 			Assert.IsTrue (dotnet.Build (), "`dotnet build` should succeed");
+			dotnet.AssertHasNoWarnings ();
 		}
 
 		[Test]
@@ -550,7 +551,7 @@ public void SupportedOSPlatformVersion ([Values (21, 30)] int minSdkVersion)
 			};
 			// Call AccessibilityTraversalAfter from API level 22
 			// https://developer.android.com/reference/android/view/View#getAccessibilityTraversalAfter()
-			proj.MainActivity = proj.DefaultMainActivity.Replace ("button.Click", "button.AccessibilityTraversalAfter.ToString ();\nbutton.Click");
+			proj.MainActivity = proj.DefaultMainActivity.Replace ("button!.Click", "button!.AccessibilityTraversalAfter.ToString ();\nbutton!.Click");
 
 			var dotnet = CreateDotNetBuilder (proj);
 			Assert.IsTrue (dotnet.Build (), "`dotnet build` should succeed");
@@ -645,7 +646,7 @@ void CreateEmptyFile (params string [] paths)
 		public void XamarinLegacySdk ()
 		{
 			var proj = new XASdkProject (outputType: "Library") {
-				Sdk = "Xamarin.Legacy.Sdk/0.1.0-alpha2",
+				Sdk = "Xamarin.Legacy.Sdk/0.1.0-alpha4",
 				Sources = {
 					new AndroidItem.AndroidLibrary ("javaclasses.jar") {
 						BinaryContent = () => ResourceData.JavaSourceJarTestJar,

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/KnownProperties.cs

@@ -36,6 +36,8 @@ public static class KnownProperties
 		public const string AndroidFastDeploymentType = "AndroidFastDeploymentType";
 		public const string AndroidClassParser = "AndroidClassParser";
 		public const string _AndroidAllowDeltaInstall = "_AndroidAllowDeltaInstall";
+		public const string Nullable = "Nullable";
+		public const string ImplicitUsings = "ImplicitUsings";
 	}
 }