Fix nullability warnings in projects targeting netstandard2.0

[captainsafia]

We have MSBuild targets in the repo that enable nullability on netstandard2.0 projects.

aspnetcore/eng/targets/CSharp.Common.targets

Lines 120 to 130 in 0793770

	<When Condition=" ('$(Nullable)' == 'annotations' OR '$(Nullable)' == 'enable') AND
	'$(SuppressNullableAttributesImport)' != 'true' AND
	(('$(TargetFrameworkIdentifier)' == '.NETStandard' AND $([MSBuild]::VersionLessThanOrEquals('$(TargetFrameworkVersion)', '2.1'))) OR '$(TargetFrameworkIdentifier)' == '.NETFramework')">
	<PropertyGroup>
	<DefineConstants>$(DefineConstants),INTERNAL_NULLABLE_ATTRIBUTES</DefineConstants>
	<NoWarn>$(NoWarn);nullable</NoWarn>
	</PropertyGroup>
	<ItemGroup>
	<Compile Include="$(SharedSourceRoot)Nullable\NullableAttributes.cs" />
	</ItemGroup>
	</When>

However, the nullability warnings are not emitted during build by default.

aspnetcore/eng/targets/CSharp.Common.targets

Line 125 in 0793770

<NoWarn>$(NoWarn);nullable</NoWarn>

Removing the property above shows that we have ~150 unhandled nullability warnings in the projects that currently "enable" nullability.

[captainsafia]

Consider making this behavior opt-out instead of opt-in so we can catch this in projects that enable it in the future:

#47144 (comment)

[mkArtakMSFT]

@captainsafia I assume this is a cross-cutting issue which is applicable to most of the areas. Should we instead file separate issues per area and track that way?

[danmoseley]

This might be a task community members would like to contribute to, folks like @maxkoshevoi and @SteveDunn did a lot of this in dotnet/runtime.

[captainsafia]

This might be a task community members would like to contribute to, folks like @maxkoshevoi and @SteveDunn did a lot of this in dotnet/runtime.

If a community member is able to help here, that would be great! Especially if we could do this work in one fell swoop and avoid having to put temporary workarounds like the one described here).

Happy to provide any interested members with guidance!

Thanks for contacting us.

We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.