Enable Nullable in Main Directory.Build.Props, Add #nullable disable to .cs Files, and Clean Up Redundant Configurations

[v-wuzhai]

1. Centralized Management: Enabling nullable reference types in the main Directory.Build.Props file ensures consistent default behavior across the entire repository.
2. Gradual Transition: By adding #nullable disable at the top of each .cs file, excluding files in projects where nullable reference types are already enabled, nullable reference types can be gradually enabled without immediately affecting all files.
3. Cleaning Up Redundant Configurations: Removing all enable and #nullable enable from the src and test directories avoids duplicate configurations, simplifies project files, and ensures consistency.

[v-wuzhai]

@ViktorHofer @marcpopMSFT To ensure consistent default behavior across the entire repository, I've enabled nullable reference types in the main Directory.Build.Props file. This allows for a gradual transition by adding #nullable disable at the top of each .cs file, excluding files in projects where nullable reference types are already enabled. Additionally, I've cleaned up redundant configurations by removing all <Nullable>enable</Nullable> and #nullable enable from the src and test directories.

Could you help to fix the VMR build failures?

[ViktorHofer]

The VMR is its own entity and doesn't inherit settings from the dotnet/sdk repo. Therefore you need to set the Nullable=enable default in src/SourceBuild/content/Directory.Build.props as well.

[v-wuzhai]

@ViktorHofer The VMR still seems to be failing, with the following error message:

CSC : error CS8630: Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language version '8.0' or greater. [/Users/runner/work/1/vmr/eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver.csproj]
##[error]CSC(0,0): error CS8630: Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language version '8.0' or greater.

The default language version for 'netstandard2.0' appears to be 7.3, which seems incompatible with enabling nullable. So, do we need to explicitly set the language version to 8.0 or higher, or should we disable nullable for netstandard2.0?

[ViktorHofer]

@v-wuzhai I pushed a commit into your branch. We had to set LangVersion explicitly in the VMR orchestrator.

[akoeplinger]

A couple more nullability errors:

##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(126,32): error CS8603: Possible null reference return.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(129,33): error CS8600: Converting null literal or possible null value to non-nullable type.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(138,37): error CS8600: Converting null literal or possible null value to non-nullable type.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(147,30): error CS8601: Possible null reference assignment.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(150,34): error CS8601: Possible null reference assignment.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(153,24): error CS8603: Possible null reference return.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(159,27): error CS8618: Non-nullable property 'Group' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(164,27): error CS8618: Non-nullable property 'Id' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(169,27): error CS8618: Non-nullable property 'Version' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(174,27): error CS8618: Non-nullable property 'Dir' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
##[error]eng/tools/tasks/Microsoft.DotNet.UnifiedBuild.MSBuildSdkResolver/UnifiedBuildSdkResolver.cs(179,27): error CS8618: Non-nullable property 'SdkDir' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

pushed a fix