Bug

Fixes: code quality

Description

Enable C# nullable annotation checking across entire repo in 3 easy steps:

1. Set <Nullable>enable</Nullable> in build/common.project.props
2. Add #nullable disable to every .cs file that fails to compile, until the solution compiles again.
   • There were a small number of other changes required when dependencies of classes that already had nullable enabled needed to resolve new nullability issues
3. Remove #nullable enable from all the code that previously opt-in individually.
4. NuGet.CommandLine.XPlat is a special case, because it uses a package that ships source code, and there's no way to exclude the package's source from nullability checks. So this project has nullable disabled, and source files in that project need #nullable enable where possible. We need to migrate the CLI parser to System.CommandLine and remove the other package to bring this project back in line with the rest of the repo.

Step 2 was performed by creating an MSBuild logger that logged every message to JSON, then doing command line builds using the custom logger. Then a second app was used to parse the json output, find files that are failing to compile, then add #nullable disable to the top of the file. Then, I just needed to keep running the two commands in a loop, with only a few manual fixes, to get the repo building again.

Step 3 was mostly a global search and replace, but it did find a few places where nullable was disabled and then re-enabled in the same file, so those were cleaned up.

Personally, I find the easiest way to review this PR is to do git diff origin/dev... on the command line. Then page through the results. The vast majority of changes are #nullable lines, so you can quickly scroll and anything that's different really pops out for more detailed investigation.

Doing git diff origin/dev... --stat shows the number of lines changed per file, and almost all files only have one or two lines changed. The files with more than 2 lines changed are:

 .../Converters/EnumerableToVisibilityConverter.cs                 | 4 ++--
 .../Commands/Package/Update/PackageUpdateCommandRunner.cs         | 6 ++----
 .../NuGet.Commands/RestoreCommand/RestoreCommandProvidersCache.cs | 4 +---
 .../NuGet.Commands/RestoreCommand/Utility/AuditUtility.cs         | 4 +---
 .../NuGet.DependencyResolver.Core/Remote/RemoteWalkContext.cs     | 4 +---
 src/NuGet.Core/NuGet.DependencyResolver.Core/ResolverUtility.cs   | 8 +++-----
 src/NuGet.Core/NuGet.Packaging/ContentModel/ContentItem.cs        | 4 +---
 .../Licenses/NuGetLicenseExpressionParsingException.cs            | 3 +++
 src/NuGet.Core/NuGet.Protocol/HttpSource/HttpRetryHandler.cs      | 6 ++----
 .../NuGet.Protocol/Providers/ReadmeUriTemplateResourceProvider.cs | 3 +--
 .../ensure-nupkg-dependencies-on-source.csproj                    | 3 +--

Doing (gci -recurse -filter *.cs).Length in PowerShell after a git clean shows there are 3882 .cs files in the repo. Since this PR changes 2176, and some of those are csproj files, and about 300 were files with #nullable enable removed, I estimate about 2,000 files now have null checking enabled, which is a little over half the files. It's also easy for someone enthusiastic about making more nullability improvements, as they can search for #nullable disable and start fixing some files.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests N/A
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. N/A