Custom rules

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A rule derives from NuGetPackageValidationRule and overrides ExecuteAsync. This one checks that the id of the package starts with a prefix:

// Reports an error when the id of the package does not start with the prefix of the company
private sealed class PackageIdPrefixRule(string prefix) : NuGetPackageValidationRule
{
    public const int ErrorCode = 1001; // Outside the ranges of the built-in error codes

    public override Task ExecuteAsync(NuGetPackageValidationContext context)
    {
        var id = context.Package.NuspecReader.GetId();
        if (!id.StartsWith(prefix, StringComparison.Ordinal))
        {
            context.ReportError(ErrorCode, $"The package id '{id}' does not start with '{prefix}'", helpText: $"Rename the package to '{prefix}{id}'");
        }

        return Task.CompletedTask;
    }
}

Add an instance to the options, with the built-in rules you want:

var packagePath = FullPath.FromPath(AppContext.BaseDirectory) / "Packages" / "Release_Author.1.0.0.nupkg";

var options = new NuGetPackageValidationOptions();
options.Rules.Add(NuGetPackageValidationRules.AuthorMustBeSet);
options.Rules.Add(new PackageIdPrefixRule("Contoso."));

var result = await NuGetPackageValidator.ValidateAsync(packagePath, options);
NuGetPackageValidationError error = Assert.Single(result.Errors);
Assert.Equal(PackageIdPrefixRule.ErrorCode, error.ErrorCode);
Assert.Equal("The package id 'Release_Author' does not start with 'Contoso.'", error.Message);

The validation context

ExecuteAsync receives a NuGetPackageValidationContext:

Member	Use
Package	The reader of the package: its metadata (`NuspecReader`), its files and their content
SymbolPackage	The reader of the `.snupkg` file next to the package, or `null` when there is none
PackagePath	The path of the package
SymbolServers	The symbol servers of the options, as URLs that end with `/`
CancellationToken	The token passed to ValidateAsync
ReportError	Reports an error, with an optional help text and file name
IsRuleExcluded	Whether an error code is excluded, to skip expensive work whose errors would be ignored

ReportError ignores the excluded codes, so a rule does not have to check them before reporting.
Use codes that the built-in rules do not use. The built-in codes are below 200: see Error codes.
The rules run one after the other, in the order of the Rules set, which is not guaranteed. Do not make a rule depend on another one.
The package and its symbol package are disposed when the validation ends: do not keep them.

The command-line tool only runs the built-in rules. To run a custom rule, call the library from your own program.