read-nullable-enable.cs

using System.Reflection.Metadata;
using System.Reflection.PortableExecutable;

Console.WriteLine(IsNullableEnabledAtBuild(@"C:\Users\jaredpar\temp\console\bin\Debug\net7.0\console.dll", out var reason));
Console.WriteLine(reason);

static bool IsNullableEnabledAtBuild(string assemblyFilePath, out string? reason)
{
    MetadataReaderProvider? pdbReaderProvider = null;
    try
    {
        using var reader = new FileStream(assemblyFilePath, FileMode.Open, FileAccess.Read, FileShare.Read);
        using var peReader = new PEReader(reader);
        if (!peReader.TryOpenAssociatedPortablePdb(assemblyFilePath, OpenPortablePdbFile, out pdbReaderProvider, out var pdbPath))
        {
            reason = $"Can't find portable pdb file for {assemblyFilePath}";
            return false;
        }

        var compilationOptionsGuid = new Guid("B5FEEC05-8CD0-4A83-96DA-466284BB4BD8");
        var pdbReader = pdbReaderProvider!.GetMetadataReader();
        if (!TryGetCustomDebugInformationBlobReader(pdbReader, compilationOptionsGuid, out var blobReader))
        {
            reason = "Cannot find compilation options section";
            return false;
        }

        return GetNullableOption(blobReader, out reason);
    }
    catch (Exception ex)
    {
        reason = ex.Message;
        return false;
    }
    finally
    {
        pdbReaderProvider?.Dispose();
    }

    static bool GetNullableOption(BlobReader blobReader, out string? reason)
    {
        for (;;)
        {
            var nullIndex = blobReader.IndexOf(0);
            if (nullIndex == -1)
            {
                break;
            }

            var key = blobReader.ReadUTF8(nullIndex);
            blobReader.ReadByte(); // Skip null terminator

            nullIndex = blobReader.IndexOf(0);
            if (nullIndex == -1)
            {
                break;
            }
            var value = blobReader.ReadUTF8(nullIndex);
            blobReader.ReadByte(); // Skip null terminator
            if (key == "nullable")
            {
                reason = null;
                return string.Equals(value, "enable", StringComparison.OrdinalIgnoreCase);
            }
        }

        reason = "Cannot find compiler option";
        return false;
    }

    static bool TryGetCustomDebugInformationBlobReader(MetadataReader pdbReader, Guid infoGuid, out BlobReader blobReader)
    {
        var blobs = from cdiHandle in pdbReader.GetCustomDebugInformation(EntityHandle.ModuleDefinition)
                    let cdi = pdbReader.GetCustomDebugInformation(cdiHandle)
                    where pdbReader.GetGuid(cdi.Kind) == infoGuid
                    select pdbReader.GetBlobReader(cdi.Value);

        if (blobs.Any())
        {
            blobReader = blobs.Single();
            return true;
        }

        blobReader = default;
        return false;
    }

    static Stream? OpenPortablePdbFile(string filePath)
    {
        if (!File.Exists(filePath))
        {
            return null;
        }

        return new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read);
    }
}