Siggingway/Siggingway/NullabilityUtil.cs

using System.Collections.ObjectModel;
using System.Reflection;

namespace Siggingway;

internal static class NullabilityUtil {
    internal static bool IsNullable(PropertyInfo property) => IsNullableHelper(property.PropertyType, property.DeclaringType, property.CustomAttributes);

    internal static bool IsNullable(FieldInfo field) => IsNullableHelper(field.FieldType, field.DeclaringType, field.CustomAttributes);

    internal static bool IsNullable(ParameterInfo parameter) => IsNullableHelper(parameter.ParameterType, parameter.Member, parameter.CustomAttributes);

    private static bool IsNullableHelper(Type memberType, MemberInfo? declaringType, IEnumerable<CustomAttributeData> customAttributes) {
        if (memberType.IsValueType) {
            return Nullable.GetUnderlyingType(memberType) != null;
        }

        var nullable = customAttributes
            .FirstOrDefault(x => x.AttributeType.FullName == "System.Runtime.CompilerServices.NullableAttribute");
        if (nullable != null && nullable.ConstructorArguments.Count == 1) {
            var attributeArgument = nullable.ConstructorArguments[0];
            if (attributeArgument.ArgumentType == typeof(byte[])) {
                var args = (ReadOnlyCollection<CustomAttributeTypedArgument>) attributeArgument.Value!;
                if (args.Count > 0 && args[0].ArgumentType == typeof(byte)) {
                    return (byte) args[0].Value! == 2;
                }
            } else if (attributeArgument.ArgumentType == typeof(byte)) {
                return (byte) attributeArgument.Value! == 2;
            }
        }

        for (var type = declaringType; type != null; type = type.DeclaringType) {
            var context = type.CustomAttributes
                .FirstOrDefault(x => x.AttributeType.FullName == "System.Runtime.CompilerServices.NullableContextAttribute");
            if (context != null &&
                context.ConstructorArguments.Count == 1 &&
                context.ConstructorArguments[0].ArgumentType == typeof(byte)) {
                return (byte) context.ConstructorArguments[0].Value! == 2;
            }
        }

        // Couldn't find a suitable attribute
        return false;
    }
}