NullableInfo.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Reflection;
using System.Text;

namespace ConsoleApp52
{
    internal static class Program
    {
        private static void Main()
        {
            var type = typeof(Test);

            var context = new NullabilityInfoContext();

            foreach (var field in type.GetFields())
            {
                var info = context.Create(field);
                Console.WriteLine($"Field {field.Name}");
                PrintNullabilityInformation(info);
            }

            foreach (var property in type.GetProperties())
            {
                var info = context.Create(property);
                Console.WriteLine($"Property {property.Name}");
                PrintNullabilityInformation(info);
            }

            foreach (var @event in type.GetEvents())
            {
                var info = context.Create(@event);
                Console.WriteLine($"Event {@event.Name}");
                PrintNullabilityInformation(info);
            }

            foreach (var method in type.GetMethods())
            {
                Console.WriteLine($"Method {method.Name}");

                Console.WriteLine("    return");
                PrintNullabilityInformation(context.Create(method.ReturnParameter), 2);

                foreach (var parameter in method.GetParameters())
                {
                    var info = context.Create(parameter);
                    Console.WriteLine($"    {parameter.Name}");
                    PrintNullabilityInformation(info, 2);
                }
            }
        }

        private static void PrintNullabilityInformation(NullabilityInfo info, int indent = 1)
        {
            Console.Write(new string(' ', indent * 4));
            Console.WriteLine(info);
        }
    }

    class Test
    {
#nullable disable
        public string PropertyUnknown { get; set; }
#nullable restore
        public string? PropertyNullable { get; set; }
        public string PropertyNonNullable { get; set; }

#nullable disable
        public string FieldUnknown;
#nullable restore
        public string? FieldNullable;
        public string FieldNonNullable;

        public IEnumerable<string>? M1() => throw null!;
        public IEnumerable<string?> M2() => throw null!;
        public IEnumerable<string?>? M3() => throw null!;

        public IEnumerable<
            #nullable disable
            string
            #nullable restore
        >? M4() => throw null!;

        public IEnumerable<KeyValuePair<(string name, object? value), object?>>? M5() => throw null!;

        public string?[] M6() => throw null!;
        public string?[]? M7() => throw null!;
        public string[]? M8() => throw null!;

    }
}

namespace System.Reflection
{
    public sealed class NullabilityInfoContext
    {
        private static NullableState GetNullableContext(MemberInfo? memberInfo)
        {
            while (memberInfo != null)
            {
                var attributes = memberInfo.GetCustomAttributesData();
                foreach (var attribute in attributes)
                {
                    if (attribute.AttributeType.Name == "NullableContextAttribute" &&
                        attribute.AttributeType.Namespace == "System.Runtime.CompilerServices" &&
                        attribute.ConstructorArguments.Count == 1)
                    {
                        return TranslateByte(attribute.ConstructorArguments[0].Value);
                    }
                }

                memberInfo = memberInfo.DeclaringType;
            }

            return NullableState.Unknown;
        }

        public NullabilityInfo Create(ParameterInfo parameterInfo)
        {
            var context = GetNullableContext(parameterInfo.Member);
            return GetNullabilityInfo(context, parameterInfo.ParameterType, parameterInfo.GetCustomAttributesData());
        }

        public NullabilityInfo Create(PropertyInfo propertyInfo)
        {
            var context = GetNullableContext(propertyInfo);
            return GetNullabilityInfo(context, propertyInfo.PropertyType, propertyInfo.GetCustomAttributesData());
        }

        public NullabilityInfo Create(EventInfo eventInfo)
        {
            var context = GetNullableContext(eventInfo);
               return GetNullabilityInfo(context, eventInfo.EventHandlerType!, eventInfo.GetCustomAttributesData());
        }

        public NullabilityInfo Create(FieldInfo parameterInfo)
        {
            var context = GetNullableContext(parameterInfo);
            return GetNullabilityInfo(context, parameterInfo.FieldType, parameterInfo.GetCustomAttributesData());
        }

        private static NullabilityInfo GetNullabilityInfo(NullableState context, Type type, IList<CustomAttributeData> customAttributes)
        {
            var offset = 0;
            return GetNullabilityInfo(context, type, customAttributes, ref offset);
        }

        private static NullabilityInfo GetNullabilityInfo(NullableState context, Type type, IList<CustomAttributeData> customAttributes, ref int offset)
        {
            NullableState state;

            if (type.IsValueType)
            {
                state = NullableState.Unknown;
            }
            else
            {
                state = context;

                foreach (var attribute in customAttributes)
                {
                    if (attribute.AttributeType.Name == "NullableAttribute" &&
                        attribute.AttributeType.Namespace == "System.Runtime.CompilerServices" &&
                        attribute.ConstructorArguments.Count == 1)
                    {
                        var o = attribute.ConstructorArguments[0].Value;

                        if (o is byte b)
                            state = TranslateByte(b);
                        else if (o is ReadOnlyCollection<CustomAttributeTypedArgument> args &&
                                 offset < args.Count &&
                                 args[offset].Value is byte elementB)
                            state = TranslateByte(elementB);
                        else
                            state = NullableState.Unknown;

                        break;
                    }
                }
            }

            // We consumed one element in the nullable array.
            offset++;

            NullabilityInfo? elementState = null;
            NullabilityInfo[]? genericArgumentStates = null;

            if (type.HasElementType)
            {
                var elementType = type.GetElementType();
                if (elementType != null)
                    elementState = GetNullabilityInfo(context, elementType, customAttributes, ref offset);
            }
            else if (type.IsGenericType)
            {
                var genericArguments = type.GetGenericArguments();
                genericArgumentStates = new NullabilityInfo[genericArguments.Length];

                for (int i = 0; i < genericArguments.Length; i++)
                {
                    genericArgumentStates[i] = GetNullabilityInfo(context, genericArguments[i], customAttributes, ref offset);
                }
            }

            return new NullabilityInfo(type, state, elementState, genericArgumentStates);
        }

        private static NullableState TranslateByte(object? singleValue)
        {
            return singleValue is byte b ? TranslateByte(b) : NullableState.Unknown;
        }

        private static NullableState TranslateByte(byte b)
        {
            return b switch
            {
                1 => NullableState.NotNull,
                2 => NullableState.MaybeNull,
                _ => NullableState.Unknown,
            };
        }
    }

    public sealed class NullabilityInfo
    {
        internal NullabilityInfo(Type type, NullableState state, NullabilityInfo? element, NullabilityInfo[]? genericTypeArguments)
        {
            Type = type;
            State = state;
            Element = element;
            GenericTypeArguments = genericTypeArguments;
        }

        public Type Type { get; }
        public NullableState State { get; }
        public NullabilityInfo? Element { get; }
        public NullabilityInfo[]? GenericTypeArguments { get; }

        public override string ToString()
        {
            var sb = new StringBuilder();
            AppendType(sb, this);
            return sb.ToString();

            static string GetTypeName(Type type)
            {
                if (type.IsArray)
                    return GetTypeName(type.GetElementType()!);

                if (type.IsGenericType && !type.IsGenericTypeDefinition)
                    return GetTypeName(type.GetGenericTypeDefinition());

                if (type.IsGenericTypeDefinition)
                {
                    var backTick = type.Name.IndexOf('`');
                    if (backTick > 0)
                        return type.Name.Substring(0, backTick);
                }

                return type.Name;
            }

            static void AppendType(StringBuilder sb, NullabilityInfo info)
            {
                sb.Append(GetTypeName(info.Type));

                if (info.State == NullableState.NotNull)
                    sb.Append('!');
                else if (info.State == NullableState.MaybeNull)
                    sb.Append('?');

                if (info.Element is not null)
                {
                    sb.Append('[');
                    AppendType(sb, info.Element);
                    sb.Append(']');
                }
                else if (info.GenericTypeArguments is not null)
                {
                    sb.Append('<');
                    var isFirst = true;

                    foreach (var arg in info.GenericTypeArguments)
                    {
                        if (isFirst)
                            isFirst = false;
                        else
                            sb.Append(", ");

                        AppendType(sb, arg);
                    }
                    sb.Append('>');
                }
            }
        }
    }

    public enum NullableState
    {
        Unknown,
        NotNull,
        MaybeNull
    }
}