using System;

using System.Threading.Tasks;

using System.Linq;

using System.Collections.ObjectModel;

using System.Reflection;

using System.Collections.Generic;

#nullable enable

public class Program

{

    public static void Main()

    {

        GenericReturnTypeIsNullable(typeof(Program).GetMethod("Foo")).Dump();

    }

    public static bool GenericReturnTypeIsNullable(MethodInfo methodInfo)

    {

        var nullableAttribute = methodInfo!.ReturnTypeCustomAttributes.GetCustomAttributes(true).FirstOrDefault(x => x.GetType().FullName == "System.Runtime.CompilerServices.NullableAttribute");

        if (nullableAttribute != null)

        {

            var flagsField = nullableAttribute.GetType().GetField("NullableFlags");

            var flags = (byte[]?)flagsField?.GetValue(nullableAttribute);

            return flags != null && flags.Length >= 2 && flags[1] == 2;

        }

        if (CheckNullableContext(methodInfo.CustomAttributes))

            return true;

        for (var type = methodInfo.DeclaringType; type != null; type = type.DeclaringType)

        {

            if (CheckNullableContext(type.CustomAttributes))

                return true;

        }

        static bool CheckNullableContext(IEnumerable<CustomAttributeData> customAttributes)

        {

            var context = customAttributes.FirstOrDefault(x => x.AttributeType.FullName == "System.Runtime.CompilerServices.NullableContextAttribute");

            return context != null &&

                context.ConstructorArguments.Count == 1 &&

                context.ConstructorArguments[0].ArgumentType == typeof(byte) &&

                (byte)context.ConstructorArguments[0].Value! == 2;

        }

​

        return false;

    }

    public Task<string>? Foo() => Task.FromResult("Foo");

​

}