UseConvertUnitAttribute.cs

using System;
using System.Reflection;
using HotChocolate.Types;
using HotChocolate.Types.Descriptors;

namespace StarWars.Characters
{
    [AttributeUsage(
        AttributeTargets.Property | AttributeTargets.Method,
        AllowMultiple = false)]
    public sealed class UseConvertUnitAttribute
        : DescriptorAttribute
    {
        protected override void TryConfigure(
            IDescriptorContext context,
            IDescriptor descriptor,
            ICustomAttributeProvider element)
        {
            if (descriptor is IObjectFieldDescriptor objectField)
            {
                objectField
                    .Argument("unit", a => a.Type<EnumType<Unit>>().DefaultValue(Unit.Meters))
                    .Use(next => async context =>
                    {
                        await next(context).ConfigureAwait(false);

                        if (context.Result is double length)
                        {
                            //NOTE: Updated to use v11 method...
                            context.Result = ConvertToUnit(length, context.ArgumentValue<Unit>("unit"));
                        }
                    });
            }
            else if (descriptor is IInterfaceFieldDescriptor interfaceField)
            {
                interfaceField
                    .Argument("unit", a => a.Type<EnumType<Unit>>().DefaultValue(Unit.Meters));
            }
        }

        private double ConvertToUnit(double length, Unit unit)
        {
            if (unit == Unit.Foot)
            {
                return length * 3.28084d;
            }
            return length;
        }
    }
}