SystemExtensions/SystemExtensions.NetStandard/Extensions/Optional.cs

namespace System.Extensions
{
    public static class Optional
    {
        public static Optional<T> None<T>()
        {
            return new Optional<T>.None();
        }

        public static Optional<T> FromValue<T>(T value)
        {
            if (value == null)
            {
                return new Optional<T>.None();
            }

            return new Optional<T>.Some(value);
        }
    }

    public abstract class Optional<T> : IEquatable<Optional<T>>
    {
        public Optional(T value)
        {
            this.Value = value;
        }

        private T Value { get; }

        public T ExtractValue()
        {
            if (this is None)
            {
                return default;
            }
            else
            {
                return Value;
            }
        }
        public Optional<T> Do(Action<T> onSome, Action onNone)
        {
            if (onSome is null) throw new ArgumentNullException(nameof(onSome));
            if (onNone is null) throw new ArgumentNullException(nameof(onNone));

            if (this is Some)
            {
                onSome.Invoke(this.Value);
            }
            else
            {
                onNone.Invoke();
            }
            return this;
        }
        public Optional<T> DoAny(Action<T> onSome = null, Action onNone = null)
        {
            if (this is Some)
            {
                onSome?.Invoke(this.Value);
            }
            else
            {
                onNone?.Invoke();
            }
            return this;
        }
        public Optional<V> Switch<V>(Func<T, V> onSome, Func<V> onNone)
        {
            if (onSome is null) throw new ArgumentNullException($"{nameof(onSome)}");
            if (onNone is null) throw new ArgumentNullException($"{nameof(onNone)}");

            if (this is Some)
            {
                return Optional.FromValue(onSome.Invoke(this.Value));
            }
            else
            {
                return Optional.FromValue(onNone.Invoke());
            }
        }
        public Optional<V> SwitchAny<V>(Func<T, V> onSome = null, Func<V> onNone = null)
        {
            if (this is Some)
            {
                return onSome != null ? Optional.FromValue(onSome.Invoke(this.Value)) : Optional.FromValue(default(V));
            }
            else
            {
                return onNone != null ? Optional.FromValue(onNone.Invoke()) : Optional.FromValue(default(V));
            }
        }

        public override bool Equals(object obj)
        {
            if (obj is Optional<T>)
            {
                return this.Equals(obj);
            }

            return base.Equals(obj);
        }

        public bool Equals(Optional<T> other)
        {
            if (this is Some && other is Some)
            {
                return this.As<Some>().Equals(other.As<Some>());
            }

            return false;
        }

        public static implicit operator Optional<T>(T value)
        {
            return Optional.FromValue(value);
        }

        public override int GetHashCode()
        {
            return this.Value == null ? base.GetHashCode() : this.Value.GetHashCode();
        }

        internal class Some : Optional<T>, IEquatable<Some>
        {
            public Some(T value) : base(value)
            {
            }

            public override bool Equals(object obj)
            {
                if (obj is Some)
                {
                    return this.Equals(obj.As<Some>());
                }

                return base.Equals(obj);
            }

            public bool Equals(Some other)
            {
                if (other is None)
                {
                    return false;
                }

                if (other is Some)
                {
                    return this.Value.Equals(other.Value);
                }

                return false;
            }

            public static bool operator ==(Some left, Some right)
            {
                return left?.Equals(right) == true;
            }

            public static bool operator !=(Some left, Some right)
            {
                return left?.Equals(right) != true;
            }

            public override int GetHashCode()
            {
                return this.Value is object ? this.Value.GetHashCode() : this.As<object>().GetHashCode();
            }
        }

        internal class None : Optional<T>
        {
            public None() : base(default)
            {
            }
        }
    }
}