Either{TL,TR}.cs

namespace Library.Functional
{
    using System;

    /// <summary>
    /// Functional data data to represent a discriminated
    /// union of two possible types.
    /// </summary>
    /// <typeparam name="TL">Type of "Left" item.</typeparam>
    /// <typeparam name="TR">Type of "Right" item.</typeparam>
    public class Either<TL, TR>
    {
        private readonly TL left;
        private readonly TR right;
        private readonly bool isLeft;

        public Either(TL left)
        {
            this.left = left;
            this.isLeft = true;
        }

        public Either(TR right)
        {
            this.right = right;
            this.isLeft = false;
        }

        public T Match<T>(Func<TL, T> leftFunc, Func<TR, T> rightFunc)
        {
            if (leftFunc == null)
            {
                throw new ArgumentNullException(nameof(leftFunc));
            }

            if (rightFunc == null)
            {
                throw new ArgumentNullException(nameof(rightFunc));
            }

            return this.isLeft ? leftFunc(this.left) : rightFunc(this.right);
        }

        /// <summary>
        /// If right value is assigned, execute an action on it.
        /// </summary>
        /// <param name="rightAction">Action to execute.</param>
        public void DoRight(Action<TR> rightAction)
        {
            if (rightAction == null)
            {
                throw new ArgumentNullException(nameof(rightAction));
            }

            if (!this.isLeft)
            {                
                rightAction(this.right);
            }
        }

        public TL LeftOrDefault() => this.Match(l => l, r => default(TL));

        public TR RightOrDefault() => this.Match(l => default(TR), r => r);

        public static implicit operator Either<TL, TR>(TL left) => new Either<TL, TR>(left);

        public static implicit operator Either<TL, TR>(TR right) => new Either<TL, TR>(right);
    }
}