Either - like in F#

Either.cs

using System;

namespace Helpers
{
    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) => this.isLeft ? leftFunc(this.left) : rightFunc(this.right);

        public Either<TL, TR> InvokeOn<T>(T param, Func<TL, T, TL> leftFunc, Func<TR, T, TR> rightFunc) => this.isLeft ? new Either<TL, TR>(leftFunc(this.left, param)) : new Either<TL, TR>(rightFunc(this.right, param));

        public Either<TL, TR> InvokeOn<T>(T[] param, Func<TL, T[], TL> leftFunc, Func<TR, T[], TR> rightFunc) => this.isLeft ? new Either<TL, TR>(leftFunc(this.left, param)) : new Either<TL, TR>(rightFunc(this.right, param));

        public bool IsOfType(Type t) => this.isLeft && typeof(TL).Equals(t) || !this.isLeft && typeof(TR).Equals(t);

        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);

        public static implicit operator TL(Either<TL, TR> e)
        {
            if (e.isLeft == true) return e.left;
            throw new InvalidCastException($"This object is not of type {typeof(TL).ToString()}");
        }

        public static implicit operator TR(Either<TL, TR> e)
        {
            if (e.isLeft == false) return e.right;
            throw new InvalidCastException($"This object is not of type {typeof(TR).ToString()}");
        }
    }
}

Copied from https://github.com/mikhailshilkov/mikhailio-samples/blob/1456e4ad61db1d0ae633f9262218aa09a9fda645/Either%7BTL,TR%7D.cs and extended with couple of new methods (casting, method invoke, etc.)