Either type in C#.cs

public class Either<TDefault, TOther> : IEquatable<Either<TDefault, TOther>>
{
    private readonly Union<TDefault, TOther> _union;

    public Either(TDefault value)
    {
        _union = value;
    }

    public Either(TOther value)
    {
        _union = value;
    }

    public static implicit operator Either<TDefault, TOther>(TDefault value)
    {
        return new Either<TDefault, TOther>(value);
    }

    public static implicit operator Either<TDefault, TOther>(TOther value)
    {
        return new Either<TDefault, TOther>(value);
    }

    public Either<TResult, TOther> Map<TResult>(Func<TDefault, TResult> map)
    {
        return _union.Match(
            x => new Either<TResult, TOther>(map(x)),
            other => new Either<TResult, TOther>(other)
        );
    }

    public Either<TResult, TOther> FlatMap<TResult>(this Either<TDefault, TOther> input, Func<TDefault, Either<TResult, TOther>> map)
    {
        return _union.Match(
            x => new Either<TResult, TOther>(map(x)),
            other => new Either<TResult, TOther>(other)
        );
    }
    ...
}