Unfold in C# .

gistfile1.cs

public sealed class Option<T> : IEquatable<Option<T>>
{
	private readonly T m_Value;
	private static readonly Option<T> g_None = new Option<T>();

	private Option() { }
	private Option(T value)
	{
		m_Value = value;
	}

	public override bool Equals(object obj)
	{
		if (obj is Option<T>)
			return Equals((Option<T>)obj);

		return false;
	}
	public bool Equals(Option<T> other)
	{
		if (IsNone)
			return other.IsNone;

		return EqualityComparer<T>.Default.Equals(m_Value, other.m_Value);
	}
	public override int GetHashCode()
	{
		if (IsNone)
			return 0;

		return EqualityComparer<T>.Default.GetHashCode(m_Value);
	}

	public bool IsNone
	{
		get { return this == g_None; }
	}
	public bool IsSome
	{
		get { return !IsNone; }
	}

	public T Value
	{
		get
		{
			if (IsSome)
				return m_Value;

			throw new InvalidOperationException();
		}
	}

	public static Option<T> None
	{
		get { return g_None; }
	}
	public static Option<T> Some(T value)
	{
		return new Option<T>(value);
	}
}

public static class Option
{
	public static Option<T> Some<T>(T value)
	{
		return Option<T>.Some(value);
	}
}

public static class Extensions
{
	public static IEnumerable<TResult> Unfold<T, TResult>(this T start, Func<T, Option<Tuple<TResult, T>>> generator)
	{
		var next = start;

		while (true)
		{
			var res = generator(next);
			if (res.IsNone)
				yield break;

			yield return res.Value.Item1;

			next = res.Value.Item2;
		}
	}
}

//Example


static void Main()
{
	var sequence = 0.Unfold(i =>
			{
			if (i < 10)
			return Option.Some(Tuple.Create(i + 1, i + 1));

			return Option<Tuple<int, int>>.None;
			}).ToList();

        var sequence = 0.Unfold(i => Option.Some(Tuple.Create(i + 1, i + 1))).Take(10).ToList();
	Console.ReadLine();
}