LINQ Y Combinator

1st YCombinate.cs

static void Main() {
	var fibonacci = from fib in YCombinate<ulong, ulong>.Definition
	                select n => n < 2 ? 1 : fib(n - 1) + fib(n - 2);
	Console.WriteLine(fibonacci(5));
}

delegate TDelegate Recursive<TDelegate>(TDelegate r);
static class YCombinate<TArg, TReturn> {
	public static class Definition {
		public static Func<TArg, TReturn> Select(Recursive<Func<TArg, TReturn>> definition) {
			Func<TArg, TReturn> retVal = null;
			retVal = arg => definition(retVal)(arg);
			return retVal;
		}
	}
}

Pure LINQ.cs

var fibonacci = from def in new Func<Func<ulong, ulong>, Func<ulong, ulong>>[] {
			fib => n => n < 2 ? 1 : fib(n - 1) + fib(n - 2)
		}
		let ret = new Func<ulong, ulong>[] { null }
		select ret[0] = a => def(ret[0])(a);
Console.WriteLine(fibonacci.First()(5));