Infinite Fibonacci sequence generator

Fibonacci1.cs

using MoreLinq;
using System.Collections.Generic;

public IEnumerable<ulong> GenerateFibonacci()
{
    ulong previous1 = 1;
    ulong previous2 = 0;

    return MoreEnumerable
        .Generate(0UL, i => previous1 + previous2)
        .Pipe(i => { previous2 = previous1; previous1 = i; });
}

Fibonacci2.cs

using System.Collections.Generic;

public IEnumerable<ulong> GenerateFibonacci()
{
    ulong previous1 = 0;
    ulong previous2 = 1;
    ulong current = 0;

    while(true)
    {
        yield return current;
        current = previous1 + previous2; 
        previous2 = previous1;
        previous1 = current;
    }       
}