In C#:

    static IEnumerable<int> Fibonacci()  
    {  
        int first = 0;  
        int second = 1;  
      
        yield return first;  
        yield return second;  
      
        while (true)  
        {  
            (first, second) = (second, second + first);  
            yield return second;  
        }  
    }