How to use Yield Return in C#

using System;
class YieldReturnEnumerable
{
    static void Main()
    {
        // foreach collects returned values in i
        // Function call is in the brackets
        foreach (int i in fibb(10)){
          // Here we can process each individual return value
          Console.Write("{0} ", i);
        }
    }
    // return type is IEnumerable<int>
    public static System.Collections.Generic.IEnumerable<int> fibb(int num)
    {
        int returnVal = 1;
        int temp;
        int prev = 0;
        for (int i = 0; i < num; i++)
        {
            // yield return returns one element at a time
            yield return returnVal;
            // Processing done for Fibonacci numbers
            temp = returnVal;
            returnVal = prev + returnVal;
            prev = temp;
        }
    }
}