Some help understanding “yield”

Question

In my everlasting quest to suck less I\'m trying to understand the \"yield\" statement, but I keep encountering the same error.

The body of [someMetho

Answer 1

@Ian P´s answer helped me a lot to understand yield and why it is used. One (major) use case for yield is in "foreach" loops after the "in" keyword not to return a fully completed list. Instead of returning a complete list at once, in each "foreach" loop only one item (the next item) is returned. So you will gain performance with yield in such cases. I have rewritten @Ian P´s code for my better understanding to the following:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace YieldReturnTest
{
    public class PrimeFinder
    {
        private Boolean isPrime(int integer)
        {
            if (0 == integer)
                return false;

            if (3 > integer)
                return true;

            for (int i = 2; i < integer; i++)
            {
                if (0 == integer % i)
                    return false;
            }
            return true;
        }

        public IEnumerable FindPrimesWithYield()
        {
            int i;

            for (i = 1; i < 2147483647; i++)
            {
                if (isPrime(i))
                {
                    yield return i;
                }
            }
        }

        public IEnumerable FindPrimesWithoutYield()
        {
            var primes = new List();
            int i;
            for (i = 1; i < 2147483647; i++)
            {
                if (isPrime(i))
                {
                    primes.Add(i);
                }
            }
            return primes;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            PrimeFinder primes = new PrimeFinder();

            Console.WriteLine("Finding primes until 7 with yield...very fast...");
            foreach (int i in primes.FindPrimesWithYield()) // FindPrimesWithYield DOES NOT iterate over all integers at once, it returns item by item
            {
                if (i > 7)
                {
                    break;
                }
                Console.WriteLine(i);
                //Console.ReadLine();

            }

            Console.WriteLine("Finding primes until 7 without yield...be patient it will take lonkg time...");
            foreach (int i in primes.FindPrimesWithoutYield()) // FindPrimesWithoutYield DOES iterate over all integers at once, it returns the complete list of primes at once
            {
                if (i > 7)
                {
                    break;
                }
                Console.WriteLine(i);
                //Console.ReadLine();
            }

            Console.ReadLine();
            Console.ReadLine();
        }
    }
}