C# Few good uses of yield keyword

Question

I want to see few good uses of yield keyword.

1) i want to know the advantage of yield and when to use yield. please discuss with particular scenario.

2) How we can create custom enumarator using yield?

3)How do i understand that yield is lazy execution ? how can one say below code is a sample of yield lazy execution?

public static IEnumerable<int> CreateCollectionWithYield()  
{  
    yield return 10;  
    for (int i = 0; i < 3; i++)   
    {  
        yield return i;  
    }  

    yield return 20;  
}  
var yieldedItems = CreateCollectionWithYield();  

another sample

IEnumerable<T> FilterCollection<T>( ReadOnlyCollection<T> input ) {  
    foreach ( T item in input )  
        if (  /* criterion is met */ )  
            yield return item;  
}  

4) How yield maintain state ? in below code runningTotal variable set 0 once but RunningTotal() function calling repeatedly but runningTotal variable not initialize with 0 again.

how it is getting possible?

var vals = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };  

foreach (int e in RunningTotal())  
{  
    Console.WriteLine(e);  
}  

IEnumerable<int> RunningTotal()  
{  
    int runningTotal = 0;  

    foreach (int val in vals)  
    {  
        runningTotal += val;  
        yield return runningTotal;  
    }  
}  

Thanks

Answer 1

If using .NET Core 5, Chunk internally uses yield. Try the following example here.

using System;  
using System.Collections.Generic;  
using System.Globalization;  
using System.Linq;  
public class Program  
{  
	public static void Main()  
	{  
        IEnumerable<string[]> chunk = DateTimeFormatInfo.CurrentInfo!.MonthNames[..^1].ToList().Chunk(2);  
        List<string[]> list = chunk.ToList();  
        for (int index = 0; index < list.LongCount(); index++)  
        {  
            Console.WriteLine(index);  
            foreach (var name in list[index])  
            {  
                Console.WriteLine($"\t{name}");  
            }  
        }  
	}  
}  

MS Code

private static IEnumerable<TSource[]> ChunkIterator<TSource>(IEnumerable<TSource> source, int size)  
{  
    using IEnumerator<TSource> e = source.GetEnumerator();  
    while (e.MoveNext())  
    {  
        TSource[] chunk = new TSource[size];  
        chunk[0] = e.Current;  
  
        int i = 1;  
        for (; i < chunk.Length && e.MoveNext(); i++)  
        {  
            chunk[i] = e.Current;  
        }  
  
        if (i == chunk.Length)  
        {  
            yield return chunk;  
        }  
        else  
        {  
            Array.Resize(ref chunk, i);  
            yield return chunk;  
            yield break;  
        }  
    }  
}  

Other uses, iterating a DataReader to return each item in the loop retrieving data.

Answer 2

the yield is used for C# implementation of the common language (dates back to the 60's) feature: generator.

https://en.wikipedia.org/wiki/Generator_(computer_programming)

Answer 3

This ^ is known as the hat operator.

If we explore the hat operator with month names shown below, indexing the names we can use conventional indexing Start Index or in reverse with the hat operator via End Index

This is the code to do what is shown above and most likely will invoke more questions as it's advance code so if there are questions, rather than ask questions perform your own research

using System.Globalization;  
using IndexMonths.Models;  
  
namespace IndexMonths.Classes;  
  
class Helpers  
{  
    public static List<string> MonthNames() =>   
        DateTimeFormatInfo.CurrentInfo!.MonthNames[..^1].ToList();  
  
    public static List<ElementContainer<T>> RangeDetails<T>(List<T> list) =>  
        list.Select((element, index) => new ElementContainer<T>  
        {  
            Value = element,  
            StartIndex = new Index(index),  
            EndIndex = new Index(Enumerable.Range(0, list.Count).Reverse().ToList()[index], true),  
            MonthIndex = index + 1  
        }).ToList();  
}  

Calling the above

using IndexMonths.Classes;  
  
namespace IndexMonths  
{  
    internal partial class Program  
    {  
        static void Main(string[] args)  
        {  
  
            var monthContainer = Helpers.RangeDetails(Helpers.MonthNames());  
            var table = CreateTable();  
            monthContainer.ForEach(x => table.AddRow(x.ItemArray));  
              
            AnsiConsole.Write(table);  
  
            ExitPrompt();  
        }  
    }  
}  

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