When to use yield return in C #

The yield keyword is used to traverse the loop, the yield return is used to return ienumerable <t>, and the yield break is used to terminate the loop traversal.

 

There is a set of int types:

static List<int> GetInitialData()        {            return new List<int>(){1,2,3,4};        }

Print all elements with a value greater than 2.

 

Implementation of yield return is not used

static IEnumerable<int> FilterWithoutYield()        {            List<int> result = new List<int>();            foreach (int i in GetInitialData())            {                if (i > 2)                {                    result.Add(i);                }             }            return result;        }

Client call:

static void Main(string[] args)        {            foreach (var item in FilterWithoutYield())            {                Console.WriteLine(item);            }            Console.ReadKey();         }

Output result: 3, 4

 

Implementation Using yeild return

Static ienumerable <int> filterwithyield () {foreach (int I in getinitialdata () {if (I> 2) {yield return I ;}} yield break; console. writeline ("code not executed here ");}

Client call:

static void Main(string[] args)        {            foreach (var item in FilterWithYield())            {                Console.WriteLine(item);            }            Console.ReadKey();         }

Output result: 3, 4

 

Summary

Through one-step debugging, we found that:

Although the output results of the two methods are the same, the operation process is quite different. The first method is to load all the result sets to the memory and traverse them. The second method is to return a value to the client every time the client calls yield return, which is "supplied on demand ".

In the first method, the client call process is roughly as follows:

 

When yield return is used, the client call process is roughly as follows:

 

Why can I use yield return to ensure that execution starts from the previous stopped place during each loop traversal?

-- The compiler generates a state machine to maintain the iterator state.

 

To put it simply, if you want to obtain a set of ienumerable <t> types instead of loading data to the memory at a time, you can use yield return to implement "On-Demand Supply ".