C#•6mo ago

yield return

i can't understand the benefits of yield return , if it's benefit to avoid fetching all data from database and storing them in the memory,and reading each data on demand. i think i can does that by stream types,so what the need to yield return?

15 Replies

Jimmacle•6mo ago

yield return is for producing sequences one element at a time, that's it it has nothing to do with databases

eidOP•6mo ago

and has nothing to do with reading file?

canton7•6mo ago

Note that Stream just works with bytes

Jimmacle•6mo ago

no, it is a standalone language feature you can use it with anything

canton7•6mo ago

Streams are specifically for reading sets of bytes, from a source which might be able to read 0, 1, or many bytes at that point in time

Jimmacle•6mo ago

public IEnumerable<int> GetSomeNumbers()
{
  yield return 1;
  yield return 2;
  yield return 3;
}

public IEnumerable<int> GetSomeNumbers()
{
  yield return 1;
  yield return 2;
  yield return 3;
}

canton7•6mo ago

Iterators are a different mechanism. They work with any element type, and they just yield a single element at a time

Jimmacle•6mo ago

public IEnumerable<int> GetPowersOf2()
{
  var current = 1;
  while (true)
  {
    yield return current;
    current *= 2;
  }
}

public IEnumerable<int> GetPowersOf2()
{
  var current = 1;
  while (true)
  {
    yield return current;
    current *= 2;
  }
}

some examples

eidOP•6mo ago

i like the example, but i'm talking about real example

Jimmacle•6mo ago

producing infinite sequences is a real example

eidOP•6mo ago

i mean how c# used in in it's built-in language, what's the apply of it in the language

canton7•6mo ago

foreach loops for one: the ability to iterate over collections

reflectronic•6mo ago

private IEnumerable<T> Where<T>(IEnumerable<T> source, Func<T, bool> predicate)
{
    foreach (var item in source)
    {
        if (predicate(item))
        {
            yield return item;
        }
    } 
}

private IEnumerable<T> Where<T>(IEnumerable<T> source, Func<T, bool> predicate)
{
    foreach (var item in source)
    {
        if (predicate(item))
        {
            yield return item;
        }
    } 
}

canton7•6mo ago

All of Linq: the ability to filter and transform sequences of elements

SpReeD•6mo ago

Also notable is that the result of an IEnumerable<T> function isn't called on it's assignment, but it's usage. Combined with a filter like Where(<predicate>) you don't need to loop over each element. And depending on the condition, not even once. Example:

private IEnumerable<int> GiveMeInts()
{
    for (int i = 0; i < 10; i++)
    {
        yield return i;
    }
}

IEnumerable<int> ints = this.GiveMeInts(); // not called
int[] intsArray = this.GiveMeInts().ToArray(); // called

foreach (int i in ints) // calls GiveMeInts
{
    Debug.Print($"{i}");
}

private IEnumerable<int> GiveMeInts()
{
    for (int i = 0; i < 10; i++)
    {
        yield return i;
    }
}

IEnumerable<int> ints = this.GiveMeInts(); // not called
int[] intsArray = this.GiveMeInts().ToArray(); // called

foreach (int i in ints) // calls GiveMeInts
{
    Debug.Print($"{i}");
}

It's mostly performance related, also by using an interface the user is free to whatever collection one wanna implement. A win-win situation.

Gaming

Programming

yield return